LM2636
5-Bit Programmable Synchronous Buck Regulator
Controller

General Description

The LM2636 is a high speed controller designed specifically
for use in synchronous DC/DC buck converters for advance
d microprocessors. A 5-bit DAC accepts the VID code di-
rectly from the CPU and adjusts the output voltage from 1.3V
to 3.5V. It provides the power good, over-voltage protection,
and output enable features as required by Intel VRM speci-
fications. Current limiting is achieved by monitoring the volt-
age drop across the r

DS_ON

of the high side MOSFET, which

eliminates an expensive current sense resistor.

The LM2636 employs a fixed-frequency voltage mode PWM
architecture. To provide a faster response to a large and fast
load transient, two ultra-fast comparators are built in to moni-
tor the output voltage and override the primary control loop
when necessary. The PWM frequency is adjustable from 50
kHz to 1 MHz through an external resistor. The wide range of
PWM frequency gives the power supply designer the flexibil-
ity to make trade-offs between load transient response per-
formance, MOSFET cost and the overall efficiency. The
adaptive non-overlapping MOSFET gate drivers help avoid
any potential shoot-through problem while maintaining high
efficiency. BiCMOS gate drivers with rail-to-rail swing ensure
that no spurious turn-on occur. When only 5V is available, a
bootstrap structure can be employed to accommodate an
NMOS high side switch. The precision reference trimmed to

2.5% over temperature is available externally for use by
other regulators. Dynamic positioning of load voltage, which
helps cut the number of output capacitors, can also be imple-
mented easily.

Features

1.3V to 3.5V 5-bit programmable output voltage

Synchronous rectification

Power Good flag and output enable

Over-voltage protection

Initial Output Accuracy: 1.5% over temperature

Current limit without external sense resistor

Adaptive non-overlapping MOSFET gate drives

Adjustable switching frequency: 50 kHz to 1 MHz

Dynamic output voltage positioning

1.256V reference voltage available externally

Plastic SO-20 package and TSSOP-20 package

Applications

Motherboard power supply/VRM for Cyrix Gxm, Cyrix
Gxi, Cyrix MII, Pentium

II, Pentium Pro, 6x86 and K6

processors

5V to 1.3V3.5V high current power supplies

Typical Application

Pentium

is a trademark of Intel Corporation.

DS100834-1

FIGURE 1. 5V to 1.3V3.5V, 14A Power Supply

October 1999

LM2636

5-Bit

Programmable

Synchronous

Buck

Regulator

Controller

DS100834

www.national.com

Connection Diagrams

Pin Descriptions

LSGATE (Pin 1): Gate drive for the low-side N-channel
MOSFET. This signal is interlocked with HSGATE (Pin 20) to
avoid a shoot-through problem.

BOOTV (Pin 2): Power supply for high-side N-channel
MOSFET gate drive. The voltage should be at least one gate
threshold above the converter input voltage to properly oper-
ate the high-side N-FET.

PGND (Pin 3): Ground for high current circuitry. It should be
connected to system ground.

SGND (Pin 4): Ground for signal level circuitry. It should be
connected to system ground.

(Pin 5): Power supply for the controller.

SENSE (Pin 6): Converter output voltage sensing. It pro-
vides input for power good, fast dual comparator control
loop, and over-voltage protection circuitry. It is recom-
mended that a 0.1 µF capacitor be connected between this
pin and ground to avoid potential noise problems.

IMAX (Pin 7): Current limit threshold setting. It sinks a fixed
180 µA current. By connecting a resistor between the high
side MOSFET drain and this pin, a fixed voltage drop can be
built across the resistor. This voltage drop is compared with
the V

of the high-side N-MOSFET to determine if an over-

current condition has occurred.

IFB (Pin 8): High-side N-MOSFET source voltage sensing.
This pin is one V

below drain voltage. When this voltage is

lower than that of IMAX pin during the time the high-side FET

is on, it means V

is higher than the preset voltage across

the IMAX resistor, which can be interpreted as an over-
current condition.

REF

(Pin 9): Bandgap reference voltage. This voltage is

mainly for use by other power supplies on the motherboard
which need a reference.

EA_OUT (Pin 10): Output of the error amplifier. The voltage
level on this pin is compared with an internally generated
ramp signal to determine the duty cycle. This pin is neces-
sary for compensating the primary control loop.

FB (Pin 11): Inverting input of the error amplifier. A pin nec-
essary for compensating the control loop.

FREQ_ADJ (Pin 12): Switching frequency adjustment.
Switching frequency can be adjusted by changing the
grounding resistance on this pin.

