October 2002
1
MIC2194
MIC2194
Micrel
MIC2194
400kHz SO-8 Buck Control IC
Final Information
General Description
Micrel's MIC2194 is a high efficiency PWM buck control IC
housed in the SO-8 package. Its 2.9V to 14V input voltage
range allows it to efficiently step down voltages in 3.3V, 5V,
and 12V systems as well as 1- or 2-cell Li Ion battery powered
applications. The flexible architecture of the MIC2194 allows
for it to be configured as a buck or a buck-boost converter.
The MIC2194 solution saves valuable board space. The
device is housed in the space-saving SO-8 package, whose
low pin-count minimizes external components. Its 400kHz
PWM operation allows a small inductor and small output
capacitors to be used. The MIC2194 can implement all-
ceramic capacitor solutions.
The MIC2194 drives a high-side P-channel MOSFET. A low
output driver impedance of 2
allows the MIC2194 to drive
large external MOSFETs to generate a wide range of output
currents. The MIC2194 can achieve maximum duty cycles of
100%, which can be useful in low headroom applications.
The MIC2194 is available in an 8 pin SOIC package with a
junction temperature range of 40
C to +125
C.
Typical Application
V
IN
12V
47
F
20V
(
2)
Si4431A
(
2)
B530 10k
3.32k
220
F
10V
(
2)
5.2
H
2k
0.012
2.2nF
MIC2194BM
VIN
EN/
UVLO
CS
OUTP
COMP
VDD
FB
GND
V
OUT
5V, 5A
1
F
Adjustable Output Buck Converter
V
IN
+3.3V
10
F
16V
Si9803
B530
3.01k
1k
220
F
10V
(
2)
22
H
4.99k
0.040
10nF
MIC2194BM
VIN
EN/
UVLO
CS
OUTP
COMP
VDD
FB
GND
V
OUT
5V, 0.6A
1
F
10
F
16V
22nF
Positive-to-Negative Buck-Boost Converter
Features
2.9V to 14V input voltage range
400kHz oscillator frequency
PWM current mode control
2
output drivers
100% maximum duty cycle
0.5
A micro-power shutdown
Programmable UVLO
Front edge blanking
Cycle-by-cycle current limiting
Frequency foldback short circuit protection
8-lead SOIC package
Applications
Point of load power supplies
Negative voltage buck-boost power supplies
Distributed power systems
Base stations
Wireless modems
ADSL line cards
Servers
Step down conversion in 3.3V, 5V, 12V systems
1-and 2-cell Li Ion battery operated equipment
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 http://www.micrel.com
MIC2194
Micrel
MIC2194
2
October 2002
Pin Configuration
1
COMP
FB
EN/UVLO
CS
8
VIN
OUTP
GND
VDD
7
6
5
2
3
4
8 Lead SOIC (M)
Ordering Information
Part Number
Output Voltage
Frequency
Junction Temp. Range
Package
MIC2194BM
Adjustable
400KHz
40
C to +125
C
8-lead SOP
Pin Description
Pin Number
Pin Name
Pin Function
1
COMP
Compensation (Output): Internal error amplifier output. Connect to a
capacitor or series RC network to compensate the regulator's control loop.
2
FB
Feedback (Input): The circuit regulates this pin to 1.245V.
3
EN/UVLO
Enable/Undervoltage Lockout (Input): A low level on this pin will power down
the device, reducing the quiescent current to under 0.5
A. This pin has two
separate thresholds, below 1.5V the output switching is disabled, and below
0.9V the device is forced into a complete micropower shutdown. The 1.5V
threshold functions as an accurate undervoltage lockout (UVLO) with
hysteresis.
4
CS
The () input to the current limit comparator. A built-in offset of 110mV
between VIN and CSL in conjunction with the current sense resistor sets the
current limit threshold level. This is also the () input to the current amplifier.
5
VDD
3V internal linear-regulator output. VDD is also the supply voltage bus for the
chip. Bypass to GND with 1
F.
6
GND
Ground.
