LT1941
Triple Monolithic
Switching Regulator
FEATURES
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DESCRIPTION
The LT
®
1941 is a triple current mode DC/DC converter
with internal power switches. Two of the regulators are
step-down converters with 3A and 2A power switches.
The third regulator can be configured as a boost, inverter
or SEPIC converter and has a 1.5A power switch. All
three converters are synchronized to a 1.1MHz oscillator.
The two step-down converters run with opposite phase,
reducing input ripple current. The output voltages are
set with external resistor dividers and each regulator
has independent shutdown and soft-start circuits. Each
regulator generates a power good signal when its output
is in regulation, easing power supply sequencing and
interfacing with microcontrollers and DSPs.
The high switching frequency offers several advantages
by permitting the use of small inductors and ceramic
capacitors, leading to a very small triple output solution.
The constant switching frequency, combined with low
impedance ceramic capacitors, result in low, predictable
output ripple. With its wide input voltage range of 3.5V to
25V, the LT1941 regulates a broad array of power sources
from 4-cell batteries and 5V logic rails to unregulated wall
transformers, lead acid batteries and distributed-power
supplies.
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Wide Input Range: 3.5V to 25V
Three Switching Regulators with Internal Power
Switches: 3A Step-Down, 2A Step-Down,
1.5A Inverting/Boost
Antiphase Switching Reduces Ripple
Independent Shutdown/Soft-Start Pins
Independent Power Good Indicators Ease Supply
Sequencing
Input Voltage Power Good Indicators Monitor Input
Supply
Uses Small Inductors and Ceramic Capacitors
Constant 1.1MHz Switching Frequency
Thermally Enhanced 28-Lead TSSOP Package
APPLICATIONS
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Cable Modems
DSL Modems
Distributed Power Regulation
Wall Transformer Regulation
Disk Drives
DSP Power
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
TYPICAL APPLICATION
V
IN
4.7V TO 14V
130k
100k
V
IN
PGOOD1
5GOOD
BOOST1
V
OUT1
1.8V
2.4A
3μH
13.7k
3300pF
33μF
7.32k
3.3k
1.5nF
0.22μF
SW1
FB1
V
C1
PGOOD2
BOOST2
0.22μF
SW2
FB2
V
C2
1000pF
10k
RUNSS1 RUNSS2
SW3
22μH
V
OUT3
–12V
350mA*
10μF
1μF
22μH
10μF
*240mA AT V
IN
= 5V, 550mA AT V
IN
= 12V
133k
13.7k
FB3
NFB
BIAS1
BIAS2
V
C3
RUNSS3
GND
1.5nF
1.5k
1941 F01
V
OUT1
100k
V
OUT2
100k
100k
PGOOD1
PGOOD2
PGOOD3
3.3μH
V
OUT2
3.3V
1.4A
22μF
5GOOD
12GOOD
12GOOD PGOOD3
LT1941
Start-Up Waveforms
with Sequencing
RUN/SS
2V/DIV
V
OUT1
2V/DIV
V
OUT2
5V/DIV
V
OUT3
10V/DIV
I
VIN(AVE)
1A/DIV
PGOOD2
5V/DIV
2ms/DIV
1941 F01b
10.7k
2.49k
1.5nF
22nF
Figure 1. Triple Output Power Supply: 3.3V, 1.8V, –12V
1941fb
1
LT1941
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
V
IN
V
IN
SW1
SW1
BOOST1
PGOOD1
V
C1
FB1
PGOOD2
1
2
3
4
5
6
7
8
9
29
28 BIAS2
27 SW3
26 PGND
25 V
IN
24 BOOST2
23 SW2
22 V
IN
21 PGOOD3
20 FB3
19 NFB
18 V
C3
17 5GOOD
16 12GOOD
15 BIAS1
V
IN
Pin ........................................................(–0.3V), 25V
BOOST Pin Voltage ...................................................35V
BOOST Above SW Pin ...............................................25V
BIAS1, BIAS2 Pins ....................................................25V
PGOOD, 5GOOD, 12GOOD Pins.................................25V
RUN/SS, V
C
, FB, NFB Pins ..........................................3V
SW1, SW2 Voltage .....................................................V
IN
SW3 Voltage .............................................................40V
Maximum Junction Temperature (Note 6) ............ 125°C
Operating Ambient Temperature Range
(Note 2)....................................................–40°C to 85°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................... 300°C
V
C2
10
FB2 11
RUN/SS1 12
RUN/SS2 13
RUN/SS3 14
FE PACKAGE
