LM2641
Dual Adjustable Step-Down Switching Power Supply
Controller
General Description
The LM2641 is a dual step-down power supply controller
intended for application in notebook personal computers and
other battery-powered equipment.
Fixed-frequency synchronous drive of logic-level N-channel
power
MOSFETs
is
combined
with
an
optional
pulse-skipping mode to achieve ultra efficient power conver-
sion over a 1000:1 load current range. The pulse-skipping
mode can be disabled in favor of fixed-frequency operation
regardless of the load current level.
High DC gain and current-mode feedback control assure
excellent line and load regulation and a wide loop bandwidth
for fast response to dynamic loads.
An internal oscillator fixes the switching frequency at
300 kHz. Optionally, switching can be synchronized to an
external clock running as fast as 400 kHz.
An optional soft-start feature limits current surges from the
input power supply at start up and provides a simple means
of start-up sequencing.
Logic-level inputs allow the controllers to be turned ON and
OFF separately.
Key Specifications
n
96% efficient
n
5.5 to 30V input range
n
Dual outputs adjustable from 2.2 to 8V
n
0.5% typical load regulation error
n
0.002%/V typical line regulation error
Features
n
300 kHz fixed-frequency switching
n
Switching synchronization with an external signal up to
400 kHz
n
Optional pulse-skipping mode
n
Adjustable secondary feedback
n
Input undervoltage lockout
n
Output undervoltage shutdown protection
n
Output overvoltage shutdown protection
n
Programmable soft-start (each controller)
n
5V, 50 mA linear regulator output
n
Precision 2.5V reference output
n
28-pin TSSOP
Applications
n
Notebook and subnotebook computers
n
Wireless data terminals
n
Battery-powered instruments
Connection Diagram and Ordering Information
28-Lead TSSOP (MTC)
10094901
Top View
Order Number LM2641MTC-ADJ
See NS Package Number MTC28
May 2001
LM2641
Dual
Adjustable
Step-Down
Switching
Power
Supply
Controller
2001 National Semiconductor Corporation
DS100949
www.national.com
Pin Description
(Refer to Typical Application Circuits)
Pin #
Name
Function
1
CSH2
The sense point for the positive side of the voltage across the current sense resistor (R13)
placed in series with output #2.
2
FB2
The regulated output voltage appearing at output #2 is sensed using this pin by connecting it to
the center of the output resistive divider (R15 and R16).
3
COMP2
An R-C network made up of R11, C10, and C12 is connected to this pin which provides loop
compensation for regulated output #2.
4
SS2
This provides programmable soft-start for the #2 output along with capacitor C15.
5
ON/OFF2
This pin turns off only output #2.
6
SD
The part can be put into "sleep" mode using this pin, where both outputs are off and the internal
chip functions are shut down.
7
SYNC
The internal oscillator may be synchronized to an external clock via this pin.
8
GND
Connect this pin to circuit Signal Ground.
9
REF
Internal 2.5V reference voltage. This voltage is turned off by the SD pin, but remains on if either
or both ON/OFF pins are pulled low, which turns off the regulated output(s).
10
2NDFB/FPWM
A 12V supply can be generated using an auxiliary winding on the 5V output inductor. Feedback
to control this 12V output is brought in through this pin. If the 12V supply is not required, this pin
can also force the chip to operate at fixed frequency at light loads by pulling the pin low (this is
the "forced-PWM" mode of operation). This will prevent the converter from operating in
pulse-skipping mode.
11
ON/OFF1
This pin turns off only output #1.
12
SS1
This provides programmable soft-start for the #1 output along with capacitor C3.
13
COMP1
An R-C network made up of R6, C5, and C7 is connected to this pin which provides loop
compensation for regulated output #1.
14
FB1
The regulated output voltage appearing at output #1 is sensed using this pin by connecting it to
the center of the output resistive divider (R1 and R2).
15
CSH1
The sense point for the positive side of the voltage across the current sense resistor (R4) placed
in series with output #1.
16
HDRV1
The drive for the gate of the high-side switching FET used for output #1.
17
SW1
This is the switching output drive point of the two power FETs which produce output #1.
18
CBOOT1
The bootstrap capacitor (C8) for output #1 is returned to this point.
19
LDRV1
The drive for the gate of the low-side switching FET (synchronous rectifier) used for output #1.