7
OUTP
High current drive for the synchronous N-channel MOSFET. Voltage swing
is from ground to VIN. On-resistance is typically 3
@ 5V
IN
.
8
VIN
Input voltage to the circuit. Also the high side input to the current sense
amplifier supplies power to the gate drive circuit.
October 2002
3
MIC2194
MIC2194
Micrel
Electrical Characteristics
V
IN
= 5V, V
OUT
= 3.3V, T
J
= 25
C, unless otherwise specified. Bold values indicate 40
C<T
J
<+125
C.
Parameter
Condition
Min
Typ
Max
Units
Regulation
Feedback Voltage Reference
(1%)
1.233
1.245
1.257
V
(2%)
1.22
1.245
1.27
V
Feedback Bias Current
50
nA
Output Voltage Line Regulation
5V
V
IN
9V
0.15
% / V
Output Voltage Load Regulation
0mV < (V
IN
V
CS
) < 75mV
0.9
%
Output Voltage Total Regulation
5V
V
IN
9V, 0mV < (V
IN
V
CS
) < 75mV (
3%)
1.208
1.282
V
Input & V
DD
Supply
V
IN
Input Current (I
Q
)
(excluding external MOSFET gate current)
1
2
mA
Shutdown Current (I
SD
)
V
EN
= 0V
0.5
5
A
Digital Supply Voltage (V
DD
)
I
L
= 0
2.82
3.0
3.18
V
Digital Supply Load Regulation
I
L
= 0 to 1mA
0.1
V
Undervoltage Lockout
V
DD
upper threshold (turn on threshold)
2.65
V
UVLO Hysteresis
100
mV
Enable/UVLO
Enable Input Threshold
0.6
0.9
1.2
V
UVLO Threshold
1.4
1.5
1.6
V
Enable Input Current
V
EN/UVLO
= 5V
0.2
5
A
Current Limit
Current Limit Threshold Voltage
V
IN
V
CS
voltage to trip current limit
90
110
130
mV
Error Amplifier
Error Amplifier Gain
20
V/V
Current Amplifier
Current Amplifier Gain
3.0
V/V
Oscillator Section
Oscillator Frequency (f
O
)
360
400
440
kHz
Maximum Duty Cycle
V
FB
= 1.0V
100
%
Minimum On Time
V
FB
= 1.5V
165
ns
Frequency Foldback Threshold
Measured on FB
0.3
V
Frequency Foldback Frequency
90
kHz
Absolute Maximum Ratings
(Note 1)
Supply Voltage (V
IN
) ..................................................... 15V
Digital Supply Voltage (V
DD
) ........................................... 7V
Enable Pin Voltage (V
EN
) ............................. 0.3V to +15V
Comp Pin Voltage (V
COMP
) ............................ 0.3V to +3V
Feedback Pin Voltage (V
FB
) .......................... 0.3V to +3V
Current Sense Voltage (V
IN
V
CS
) ................. 0.3V to +1V
Power Dissipation (P
D
) ..................... 285mW @ T
A
= 85
C
Ambient Storage Temp ............................ 65
C to +150
C
ESD Rating, Note 3 ...................................................... 2kV
Operating Ratings
(Note 2)
Supply Voltage (V
IN
) .................................... +2.9V to +14V
Junction Temperature ....................... 40
C
T
J
+125
C
Package Thermal Resistance
JA
8-lead SOP ................................................. 140
C/W
MIC2194
Micrel
MIC2194
4
October 2002
Parameter
Condition
Min
Typ
Max
Units
Gate Drivers
Rise/Fall Time
C
L
= 3300pF
25
ns
Output Driver Impedance
Source, V
IN
= 12V
2
6
Sink, V
IN
= 12V
2
6
Source, V
IN
= 5V
3
7
Sink, V
IN
= 5V
3
7
Note 1.
Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when
operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction
temperature, T
J(Max)
, the junction-to-ambient thermal resistance,
JA
, and the ambient temperature, T
A
.
Note 2.