PWRGD (Pin 13): Power Good. There are two windows
around the DAC output voltage that are associated with
PWRGD pin, the

10% window and the

8% window. If

PWRGD is initially high (open drain state) and output voltage
travels out of

10% window, PWRGD goes to low (low im-

pedance to ground). If PWRGD is initially low and output
voltage travels into the

8% window and has stayed within

the window for at least 10 ms, PWRGD goes to high. A
PWRGD high means the output voltage is at least within the

10% window whereas a PWRGD low indicates the output

voltage is definitely outside the

8% window.

VID4:0 (Pins 14, 15, 16, 17, 18): Voltage Identification
Code. The five pins accept an open-ground pattern 5-bit bi-
nary code from outside the chip (typically from the CPU) for
generating the desired output voltage. Each VID pin is inter-
nally pulled up to V

via a 90 µA current source.

Table 1

shows the code table.

OUTEN (Pin 19): Output Enable. The output voltage is dis-
abled when this pin is pulled low. It is internally pulled up to
V

via a 90 µA current source.

HSGATE (Pin 20): Gate drive for the high-side N-channel
MOSFET. This signal is interlocked with LSGATE (Pin 1) to
avoid a shoot-through problem.

TABLE 1. VID Code and DAC Output

ID4

ID3

ID2

ID1

ID0

Rated Output

Voltage (V)

1.30

1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

TOP VIEW

DS100834-3

Plastic SO-20

Order Number LM2636M

See NS Package Number M20B

TOP VIEW

DS100834-3

Plastic TSSOP-20

Order Number LM2636MTC

See NS Package Number MTC20

LM2636

www.national.com

Absolute Maximum Ratings

(Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

(All voltages are referenced to the PGND and SGND pins.)

BOOTV

18V

Junction Temperature

150°C

DC Power Dissipation (Note 2)

1.42W

Storage Temperature

-65°C to +150°C

Soldering Time, Temperature

Wave (4 seconds)
Infrared (10 seconds)
Vapor Phase (75 seconds)

260°C
240°C
219°C

ESD Susceptibility (Note 3)

2 kV

Recommended Operating
Conditions

(Note 1)

Supply Voltage Range (V

)

4.5V to 5.5V

Junction Temperature Range

0°C to +125°C

Electrical Characteristics

= 5V unless otherwise indicated under the Conditions column. Typicals and limits appearing in plain type apply for T

= +25°C. Limits appearing in boldface type apply over 0°C to +70°C.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

BOOTV

FET Driver Supply
Voltage

DACOUT

5-Bit DAC Output
Voltage

VID4:0=01111

1.284

1.304

1.324

VID4:0=01101

1.385

1.406

1.427

VID4:0=01011

1.483

1.506

1.529

VID4:0=01001

1.585

1.609

1.633

VID4:0=00111

1.683

1.709

1.735

VID4:0=00101

1.784

1.811

1.838

VID4:0=00001

1.983

2.013

2.043

VID4:0=11101

2.173

2.206

2.239

VID4:0=11010

2.471

2.509

2.547

VID4:0=10111

2.768

2.81

2.852

OUT

DC Load Regulation

OUT

=0 to 14A

Figure 2

-5

DC Line Regulation

=4.75V to 5.25V

Figure 2

Error Amplifier DC Gain

Error Amplifier Slew
Rate

V/µs

Error Amplifier Unity
Gain Bandwidth

MHz

Q_VCC

Operating V

Current

OUTEN=V

=5V, VID=10111

1.5

2.5

Shutdown V

Current

OUTEN Floating, VID0:4 Floating

1.5

Q_BOOTV

BOOTV Pin Quiescent
Current

BOOTV=12V, OUTEN=0, VID0:4
Floating

µA

MAX

Maximum Duty Cycle

MIN

Minimum Duty Cycle

SENSE

SENSE Pin Resistance
to Ground

11.5

DS_SRC

FET Driver
Drain-Source ON
Resistance when
Sourcing Current

BOOTV=5V

DS_SINK

FET Driver
Drain-Source ON
Resistance when
Sinking Current

(Independent of BOOTV Voltage)

1.7

OSC

Oscillator Frequency

= 84 k

250

300

350

kHz

= 22 k

1000

= 10.5 k

2000

MAX

IMAX Pin Sink Current

IMAX

= 5V, V

IFB

= 6V, V

= 5V

130

180

230

µA

LM2636

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Electrical Characteristics

(Continued)