28-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 25°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT1941EFE#PBF
LEAD BASED FINISH
LT1941EFE
TAPE AND REEL
LT1941EFE#TRPBF
TAPE AND REEL
LT1941EFE#TR
PART MARKING
LT1941EFE
PART MARKING
LT1941EFE
PACKAGE DESCRIPTION
28-LEAD PLASTIC TSSOP
PACKAGE DESCRIPTION
28-LEAD PLASTIC TSSOP
TEMPERATURE RANGE
–40°C to 85°C
TEMPERATURE RANGE
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Operating Voltage
V
IN
Quiescent Current
BIAS1 Quiescent Current
BIAS2 Quiescent Current
Shutdown Current
Reference Voltage Line Regulation
V
C
Source Current
V
C
Sink Current
V
C
Clamp Voltage
Switching Frequency
The
l
denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at T
A
= 25°C. V
IN
, V
BIAS1
, V
BIAS2
= 5V, V
BOOST1
, V
BOOST2
= 8V, unless otherwise noted. (Note 2)
CONDITIONS
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MIN
TYP
3.3
2
5
1.6
50
0.01
100
100
1.7
MAX
3.5
3.5
7.5
2.2
75
UNITS
V
mA
mA
mA
μA
%/V
μA
μA
V
Not Switching
Not Switching
Not Switching
V
RUNSS1,2,3
= 0V
5V < V
IN
< 25V
V
C
= 0.6V
V
C
= 0.6V
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0.9
1.1
1.35
MHz
1941fb
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LT1941
ELECTRICAL CHARACTERISTICS
PARAMETER
Switching Phase
Foldback Frequency
RUN/SS Current
RUN/SS Threshold
5GOOD Threshold
5GOOD Voltage Output Low
5GOOD Leakage
12GOOD Threshold
12GOOD Voltage Output Low
12GOOD Leakage
PGOOD Voltage Output Low
PGOOD Pin Leakage
3A Step-Down
FB1 Voltage
FB1 Pin Bias Current
PGOOD1 Threshold Offset
Frequency Shift Threshold on FB1
Error Amplifier Transconductance
Error Amplifier Voltage Gain
V
C
Switching Threshold
V
C1
to Switch Current Gain
Switch 1 Current Limit (Note 3)
Switch 1 V
CESAT
(Note 7)
BOOST1 Pin Current
Switch 1 Leakage Current
Minimum Boost Voltage Above Switch (Note 4)
Maximum Duty Cycle
2A Step-Down
FB2 Voltage
FB2 Pin Bias Current
PGOOD2 Threshold Offset
Frequency Shift Threshold on FB2
Error Amplifier Transconductance
Error Amplifier Voltage Gain
V
C
Switching Threshold
V
C2
to Switch Current Gain
Switch 2 Current Limit (Note 3)
Switch 2 V
CESAT
(Note 7)
BOOST2 Pin Current
V
IN
= 12V, V
BOOST1
, V
BOOST2
= 15V
I
SW
= 1.5A
I
SW
= 1.5A
l
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l
l
l
l
The
l
denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at T
A
= 25°C. V
IN
, V
BIAS1
, V
BIAS2
= 5V, V
BOOST1
, V
BOOST2
= 8V, unless otherwise noted. (Note 2)
CONDITIONS
SW1 to SW2
SW1 to SW3
V
FB
= 0V
1
0.4
V
IN
Rising
I
5GOOD
= 125μA, V
IN
= 4V
V
5GOOD
= 2V
V
IN
Rising
I
12GOOD
= 125μA
V
12GOOD
= 2V, V
IN
= 12V
I
PGOOD
= 200μA
V
PGOOD
= 2V
618
613
MIN
150
–30
TYP
180
0
200
2
0.6
4.5
0.2
10
10.8
0.2
10
0.2
10
628
50
54
0.35
1700
500
0.9
5
V
IN
= 12V, V
BOOST1
, V
BOOST2
= 15V
I
SW
= 2.5A
I
SW
= 2.5A
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MAX
210
30
3
UNITS
Deg
Deg
kHz
μA
V
V
0.4
400
0.4
400
0.4
400
638
638
500
V
nA
V
V
nA
V
nA
mV
mV
nA
mV
V
μMhos
V/V
V
A/V
V
FB
Rising
3
4.3
400
40
0.01
1.8
6
600
60
10
2.5
A
mV
mA
μA
V
%
78
618
613
88
628
50
54
0.35
1700
500
0.9
3.6
638
638
500
mV
mV
nA
mV
V
μMhos
V/V
V
A/V
V
FB
Rising
2
2.9
450
26
4.1
600
40
A
mV
mA
1941fb
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LT1941
ELECTRICAL CHARACTERISTICS
PARAMETER
Switch 2 Leakage Current
Minimum Boost Voltage Above Switch (Note 4)
Maximum Duty Cycle
1.5A Inverting/Boost
FB3 Voltage
FB3 Pin Bias Current
NFB Voltage
NFB Pin Bias Current
NFB3 Voltage (V
FB3
-V
NFB
)
FB3 Pin Output Current
PGOOD3 Threshold Offset
Error Amplifier Transconductance
Error Amplifier Voltage Gain
V
C
Switching Threshold
V
C3
to Switch Current Gain
Frequency Shift Threshold on FB3
Switch 3 Current Limit (Note 5)
Switch 3 V
CESAT
BIAS2 Pin Current
Switch 3 Leakage Current
Maximum Duty Cycle
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LT1941E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Current limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at higher duty cycles.