20
PGND
Connect this pin to circuit Power Ground.
21
CSL1
The sense point for the negative side of the voltage across the current sense resistor (R4)
placed in series with output #1.
22
LIN
This pin provides a low-current (50 mA max) 5V output. This output is always on, and can not be
turned off by either the SD or ON/OFF pins.
23
IN
This is the connection for the main input power.
24
LDRV2
The drive for the gate of the low-side switching FET (synchronous rectifier) used for output #2.
25
CBOOT2
The bootstrap capacitor (C9) for output #2 is returned to this point.
26
SW2
This is the switching output drive point of the two power FETs which produce output #2.
27
HDRV2
The drive for the gate of the high-side switching FET used for output #2.
28
CSL2
The sense point for the negative side of the voltage across the current sense resistor (R13)
placed in series with output #2.
LM2641
www.national.com
2
Absolute Maximum Ratings
(Notes 2,
1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
IN, SW1, and SW2
-0.3 to 31V
FB1 and FB2
-0.3 to 3V
SD, ON/OFF1, ON/OFF2,
2NDFB/FPWM, SYNC, REF,
SS1, SS2, COMP1,
COMP2 and CSL1
-0.3 to (V
LIN
+0.3)V
LIN
-0.3 to 6V
CSH1, CSH2, and CSL2 (Note
12)
-0.3 to 9V
Voltage from CBOOT1 to SW1
and from CBOOT2 to SW2
-0.3 to 5V
Voltage from HDRV1 to SW1
and from HDRV2 to SW2
-0.3V
Voltage from CBOOT1 to
HDRV1 and from CBOOT2
HDRV2
-0.3V
Junction Temp.
+150C
Power Dissipation (Note 3)
883 mW
Ambient Storage Temp. (T
J
)
-65 to +150C
Soldering Dwell Time, Temp. (Note 4)
Wave
4 sec, 260C
Infrared
10 sec, 240C
Vapor Phase
75 sec, 219C
ESD Rating (Note 5)
2 kV
Operating Ratings
(Notes 1, 2)
V
IN
5.5 to 30V
Junction Temp. (T
J
)
0 to +125C
Electrical Characteristics
Typicals and limits appearing in regular type apply for T
J
= 25C. Limits appearing in boldface type apply over the entire junc-
tion temperature range for operation, 0 to +125C. Unless otherwise specified under the Parameter or Conditions columns, V
IN
= 10V, and V
SD
= V
ON/OFF1
= V
ON/OFF2
= 5V. (Notes 2, 6, 7)
Symbol
Parameter
Conditions
Typical
Limit
Units
System
V
IN
Input Supply Voltage Range
5.5
V(min)
30
V(max)
V
OUT1
Output Voltage Adjustment
Range
2.2
V(min)
6.0
V(max)
V
OUT2
Output Voltage Adjustment
Range
2.2
V(min)
8.0
V(max)
V
OUT
/V
OUT
Load Regulation
0 mV
(CSH1-CSL1)
80 mV,
0 mV
(CSH2-CSL2)
80 mV
0.5
%
V
OUT
/
V
IN
Line Regulation
5.5V
V
IN
30V
0.002
%/V
I
IN
Input Supply Current
ON (Note 8)
0.6
mA
V
FB1
= V
FB2
= 1.4V,
1
mA(max)
V
CSH1
= 5.2V, V
CSL1
= 5V,
V
CSH2
= 3.5V, V
CSL2
= 3.3V
Standing By (Note 9)
80
A
V
ON/OFF1
= V
ON/OFF2
= 0V
150
A(max)
Shut Down (Note 10)
25
A
V
SD
= 0V
60
A(max)
I
SS1
, I
SS2
Soft-Start Source Current
V
SS1
= V
SS2
= 1V
4.75
A
2.0
A(min)
7.0
A(max)
Soft-Start Sink Current
10
A
V
PCL
Positive Current Limit Voltage
(Voltage from CSH1 to CSL1
and from CSH2 to CSL2)
100
mV
80
mV(min)
140
mV(max)
V
NCL
Negative Current Limit Voltage
(Voltage from CSH1 to CSL1
and from CSH2 to CSL2)
V
2NDFB/FPWM
= 0.8V
-100
mV
-80
mV(min)
-140
mV(max)
LM2641
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