= 5V unless otherwise indicated under the Conditions column. Typicals and limits appearing in plain type apply for T

= +25°C. Limits appearing in boldface type apply over 0°C to +70°C.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

OUTEN_IH

OUTEN Pin Input Logic
Low to Logic High Trip
Point

OUTEN Voltage

3.5

3.0

OUTEN_IL

OUTEN Pin Input Logic
High to Logic Low Trip
Point

OUTEN Voltage

1.8

1.5

REF

Band Gap Reference

VREF

= 0 mA

1.225

1.256

1.287

REF_LOAD

Reference Voltage at
Full Load

VREF

= 0.5 mA, Sourcing

1.223

1.254

1.285

REF_525

Reference Voltage at
High Line

VREF

= 0 mA, V

= 5.25V

1.226

1.257

1.288

REF_475

Reference Voltage at
Low Line

VREF

= 0 mA, V

= 4.75V

1.224

1.255

1.286

REF_LOAD

Reference Voltage
Load Regulation

VREF

= 0.5 mA, Sourcing

-2

REF_LINE

Reference Voltage Line
Regulation

VREF

= 0 mA, V

Changes from

5.25V to 4.75V

-0.5

SAWL

Ramp Signal Valley
Voltage

1.25

SAWH

Ramp Signal Peak
Voltage

3.25

PWRBAD_GD

PWRGD Pin

Trip

Points (see Pin
Description for Pin 13)

% above DAC Output Voltage, when
Output Voltage

% below DAC Output Voltage, when
Output Voltage

-10

PWRGD_BAD

PWRGD Pin

Trip

Points (see Pin
Description for Pin 13)

% above DAC Output Voltage, when
Output Voltage

% below DAC Output Voltage, when
Output Voltage

-8

OVP

Over-voltage Protection
Trip Point

% above DAC Output Voltage

PWRGD

Power Good Response
Time

SENSE

Rises from 0V to Rated

OUT

µs

PWRBAD

Power Not Good
Response Time

SENSE

Falls from Rated V

OUT

to 0V

µs

OUTEN

OUTEN Pin Internal
Pull-Up Current

130

µA

VID_IH

VID Pins Logic High
Trip Point

3.5

3.0

VID_IL

VID Pins Logic Low
Trip Point

1.8

1.3

VID

VID0:4 Internal Pull-Up
Current

130

µA

Soft Start Duration

2048

clock

cycles

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Recommended Operating Conditions are conditions under which
the device operates correctly. Recommended Operating Conditions do not imply guaranteed performance limits.

Note 2: Maximum allowable DC power dissipation is a function of the maximum junction temperature, T

JMAX

, the junction-to-ambient thermal resistance,

, and

the ambient temperature, T

. The maximum allowable power dissipation at any ambient temperature is calculated using:

The junction-to-ambient thermal resistance,

, for LM2636 in the M20B package is 88°C/W, and 120°C/W for the MTC20 package.

LM2636

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Applications Information

OVERVIEW

The LM2636 is a high speed synchronous PWM buck regu-
lator controller designed for VRM vendors or motherboard
manufacturers who need to build on-board power supplies
for Cyrix MII, Pentium II or Deschutes microprocessors. It
has a built-in 5-bit DAC to decode the 5-bit word provided by
the CPU and supply the corresponding voltage. It also has
the power good (PWRGD) and output enable (OUTEN) func-
tions required by the VRM specification. It employs a voltage
mode control scheme plus two fast responding comparators
to quickly respond to large load transients. It has two fast
FET drivers to drive the high-side and low-side NMOS
switches of a synchronous buck regulator. The PWM fre-
quency is adjustable from 50 kHz to 1 MHz through an exter-
nal resistor. Over-voltage protection is achieved by shutting
off the high-side driver and turning on the low-side driver
100% of the time. Current limiting is implemented by sensing
V

of the high-side NMOS switch and shutting it off for the

present switching cycle when an over current condition is de-
tected. Soft start functionality is realized through an internal
digital counter and an internal DAC.

THEORY OF OPERATION

Start Up

When V

voltage exceeds 4.2V, OUTEN pin is a logic high

and the VID code is valid, the soft start circuitry starts to
work. The duration of the soft start is determined by an inter-
nal digital counter and the switching frequency. During soft
start, the output of the error amplifier is allowed to increase
gradually. When the counter has counted 2,048 clock cycles,
the soft start session ends and the output voltage level of the
error amplifier is released and allowed to go to a value that is
determined by the feedback loop. PWRGD pin is forced low
during soft start and is turned over to output voltage monitor-
ing circuitry after that. Before V

reaches 4.2V, all internal

logic is in a power on reset state and the two FET drivers are
disabled.

