LM2631
Synchronous Step-Down Power Supply Controller
General Description
The LM2631 controller provides all the active functions for
step-down (buck) switching converters. These dc-to-dc con-
verters provide core CPU power in battery-operated sys-
tems.
High efficiency is achieved by using synchronous rectifica-
tion and pulse-skipping mode operation at light load. Inex-
pensive N-channel MOSFETs are used to reduce system
cost. Bootstrap circuit is used to drive the high-side
N-channel MOSFET.
Current mode control scheme is used to improve line regula-
tion and transient response, also provides cycle-by-cycle
current limiting.
The operating frequency is adjustable between 200 kHz and
400 kHz. An external shutdown pin can be used to disable
the device and reduce the quiescent current to 0.1 A. In low
noise applications, bringing the FPWM pin high can force the
device to operate in constant frequency mode. Other fea-
tures include the external synchronization pin, and the
PGOOD pin to indicate the state of the output voltage.
Protection circuitry includes thermal shutdown, undervoltage
and overvoltage shutdown protection, soft-start capability,
and two levels of current limits: The first level simply limits
the load current directly; at the second level, if the load pulls
the output voltage down below 80% of the regulated value,
the chip will shut down. This operation is disabled during
startup, but an internal timer will enable it if the output does
not come up in the preset time.
Features
n
4.5V to 30V input range
n
Adjustable output (1.5V to 7V)
n
200 kHz to 400 kHz adjustable operating frequency
n
Externally synchronizable
n
On-board power good function
n
Precision 1.24V reference output
n
0.8 mA typical quiescent current
n
0.1 A shutdown current
n
Thermal shutdown
n
Direct current limit protection
n
Input undervoltage lockout
n
Output undervoltage shutdown protection
n
Output overvoltage shutdown protection
n
Programmable soft-start function
n
Tiny TSSOP package
Applications
n
Notebook and subnotebook computers
n
Cellular phones
n
Portable instruments
n
Battery-powered digital devices
Typical Application Circuit
DS100937-1
April 1999
LM2631
Synchronous
Step-Down
Power
Supply
Controller
1999 National Semiconductor Corporation
DS100937
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.
Voltages from the indicated
pins to GND and PGND:
V
IN
-0.3V to 32V
CBOOT
-0.3V to 37V
SD
-0.3V to 32V
SW
-0.3V to 32V
CSH, CSL
-0.3V to 8V
FPWM, SYNC
-0.3V to 10V
Power Dissipation (T
A
=
70C), (Note 2)
720mW
Storage Temperature Range
-65C to +150C
Soldering Dwell Time,
Temperature (Note 3)
Wave
4 sec, 260C
Infrared
10 sec, 240C
Vapor Phase
75 sec, 219C
ESD Rating (Note 4)
1.5 kV
Operating Ratings
V
IN
4.5V to 30V
Junction Temperature
-25C to +125C
Electrical Characteristics
Specifications in standard type face are for T
j
= 25C and those with boldface type apply over full operating junction tem-
perature range. V
IN
=10V, GND = PGND = 0V,unless otherwise stated. (Notes 5, 6)
Symbol
Parameter
Conditions
Typical
Limit
Units
System
V
IN
Input Supply Voltage
4.5
30
V(min)
V(max)
V
OUT
Output Voltage Adjustment Range
1.5
7.0
V(min)
V(max)
V
OUT
/
V
OUT
Load Regulation
0 mV
(CSH-CSL)