The device is not guaranteed to function outside its operating rating.
Note 3.
Devices are ESD sensitive, handling precautions required. Human body model, 1.5k
in series with 100pF.
October 2002
5
MIC2194
MIC2194
Micrel
Typical Characteristics
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0
5
10
15
QUIESCENT CURRENT (mA)
INPUT VOLTAGE (V)
Quiescent Current
vs. Input Voltage
Standby
Switching
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
-40 -20 0
20 40 60 80 100 120
QUIESCENT CURRENT (mA)
TEMPERATURE (
C)
Quiescent Current
vs. Temperature
V
IN
= 5V
2.80
2.85
2.90
2.95
3.00
3.05
0
5
10
15
V
DD
(V)
INPUT VOLTAGE (V)
V
DD
vs. Input Voltage
2.50
2.60
2.70
2.80
2.90
3.00
3.10
3.20
3.30
3.40
3.50
-40 -20 0
20 40 60 80 100 120
VDD (V)
TEMPERATURE (
C)
V
DD
vs. Temperature
V
IN
= 5V
2.85
2.87
2.89
2.91
2.93
2.95
2.97
2.99
3.01
3.03
3.05
0
0.2
0.4
0.6
0.8
1
1.2
V
DD
(V)
V
DD
LOAD CURRENT (mA)
V
DD
vs. Load
V
IN
= 3.3V
V
IN
= 12V
V
IN
= 5V
1.2430
1.2435
1.2440
1.2445
1.2450
1.2455
0
2
4
6
8
10 12 14
REFERENCE VOLTAGE (V)
INPUT VOLTAGE (V)
Error Amp Reference Voltage
vs. Input Voltage
1.2
1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28
1.29
1.3
-40 -20 0
20 40 60 80 100 120
REFERENCE VOLTAGE (V)
TEMPERATURE (
C)
Error Amp Reference Voltage
vs. Temperature
V
IN
= 5V
-2
-1.5
-1
-0.5
0
0.5
0
2
4
6
8
10
12
14
FREQUENCY VARIATION (%)
INPUT VOLTAGE (V)
Frequency Variation
vs. Input Voltage
350
360
370
380
390
400
410
420
430
440
450
-50 -30 -10 10 30 50 70 90 110
SOFT START CURRENT (
A)
TEMPERATURE (
C)
Frequency Variation vs.
Temperature
V
IN
= 5V
90
95
100
105
110
115
120
125
130
0
2
4
6
8
10 12 14
THRESHOLD (mV)
INPUT VOLTAGE (V)
Overcurrent Threshold vs.
Input Voltage
80
85
90
95
100
105
110
115
120
-40 -20 0
20 40 60 80 100 120
CURRENT LIMIT (mV)
TEMPERATURE (
C)
Current Limit Threshold
vs. Temperature
V
IN
= 5V
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0
2
4
6
8
10 12 14
IMPEDANCE (
)
INPUT VOLTAGE (V)
OUTP Drive Impedance
vs. Input Voltage
Sink (
)
Source(
)
MIC2194
Micrel
MIC2194
6
October 2002
Functional Characteristics
Controller Overview and Functional Description
The MIC2194 is a BiCMOS, switched-mode, step down
(buck) converter controller. It uses a P-channel MOSFET,
which allows the controller to operate at 100% duty cycle and
eliminates the need for a high side drive bootstrap circuit.
Current mode control is used to achieve superior transient
line and load regulation. An internal corrective ramp provides
slope compensation for stable operation above a 50% duty
cycle. The controller is optimized for high efficiency, high
performance DC-DC converter applications.
Figure 1 is a block diagram of the MIC2194 configured as a
buck converter. At the beginning of the switching cycle, the
OUTP pin pulls low and turns on the high-side P-channel
MOSFET, Q1. Current flows from the input to the output
through the current sense resistor, MOSFET and inductor.
The current amplitude increases, controlled by the inductor.