During normal operation, if V

voltage drops below 3.8V,

the internal circuitry will go into power on reset again. The
hysteresis helps decrease the noise sensitivity on the V

pin. After soft starts ends and during normal operation, if the
converter output voltage exceeds 115% of the DAC output
voltage, the LM2636 will lock into over voltage protection
mode. The high side drive will be disabled, and the low side
drive will be high. There are two ways to clear the mode. One
is to cycle V

voltage once. The other is to toggle the

OUTEN level. After the over voltage protection mode is
cleared, the LM2636 will enter the soft start session and start
over.

Normal Operation

During the normal operation mode, the LM2636 regulates
the converter output voltage by adjusting the duty ratio. The
output voltage is determined by the 5-bit VID code set by the
user/load.

The PWM frequency is set by the external resistor between
FREQ_ADJ pin and ground. The resistance needed for a de-
sired switching frequency is:

For example, if the desired switching frequency is 300 kHz,
the resistance should be around 84 k

The minimum allowable PWM frequency is 5 kHz.

MOSFET Gate Drive

The LM2636 has two gate drives that are suitable for driving
external N-MOSFETs in a synchronous buck topology. The
power for the two FET drivers is supplied by the BOOTV pin.
This BOOTV voltage needs to be at least one V

GS(th)

higher

than the converter input voltage for the high side FET to be
fully turned on. The voltage can be either supplied from
a separate source other than the input voltage or can be
generated locally by utilizing a charge pump structure. In a
typical desktop microprocessor application, if 5V is chosen
to be the input voltage, then 12V can be used for the
BOOTV. If 12V is not available, a simple charge pump cir-
cuitry consisting of a diode and a small capacitor can be
used, as shown in

Figure 3.

When the low side FET is on, the charge pump capacitor is
charged to near the input voltage through the diode. When
low side FET is turned off, the high side FET driver is en-
abled, and the charge pump capacitor starts to charge the
high side FET gate until it is fully on. By this time the high
side FET source node will fly to close to input voltage level
and the upper node of the capacitor will also fly to one input
voltage higher than the input voltage, enabling the high side
FET driver to continue working.

For a BOOTV of 12V, the initial gate charging current is typi-
cally 2A, and the initial gate discharging current is typically
6A, good for high speed switching.

The LM2636 gate drives are of BiCMOS design. Unlike
some other bipolar VRM control ICs, the gate drive has rail-
to-rail swing that ensures no spurious turn-on due to capaci-
tive coupling.

Another feature of the FET gate drives is the adaptive non-
overlapping mechanism. A gate driver is not turned on until
the other is fully off. The dead time in between is typically 20
ns. This avoids the potential shoot-through problem and
helps improve efficiency.

Load Transient Response

In a typical modern MPU application such as the Pentium II
core voltage power supply, load transient response is a criti-
cal issue. The LM2636 utilizes the conventional voltage
feedback technology as the primary feedback control
method. When the load transient happens, the error in the
output voltage level is fed to the error amplifier. The output of

DS100834-6

FIGURE 3. BOOTV Voltage Supplied by a Charge Pump

LM2636

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Applications Information

(Continued)

the

error

amplifier

then

compared

with

internally generated PWM ramp signal and the result of the
comparison is a series of pulses with certain duty ratios.
These pulses are used to control the turn-on and turn-off of
the MOSFET gate drivers. In this way, the error in the output
voltage gets "compensated" or cancelled by the change in
the duty ratio of the FET switches. During a large load tran-
sient, depending on the compensation design, the change in
duty ratio can be as fast as less than one switching cycle.
Refer to Design Considerations section for more details.

Besides the usual voltage mode feedback control loop, the
LM2636 also has a pair of fast comparators (the MIN and
MAX comparators) to help maintain the output voltage dur-
ing a large and fast load transient. The trip points of the com-
parators are set to

5% of the DAC output voltage. When

the load transient is so large that the output voltage goes
outside the

5% window, the MIN or MAX comparator will

bypass the primary voltage control loop and immediately set
the duty ratio to either maximum value or to zero. This pro-
vides the fastest possible way to react to such a large load
transient in a classical buck converter.

Power Good Signal

The power good signal is used to indicate that the output
voltage is within specified range. In the LM2636, the range is
set to a

10% window of the DAC output voltage. During soft

start, the power good signal is always low. At the end of the
soft start session,the output voltage is checked and the
PWRGD pin will be asserted if the voltage is within specified
range.