75 mV
0.3
%
V
OUT
/
V
OUT
Line Regulation
4.5
V
IN
30V
0.002
%/V
I
IN
Input Supply Current with the
Switching Controller ON
V
FB
= 1V, V
CSH
= 2.15V, V
CSL
=
2.1V
0.8
mA
1.2/1.4
mA(max)
Input Supply Current with the
Switching Controller ON (Internal
Rail is Supplied from CSL Pin)
V
FB
= 1V, V
CSH
= 5.15V, V
CSL
=
5V
0.15
mA
Input Supply Current with the IC
Shut Down
V
SD
= 0V, V
IN
= 30V
0.1
A
3 (Note 7)
A(max)
Minimum Output Voltage for CSL
Providing the Internal Rail
3
V
I
SS
Soft Start Source Current
V
SS
= 1.5V
10
A
5
A(min)
13
A(max)
Soft Start Sink Current
V
SS
= 1.5V
20
A
V
CL
Current Limit Voltage (Voltage
from CSH to CSL)
V
FB
= 1V, V
CSL
= 1.8V
110
mV
90/80
mV(min)
130/140
mV(max)
V
IN
Undervoltage Shutdown Latch
Threshold
Rising Edge
3.5
V
2.6
V(min)
V
OUT
Undervoltage Shutdown
Latch Threshold (Note 8)
80
% V
OUT
72/70
% V
OUT
(min)
89/90
%V
OUT
(max)
V
OUT
Overvoltage Shutdown
Latch Threshold (Note 8)
120
% V
OUT
113/110
% V
OUT
(min)
129/130
% V
OUT
(max)
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2
Electrical Characteristics
(Continued)
Specifications in standard type face are for T
j
= 25C and those with boldface type apply over full operating junction tem-
perature range. V
IN
=10V, GND = PGND = 0V,unless otherwise stated. (Notes 5, 6)
Symbol
Parameter
Conditions
Typical
Limit
Units
System
V
OUT
Low Regulation Comparator
Enable Threshold
97
% V
OUT
Hysteresis of Low Regulation
Comparator
2
% V
OUT
Regulator Window Detector
Thresholds (PGOOD from High to
Low)
91 or 109
% V
OUT
Regulator Window Detector
Thresholds (PGOOD from Low to
High)
97 or 103
% V
OUT
Gate Drive
V
BOOT
Bootstrap Voltage (Voltage from
CBOOT to SW)
CBOOT Sourcing 100 A
4.5
V
4.0
V(min)
I
BOOT
CBOOT Leakage Current
V
CBOOT
= 7V
100
nA
High Drive Source Current
V
HDRV
= 0V, V
CBOOT
= 5V
0.3
A
High Drive Sink Current
HDRV Forced to 5V
0.45
A
Low Drive Source Current
LDRV Forced to 0V
0.35
A
Low Drive Sink Current
LDRV Forced to 5V
0.55
A
High-Side FET On-Resistance
HDRV or LDRV
8
Low-Side FET On-Resistance
HDRV or LDRV
4
Oscillator
F
OSC
Oscillator Frequency
FADJ Open
200
kHz
172/162
kHz(min)
228/230
kHz(max)
Oscillator Frequency
FADJ Sourcing 2.94 A (Note 9)
300
kHz
255
kHz(min)
345
kHz(max)
V
FADJ
Voltage at FADJ pin
1.03
V
D
MAX
Maximum Duty Cycle
FADJ Open
96
%
92
%(min)
Maximum Frequency of
Synchronization
Low-Going 200 ns Wide
Rectangular Pulses Applied at
400 kHz at the SYNC Input
400
kHz(min)
Minimum Pulse Width of the
SYNC Signal
SYNC Pulses are Low-Going
200
ns(min)
Error Amplifier
I
FB
Feedback Input Bias Current
V
FB
= 1.3V, V
CSH
= 5.15V, V
CSL
= 5V
100
nA
I
COMP
COMP Output Source Current
V
COMP
= 0.2V, V
FB
= 1V
50
A
COMP Output Sink Current
V
COMP
= 1.2V, V
FB
= 1.4V
50
A
Voltage Reference
V
REF
Reference Voltage (Nominal))
I
REF
= 0A
1.238
V
1.219/1.219
V(min)
1.251/1.262
V(max)
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3
Electrical Characteristics
(Continued)
Specifications in standard type face are for T
j
= 25C and those with boldface type apply over full operating junction tem-
perature range. V
IN
=10V, GND = PGND = 0V,unless otherwise stated. (Notes 5, 6)
Symbol
Parameter
Conditions
Typical
Limit
Units
Voltage Reference
V
REF
Reference Voltage (Line