The voltage developed across the current sense resistor,
R
SENSE
, is amplified inside the MIC2194 and combined with
an internal ramp for stability. This signal is compared to the
output of the error amplifier. When the current signal equals
the error voltage signal, the P-channel MOSFET is turned off.
The inductor current flows through the diode, D1. At the
beginning of the next switching cycle, the P-channel MOSFET
is turned on which turns off the diode, D1.
Functional Diagram
FREQUENCY
FOLDBACK
ERROR
AMP
PWM
COMPARATOR
OVERCURRENT
COMPARATOR
0.1V
Threshold
L1
CURRENT
SENSE
AMP
0.3V
100k
V
REF
gm = 0.0002
gain = 20
fs/4
COMP
ON
GAIN
3
RESET
VREF
1.245V
1
VDD
SLOPE
COMPENSATION
OSC
fs/4
CONTROL
BIAS
VDD
5
EN/UVLO
3
CSH
9
CSL
4
VIN
OUTP
7
Q1
D1
FB
2
VIN
C
DECOUP
8
C
IN
GND
6
C
OUT
V
OUT
R
SENSE
V
IN
Figure 1. MIC2194 Block Diagram
October 2002
7
MIC2194
MIC2194
Micrel
The MIC2194 controller is broken down into several func-
tions.
Control loop
PWM operation
Current mode control
Current limit
Reference, enable and UVLO
MOSFET gate drive
Oscillator
Control Loop
PWM Control Loop
The MIC2194 uses current mode control to regulate the
output voltage. This dual control loop method (illustrated in
Figure 2) senses the output voltage (outer loop) and the
inductor current (inner loop). It uses inductor current and
output voltage to determine the duty cycle of the buck
converter. Sampling the inductor current effectively removes
the inductor from the control loop, which simplifies compen-
sation.
Switching
Converter
Voltage
Divider
V
REF
V
ERROR
V
ERROR
t
ON
t
PER
D = t
ON
/t
PER
I
INDUCTOR
I
INDUCTOR
Switch
Driver
V
OUT
V
IN
Figure 2. Current Mode Control Example
As shown in Figure 1, the inductor current is sensed by
measuring the voltage across the resistor, R
SENSE
. A ramp is
added to the amplified current sense signal to provide slope
compensation, which is required to prevent unstable opera-
tion at duty cycles greater than 50%.
A transconductance amplifier is used for the error amplifier,
which compares an attenuated sample of the output voltage
with a reference voltage. The output of the error amplifier is
the compensation pin (COMP), which is compared to the
current sense waveform in the PWM block. When the current
signal becomes greater than the error signal, the comparator
turns off the high side drive. The COMP pin provides access
to the output of the error amplifier and allows the use of
external components to stabilize the voltage loop.
Current Limit
The output current is detected by the voltage drop across the
external current sense resistor (R
SENSE
in Figure 1.). The
current sense resistor must be sized using the minimum
current limit threshold. The external components must be
designed to withstand the maximum current limit. The current
sense resistor value is calculated by the equation below:
R
MIN CURRENT
SENSE THRESHOLD
I
SENSE
OUT
MAX
=
_
_
_
_
The maximum output current is:
I
MAX
CURRENT
SENSE THRESHOLD
R
OUT MAX
SENSE
_
_
_
_
=
The current sense pins VIN (pin 8) and CSL (pin 4) are noise
sensitive due to the low signal level, high input impedance
and input ripple voltage. The PCB traces should be short and
routed close to each other. A 0.1
F capacitor across the pins
will attenuate high frequency switching noise.
When the peak inductor current exceeds the current limit
threshold, the overcurrent comparator turns off the high-side
MOSFET for the remainder of the switching cycle, effectively
decreasing the duty cycle. The output voltage drops as
additional load current is pulled from the converter. When the
voltage at the feedback pin (FB) reaches approximately 0.3V,
the circuit enters frequency foldback mode and the oscillator
frequency will drop to 1/4 of the switching frequency. This
limits the maximum output power delivered to the load under
a short circuit condition.