Over Voltage Protection

When the output voltage exceeds 115% of the DAC output
voltage after the end of soft start, the LM2636 will enter over
voltage protection mode in which it shuts itself down. The up-
per gate driver is held low while the lower gate driver is held
high. PWRGD will be low. For LM2636 to recover from OVP
mode, either OUTEN or V

voltage has to be toggled. An-

other more subtle way to recover is to float all the VID pins
and reapply the correct code.

Current Limit

Current limit is realized by sensing the V

voltage of the

high side MOSFET when it is on. Since the r

DS_ON

of a

MOSFET is a known value, current through the MOSFET
can be known by monitoring V

. The relationship between

the three parameters is:

To implement the current limit function, an external resistor
R

IMAX

is need. The resistor should be connected between

the drain of the high side MOSFET and the IMAX pin. A con-
stant current of around 180 µA is forced into the IMAX pin
and causes a fixed voltage drop across the R

IMAX

resistor.

This voltage drop is then compared with the V

of the high

side MOSFET and if the latter is higher, over current is
reached. So the appropriate value of R

IMAX

for a pre-

determined current limit level I

LIM

can be calculated by the

following equation:

For example, if we know that the r

DS_ON

of the MOSFET is

20 m

, and the current limit we want to set is 20A, then we

should choose the value of R

IMAX

to be 2.2 k

To provide the greatest protection over the high side MOS-
FET, cycle by cycle protection is implemented. The sampling
of the V

starts as early as about 300 ns after the switch is

turned on. Whenever an over current condition is detected,
the high side switch is immediately turned off and the low
side switch turned on, until the next switching cycle comes.
The delay of 300 ns is to circumvent switching noise when
the MOSFET is first turned on.

DESIGN CONSIDERATIONS

Control Loop Compensation

A switching regulator should be properly compensated to
achieve a stable condition. For a synchronous buck regula-
tor that needs to meet stringent load transient requirement
such as a Pentium II MPU core voltage supply, a simple
2-pole-1-zero compensation network should suffice, such as
the one shown in

Figure 4 (C

, C

, R

and R

). This is be-

cause the ESR zero of the typical output capacitors is low
enough to make the control-to-output transfer function a
single-pole-roll-off.

As an example, let us figure out the values of the compensa-
tion network components in

Figure 4. Assume the following

parameters: R = 20

, R

= 20 m

, R

= 9 m

, L = 2 µH,

C = 7.5 mF, V

= 5V, V

= 2V and switching frequency

= 300 kHz. These parameters are based on the typical appli-
cation in

Figure 1. Notice R

is the sum of the inductor DC re-

sistance and the on resistance of the MOSFETs.

DS100834-9

FIGURE 4. Buck Converter from a Control Point of

View

LM2636

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Applications Information

(Continued)

The control-to-output transfer function is

The ESR zero frequency is:

The power stage double pole frequency is:

The corresponding Bode plots are shown in

Figure 5.

Since the ESR zero frequency is so low, it effectively cancels
the phase shift from one of the power stage poles. This limits
the total phase shift to 90%.

Although this regulator design is stable (phase shift is

90°

when gain = 0dB), it needs compensation to improve the DC
gain and cut off frequency (0dB frequency). Otherwise, the
low DC gain may cause a poor line regulation, and the low
cutoff frequency will hurt transient response performance.

The transfer function for the 2-pole-1-zero compensation
network shown in

Figure 4 is:

where

One of the poles is located at origin to help achieve the high-
est DC gain. So there are three parameters to determine, the
position of the zero, the position of the second pole, and the
constant A. To determine the cutoff frequency and phase
margin, the loop bode plots need to be generated. The loop
transfer function is:

TF = -TF1 x TF2

By choosing the zero close to the double pole position and
the second pole to half of the switching frequency, the closed
loop transfer function turns out to be very good.

That is, if f

= 1.32 kHz, f

= 153 kHz, and A = 4.8 x 10

-6

then the cutoff frequency will be 50 kHz, the phase margin
will be 72°, and the DC gain will be that of the error amplifier.
See

Figure 6 below.