l
l
l
l
l
l
l
l
The
l
denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at T
A
= 25°C. V
IN
, V
BIAS1
, V
BIAS2
= 5V, V
BOOST1
, V
BOOST2
= 8V, unless otherwise noted. (Note 2)
CONDITIONS
MIN
TYP
0.01
1.8
78
1.23
1.22
–15
1.212
1.205
150
88
1.25
800
0
60
1.24
350
120
800
150
1.1
5
0.65
1.5
2
240
30
0.01
77
86
2.9
320
45
10
I
SW
= 1A
I
SW
= 1A
1.27
1.27
1400
15
500
1.258
1.260
MAX
10
2.5
UNITS
μA
V
%
V
V
nA
mV
nA
V
V
μA
mV
μMhos
V/V
V
A/V
V
A
mV
mA
μA
%
V
FB3
= 1.35V, V
NFB
= –0.1V
V
FB
Rising
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Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 5:
Current limit is guaranteed by design and/or correlation to static
test.
Note 6:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 7:
Guaranteed by design, not 100% tested.
1941fb
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LT1941
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency, V
OUT1
= 1.8V
90
V
IN
= 5V
T
A
= 25°C
90
Efficiency, V
OUT2
= 3.3V
V
IN
= 5V
T
A
= 25°C
90
Efficiency, V
OUT3
= –12V
V
IN
= 5V
T
A
= 25°C
80
EFFICIENCY (%)
EFFICIENCY (%)
80
EFFICIENCY (%)
1.25
1.5
80
70
70
70
60
60
60
50
0
0.5
1.5
1
LOAD CURRENT (A)
2
2.5
1941 G01
50
0
0.25
0.5
0.75
1
LOAD CURRENT (A)
50
0
50
100
150
200
LOAD CURRENT (mA)
250
300
1941 G07
1941 G08
SW1 V
CESAT
500
T
A
= 25°C
400
SWITCH VOLTAGE (mV)
SWITCH VOLTAGE (mV)
500
400
300
200
100
0
600
SW2 V
CESAT
T
A
= 25°C
500
SW3 V
CESAT
T
A
= 25°C
400
SWITCH VOLTAGE (mV)
300
300
200
200
100
100
0
0
0.5
1
1.5
2
SWITCH CURRENT (A)
2.5
3
1941 G02
0
0.5
1
1.5
SWITCH CURRENT (A)
2
1941 G09
0
0
0.25
0.5
0.75
1
1.25
SWITCH CURRENT (A)
1.5
1941 G10
SW1 Current Limit vs Duty Cycle
5.0
4.5
4.0
CURRENT LIMIT (A)
CURRENT LIMIT (A)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
0
20
60
40
DUTY CYCLE (%)
80
100
1941 G03
SW2 Current Limit vs Duty Cycle
3.0
40
2.5
2.0
1.5
1.0
0.5
0
BOOST2 Pin Current
T
A
= 25°C
TYPICAL
TYPICAL
MINIMUM
BOOST CURRENT (mA)
30
MINIMUM
20
10
0
20
40
60
DUTY CYCLE (%)
80
100
1941 G06
0
1
1.5
0.5
SW2 PIN CURRENT (A)
2
1941 G11
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