Reference, Enable and UVLO Circuits
The output drivers are enabled when the following conditions
are satisfied:
The V
DD
voltage (pin 5) is greater than its
undervoltage threshold.
The voltage on the enable pin (pin 3) is greater
than the enable UVLO threshold.
The enable pin (pin 3) has two threshold levels, allowing the
MIC2194 to shut down in a low current mode, or turn off output
switching in standby mode. An enable pin voltage lower than
the shutdown threshold turns off all the internal circuitry and
places the MIC2194 in a micropower shutdown mode.
If the enable pin voltage is between the shutdown and
standby thresholds, the internal bias, V
DD
and reference
voltages are turned on. The output drivers are inhibited from
switching. The OUTP pin is in a high state. Raising the enable
voltage above the standby threshold enables the output
driver. The standby threshold is specified in the electrical
characteristics. A resistor divider can be used with the enable
pin to prevent the power supply from turning on until a
specified input voltage is reached. The circuit in Figure 3
shows how to connect the resistors.
MIC2194
Micrel
MIC2194
8
October 2002
1.5V
Typical
140mV
Hysteresis
(typical)
EN/UVLO
(3)
MIC2194
V
IN
R1
R2
Bias
Circuitry
Figure 3. UVLO Circuitry
The line voltage turn on trip point is:
V
V
R
R
R
INPUT ENABLE
THRESHOLD
_
=
+
2
1
2
where:
V
THRESHOLD
is the voltage level of the internal
comparator reference, typically 1.5V.
The input voltage hysteresis is equal to:
V
V
R
R
R
INPUT HYST
HYST
_
=
+
1
2
2
where:
V
HYST
is the internal comparator hysteresis level,
typically 140mV.
V
INPUT_HYST
is the hysteresis at the input voltage
The MIC2194 will be disabled when the input voltage drops
back down to:
V
INPUT_OFF
=
V
INPUT_ENABLE
V
INPUT_HYST
=
(V
THRESHOLD
V
HYST
)
+
R
R
R
2
1
2
Either of 2 UVLO conditions will pull the soft start capacitor
low:
When the V
DD
voltage drops below its
undervoltage lockout level.
When the enable pin drops below the its enable
threshold
The internal bias circuit generates an internal 1.245V band-
gap reference voltage for the voltage error amplifier and a 3V
VDD voltage for the internal control circuitry. The VDD pin
must be decoupled with a 1
F ceramic capacitor. The capaci-
tor must be placed close to the VDD pin. The other end of the
capacitor must be connected directly to the ground plane.
MOSFET Gate Drive
The MIC2194 is designed to drive a high-side P-channel
MOSFET. The source pin of the P-channel MOSFET is
connected to the input of the power supply. It is turned on
when OUTP pulls the gate of the MOSFET low. The advan-
tage of using a P-channel MOSFET is that it does not require
a bootstrap circuit to boost the gate voltage higher than the
input, as would be required for an N-channel MOSFET. The
VIN pin (pin 8) supplies the drive voltage to the gate drive pin,
OUTP.
MOSFET Selection
The P-channel MOSFET must have a V
GS
threshold voltage
equal to or lower than the input voltage when used in a buck
converter topology. There is a limit to the maximum gate
charge the MIC2194 will drive. MOSFETs with high gate
charge will have slower turn-on and turn-off times. Slower
transition times will cause higher power dissipation in the
MOSFET due to higher switching transition losses.
The MOSFET gate charge is also limited by power dissipation
in the MIC2194. The power dissipated by the gate drive
circuitry is calculated below:
P
GATE_DRIVE
= Q
GATE
V
IN
f
S
where: Q
GATE
is the total gate charge of both the N- and P-
channel MOSFETs.
f
S
is the switching frequency
V
IN
is the gate drive voltage
The graph in Figure 4 shows the total gate charge that can be
driven by the MIC2194 over the input voltage range, for
different values of switching frequency.