The compensation network component values can be deter-
mined by the following equations:

Notice there are three equations but four variables. So one
of the variables can be chosen arbitrarily. Since the current
driving capability of the error amplifier is limited to around
3 mA, it is a good idea to have a high impedance path from
the output of the error amplifier to the output of the converter.
From the above equations it can be told that a larger R

will

DS100834-13

FIGURE 5. Control-to-Output Bode Plots

DS100834-17

FIGURE 6. Loop Bode Plots

LM2636

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Applications Information

(Continued)

result in a smaller C

, C

and a larger R

. However, too large

an R

can also bring error due to the bias current required by

the inverting input pin of the error amplifier. Calculations
show that the following combination is a good one: R

= 51

= 0.022 µF, R

= 5.6 k

, C

= 820 pF.

For a different application or different type of output capaci-
tors, a different compensation scheme may be necessary.
The user can either follow the steps above to figure the ap-
propriate component values or contact the factory for help.

MOSFET SELECTION

The selection of MOSFET switches affects both the effi-
ciency of the whole converter and the current limit setting.
From an efficiency point of view it is suggested that for the
high-side switch, only logic level MOSFETs be used. Stan-
dard MOSFETs can be used for the low side switch when
12V is used to power the BOOTV pin. The lower loss asso-
ciated with the MOSFETs is two-fold -- Ohmic loss and
switching loss. The Ohmic loss is easy to calculate whereas
the switching loss is much more difficult to estimate. In gen-
eral the switching loss is directly proportional to the switching
frequency. As the power MOSFET technology advances,
lower and lower gate charge devices will be available. That
should allow the user to go to higher switching frequencies
without the penalty of losing too much efficiency.

As an example, let us select the MOSFETs for a converter
with a target efficiency of 80% at a load of 2.8V, 14A. As-
sume the inductors lose 1W, the capacitors lose 0.75W and
the total switching loss at 300 kHz is 3.2W. The total allowed
power loss is 9.8W, so the MOSFET Ohmic loss should not
exceed 4.9W. Assume the two switches have the same con-
duction loss, i.e., 2.5W each, then the ON resistance for the
two switches is:

The low side switch ON resistance is much higher than the
high side because at 2.8V the duty cycle is higher than 50%
and becomes even larger at full load. For the high side
switch, an IRL3202 (TO-220 package) or IRL3202S (D

PAK)

should be sufficient. For the low side switch, an IRL3303
(TO-220 package) or IRL3303S (D

PAK) should be suffi-

cient. Since each FET is dissipating 3.2W/2 + 2.5W = 4.1W,
it is suggested that appropriate heat sinks be used in the
case of TO-220 package or large enough copper area be
connected to the drain in the case of surface mount pack-
age.

CAPACITOR SELECTION

The selection of capacitors is an extremely important step
when designing a converter for a load such as the
Pentium II. Since the typical slew rate of the load current dur-
ing a large load transient is around 20A/µs to 30A/µs, the
switching converter has to rely on the output capacitors to
take care of the first few microseconds. Under such a current
slew rate, ESR of the output capacitors is more of a concern
than the ESL. Depending on the kind of capacitors being
used, capacitance of the output capacitors may or may not
be an important factor. When the output capacitance is too
low, the converter may have to have a small output inductor

to quickly supply current to the output capacitors when the
load suddenly kicks in and to quickly stop supplying current
when the load is suddenly removed.

Multilayer ceramic (MLC) capacitors can have very low ESR
but also a low capacitance value compared to other kinds of
capacitors. Low ESR aluminum electrolytic capacitors tend
to have large sizes and capacitances. Tantalum electrolytic
capacitors can have a fairly low ESR with a much smaller
size and capacitance than the aluminum capacitors. Certain
OSCON capacitors present ultra low ESR and long life span.
By the time the total ESR of the output capacitor bank
reaches around 9 m

, the capacitance of the aluminum/

tantalum/OSCON capacitors is usually already in the milli-
farad range. For those capacitors, ESR is the only factor to
consider. MLCs can have the same amount of total ESR with
much less capacitance, most probably under 100 µF. A very
small inductor, ultra fast control loop and a high switching
frequency become necessary in such a case to deal with the
fast charging/discharging rate of the output capacitor bank.

From a cost savings point of view, aluminum electrolytic ca-
pacitors are the most popular choice for output capacitors.
They have reasonably long life span and they tend to have
huge capacitance to withstand the charging or discharging
process during a load transient for a fairly long period. Sanyo
MV-GX series gives good performance when enough of the
capacitors are paralleled. The 6MV1500GX capacitor has a
typical ESR of 44 m

. Five of these capacitors should be

sufficient in the case of on-board power supply for a Pentium
II motherboard.