Regulation)
4.5V
<
V
IN
<
30V
1.238
V
1.219/1.219
V(min)
1.251/1.262
V(max)
Reference Voltage (Load
Regulation)
0 A
<
I
REF
<
50 A
1.238
V
1.219/1.219
V(min)
1.251/1.262
V(max)
Logic Inputs and Outputs
V
IH
Minimum High Level Input
Voltage (SD, FPWM and SYNC)
2.4
V(min)
V
IL
Maximum Low Level Input
Voltage (FPWM and SYNC)
0.8
V(max)
Maximum Low Level Input
Voltage (SD)
0.5
V(max)
Maximum Input Leakage Curren1t
(SD , FPWM and SYNC)
Logic Input Voltage 0V or 5V
0.1
A
V
OH
PGOOD High Level Output
Voltage
PGOOD Sourcing 50 A
2.7
V
2.4
V(min)
V
OL
PGOOD Low Level Output
Voltage
PGOOD Sinking 50 A
0
V
0.5
V(max)
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device
outside of its rated operating conditions.
Note 2: The maximum allowable power dissipation is calculated by using P
Dmax
= (T
Jmax
- T
A
)/
JA
, where T
Jmax
is the maximum junction temperature, T
A
is the
ambient temperature, and
JA
is the junction-to-ambient thermal resistance of the specified package. The 720 mW rating results from using 160C, 70C, and
125C/W for T
Jmax
, T
A
, and
JA
respectively. A
JA
of 125C/W represents the worst-case condition of no heat sinking of the 20-pin TSSOP. Heat sinking allows the
safe dissipation of more power. The Absolute Maximum power dissipation must be derated by 8 mW per C above 70C ambient. The LM263 actively limits its junction
temperature to about 160C.
Note 3: For detailed information on soldering plastic small-outline packages, refer to the Packaging Databook available from National Semiconductor Corporation.
Note 4: For testing purposes, ESD was applied using the human-body model, a 100 pF capacitor discharged through a 1.5 k
resistor.
Note 5: A typical is the center of characterization data taken with T
A
= T
J
= 25C. Typicals are not guaranteed.
Note 6: All limits are guaranteed. All electrical characteristics having room-temperature limits are tested during production with T
A
= T
J
= 25C. All hot and cold limits
are guaranteed by correlating the electrical characteristics to process and temperature variations and applying statistical process control.
Note 7: This limit is guaranteed by design.
Note 8: Percentage limits are determined by measuring the shutdown latch threshold at the FB pin, and dividing it by the nominal reference voltage.
Note 9: Pulling 2.94 A out of FADJ pin simulates adjusting the oscillator frequency with a 350 k
resistor connected from FADJ to GND.
Typical Performance Characteristics
Efficiency vs Load Current
(FPWM = Low, V
OUT
= 3.3V)
DS100937-11
Efficiency (FPWM = High, Input Voltage = 16V
V
OUT
= 2.9V)
DS100937-12
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4
Typical Performance Characteristics
(Continued)
Quiscent Supply Current vs Supply Voltage
(Not Switching, FPWM = Low, V
OUT
= 2.0V)
DS100937-15
Quiscent Supply Current vs Supply Voltage
(FPWM = Low, V
OUT
= 3.3V)
DS100937-16
Supply Current vs Oscillator Frequency
(FPWM = High)
DS100937-17
Oscillator Frequency vs Adjusting Resistor
DS100937-18
Oscillator Frequency vs Junction Temperature
DS100937-13
Reference Voltage vs Junction Temperature
DS100937-14
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5
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