0
50
100
150
200
250
0
5
10
15
MAMIMUM GATE CHARGE (nC)
INPUT VOLTAGE (V)
Max Gate Charge
Figure 4. MIC2194 V
IN
vs Max. Gate Charge
Oscillator
The internal oscillator is free running and requires no external
components. The maximum duty cycle for both frequencies
is 100%. This is another advantage of using a P-channel
MOSFET for the high-side drive; it can be continuously turned
on.
A frequency foldback mode is enabled if the voltage on the
feedback pin (pin 2) is less than 0.3V. In frequency foldback,
the oscillator frequency is reduced by approximately a factor
of 4. Frequency foldback is used to limit the energy delivered
to the output during a short circuit fault condition.
Voltage Setting Components
The MIC2194 requires two resistors to set the output voltage
as shown in Figure 5.
October 2002
9
MIC2194
MIC2194
Micrel
V
REF
1.245V
Voltage
Amplifier
Pin 2
MIC2194
V
OUT
R1
R2
Figure 5
The output voltage is determined by:
V
V
R
R
OUT
REF
=
+
1
1
2
Where: V
REF
for the MIC2194 is typically 1.245V.
Lower values of R1 are preferred to prevent noise from
appearing on the FB pin. A typically recommended value is
10k
. If R1 is too small in value it will decrease the efficiency
of the power supply, especially at low output loads.
Once R1 is selected, R2 can be calculated with the following
formula:
R
V
R
V
V
REF
OUT
REF
2
1
=
Efficiency Considerations
Efficiency is the ratio of output power to input power. The
difference is dissipated as heat in the buck converter. Under
light output load, the significant contributors are:
The V
IN
supply current, which includes the current
required to switch the external MOSFET.
Core losses in the output inductor.
To maximize efficiency at light loads:
Use a low gate charge MOSFET or use the smallest
MOSFET, which is still adequate for maximum output
current.
Use a ferrite material for the inductor core, which has
less core loss than an MPP or iron power core.
Under heavy output loads the significant contributors to
power loss are (in approximate order of magnitude):
Resistive on time losses in the MOSFET
Switching transition losses in the MOSFET
Inductor resistive losses
Current sense resistor losses
Input capacitor resistive losses (due to the capacitors
ESR)
To minimize power loss under heavy loads:
Use low on-resistance MOSFETs. Use low threshold
logic level MOSFETs when the input voltage is below
5V. Multiplying the gate charge by the on-resistance
gives a figure of merit, providing a good balance
between low load and high load efficiency.
Slow transition times and oscillations on the voltage
and current waveforms dissipate more power during
the turn on and turn off of the MOSFET. A clean
layout will minimize parasitic inductance and capaci-
tance in the gate drive and high current paths. This
will allow the fastest transition times and waveforms
without oscillations. Low gate charge MOSFETs will
transition faster than those with higher gate charge
requirements.
For the same size inductor, a lower value will have
fewer turns and therefore, lower winding resistance.
However, using too small of a value will require more
output capacitors to filter the output ripple, which will
force a smaller bandwidth, slower transient response
and possible instability under certain conditions.
Lowering the current sense resistor value will de-
crease the power dissipated in the resistor. However,
it will also increase the overcurrent limit and will
require larger MOSFETs and inductor components.
Use low ESR input capacitors to minimize the power
dissipated in the capacitors ESR.
MIC2194
Micrel
MIC2194
10
October 2002
Package Information
45
0
8
0.244 (6.20)
0.228 (5.79)
0.197 (5.0)
0.189 (4.8)
SEATING
PLANE
0.026 (0.65)
MAX
)
0.010 (0.25)
0.007 (0.18)
0.064 (1.63)
0.045 (1.14)
0.0098 (0.249)
0.0040 (0.102)
0.020 (0.51)
0.013 (0.33)
0.157 (3.99)
0.150 (3.81)
0.050 (1.27)
TYP
PIN 1
DIMENSIONS:
INCHES (MM)
0.050 (1.27)
0.016 (0.40)
8-Pin SOP (M)
MICREL, INC.
1849 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
TEL
+ 1 (408) 944-0800
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2002 Micrel, Incorporated
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