The challenge for input capacitors is the ripple current. The
large ripple current drawn by the high side switch tends to
generate quite some heat due to the capacitor ESR. The
ripple current ratings in the capacitor catalogs are usually
specified under the highest allowable temperature. In the
case of desktop applications, those ratings seem too conser-
vative. A good way to ensure enough number of capacitors is
through lab evaluation. The input current RMS ripple value
can be determined by the following equation:

and the power loss in each input capacitor is:

In the case of Pentium II power supply, the maximum output
current is around 14A. Under the worst case when duty cycle
is 50%, the maximum input capacitor RMS ripple current is
half of output current, i.e., 7A. It is found that three Sanyo
16MV820GX capacitors are enough under room tempera-
ture. The typical ESR of those capacitors is 44 m

. So the

power loss in each of them is around (7A)

x 44 m

0.24W. Note that the power loss in each capacitor is in-
versely proportional to the square of the total number of ca-
pacitors, which means the power loss in each capacitor
quickly drops when the number of capacitors increases.

INDUCTOR SELECTION

The size of the output is determined by a number of param-
eters. Basically the larger the inductor, the smaller the output
ripple voltage, but the slower the converter's response
speed during a load transient. On the other hand, a smaller
inductor requires higher switching frequency to maintain the
same level of output ripple, and probably results in a more
lossy converter, but has less inertia responding to load tran-

LM2636

www.national.com

Applications Information

(Continued)

sient. In the case of Pentium II power supply, fast recovery of
the load voltage from transient window back to the steady
state window is considered important. This limits the highest
inductance value that can be used. The lowest inductance
value is limited by the highest switching frequency that can
be practically employed. As the switching frequency in-
creases, the switching loss in the MOSFETs tends to in-
crease, resulting in less converter efficiency and larger heat
sinks. A good switching frequency is probably a frequency
under which the MOSFET conduction loss is higher than the
switching loss because the cost of the MOSFET is directly
related to its R

DSON

. The inductor size can be determined by

the following equation:

where V

O_RIP

is the peak-to-peak output ripple voltage, f is

the switching frequency. For commonly used low R

DSON

MOSFETs, a reasonable switching frequency is 300 kHz. As-
sume an output peak-peak ripple voltage of 18 mV is to be
guaranteed, the total output capacitor ESR is 9 m

, the input

voltage is 5V, and output voltage is 2.8V. The inductance
value according to the above equation will then be 2 µH. The
highest slew rate of the inductor current when the load
changes from no load to full load can be determined as fol-
lows:

where D

MAX

is the maximum allowed duty cycle, which is

around 0.9 for LM2636. For a load transient from 0A to 14A,
the highest current slew rate of the inductor, according to the
above equation, is 0.85A/µs, and therefore the shortest pos-
sible total recovery time is 14A/(0.85A/µs) = 16.5 µs. Notice
that the output voltage starts to recover whenever the induc-
tor starts to supply current.

The highest slew rate of the inductor current when the load
changes from full load to no load can be determined from the
same equation, but use D

MIN

instead of D

MAX

Since the D

MIN

of LM2636 at 300 kHz is 0%, the slew rate is

therefore -1.4A/µs. So the approximate total recovery time
will be 14A/(1.4A/µs) = 10 µs.

The input inductor is for limiting the input current slew rate
during a load transient. In the case that low ESR aluminum
electrolytic capacitors are used for the input capacitor bank,
voltage change due to capacitor charging/discharging is usu-
ally negligible for the first 20 µs. ESR is by far the dominant
factor in determining the amount of capacitor voltage
undershoot/overshoot due to load transient. So the worst
case is when the load changes between no load and full
load, under which condition the input inductor sees the high-
est voltage change across the input capacitors. Assume the
input capacitor bank is made up of three 16MV820GX, i.e.,
the total ESR is 15 m

. Whenever there is a sudden load

current change, it has to initially be supported by the input
capacitor bank instead of the input inductor. So for a full load
swing between 0A and 14A, the voltage seen by the input in-
ductor is

V = 14A x 15 m

= 210 mV. Use the following

equation to determine the minimum inductance value:

where (di/dt)

MAX

is the maximum allowable input current

slew rate, which is 0.1A/µs in the case of the Pentium II
power supply. So the input inductor size, according to the
above equation, should be 2.1 µH.

DYNAMIC POSITIONING OF LOAD VOLTAGE

Since the Intel VRM specifications have defined two operat-
ing windows for the MPU core voltage, one being the steady
state window and the other the transient window, it is a good
idea to dynamically position the steady state output voltage
in the steady state window with respect to load current level
so that the output voltage has more headroom for load tran-
sient response. This requires information about the load cur-
rent. There are at least two simple ways to implement this
idea with LM2636. One is to utilize the output inductor DC re-
sistance, see

Figure 7. The average voltage across the out-

put inductor is actually that across its DC resistance. That
average voltage is proportional to load current.

Since the switching node voltage V

bounces between the

input voltage and ground at the switching frequency, it is im-
possible to choose point A as the feedback point, otherwise
the dynamic performance will suffer and the system may
have some noise problems. Using a low pass filter network
around the inductor, such as the one shown in the figure,
seems to be a good idea. The feedback point is C.

Since at the switching frequency the impedance of the 0.1
µF is much less than 5 k

, the bouncing voltage at point A

will be mainly applied across the resistor 5 k

, and point C

will be much quieter than A. However, V

average is still the

majority of V

average, because of the resistor divider. So

in steady state V

= I

x r

+ V

CORE

, where r

is the inductor

DC resistance. So at no load, output voltage is equal to V

and at full load, output voltage is I

x r

lower than V

. To fur-

ther utilize the steady state window, a resistor can be con-
nected between the FB pin and ground to increase the no
load output voltage to close to the upper limit of the window.

DS100834-26

FIGURE 7. Dynamic Voltage Positioning by Utilizing

Output Inductor DC Resistance

LM2636

www.national.com

Applications Information

(Continued)

A possible drawback of the scheme in

Figure 7 is slow tran-

sient recovery speed. Since the 5 k

resistor and the 0.1 µF

capacitor have a large time constant, the settling of point C
to its steady state value during a load transient may take a
few milliseconds. Depending on the interaction between the
compensation network and the 0.1 µF capacitor, V

core

may

take different routes to reach its steady state value. This is
undesired when the load transients happens more than 1000
times per second. Reducing the time constant will result in a
more fluctuating V

due to a less effective low pass filter.

Fine tuning the parameters may balance the tradeoffs.

Another way to implement the dynamic voltage positioning is
through the use of a stand-alone resistor, such as the 4 m

resistor in

Figure 8 above. The advantage of this implemen-

tation over the previous one is a much faster speed of V

CORE

from transient level to steady state level. The disadvantage
is less efficiency. The total power loss can be 0.78W at 14A
of load current. The cost of the resistor can be minimized by
implementing it through a PCB trace.

REFERENCE VOLTAGE

The V

REF

pin can have many uses, such as in the watchdog

circuitry and in an LDO controller.

Figure 9 shows an applica-

tion where V

REF

is used to build a N-FET LDO controller. An

appropriate compensation network is necessary to tailor the
dynamic performance of the whole power supply.

PCB LAYOUT CONSIDERATIONS

There are several points to consider.

Try to use 2 oz. copper for the ground plane if tight load
regulation is desired. In the case of dynamic voltage po-
sitioning, this may not be a concern because the loose
load regulation is desired anyway. However, do not for-
get to take into consideration the voltage drop caused by
the ground plane when calculating dynamic voltage po-
sitioning parameters.

Try to keep gate traces short. However, do not make
them too short or else the LM2636 may stay too close to
the MOSFETs and get heated up by them. For the same
reason, do not use wide traces, 10 mil traces should be
enough.

When not employing dynamic voltage positioning, place
the feedback point at the VRM connector pins to have a
tight load regulation. If it is an on-board power supply,
place the feedback point at Slot I connector or wherever
is closest to the MPU.

Start component placement with the power devices such
as MOSFETs and inductors.

Do not place the LM2636 directly underneath the MOS-
FETs when when surface mount MOSFETs are used.

If possible, keep the capacitors some distance away
from the inductors so that the capacitors will have a
lower temperature environment.

When

implementing

dynamic

voltage

positioning

through a PCB trace, be aware that the PCB trace is a
heat source and try to avoid placing the trace directly un-
derneath the LM2636.

DS100834-27

FIGURE 8. Dynamic Voltage Positioning by Using A

Stand-Alone Resistor

DS100834-28

FIGURE 9. V

REF

Used in an N-FET LDO Supply

LM2636

www.national.com

Physical Dimensions

inches (millimeters) unless otherwise noted (Continued)

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whose failure to perform when properly used in
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significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform
can be reasonably expected to cause the failure of
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www.national.com

20-Lead Plastic TSSOP (MTC)

Order Number LM2636MTC

NS Package Number MTC20

LM2636

5-Bit

Programmable

Synchronous

Buck

Regulator

Controller

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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