LT1940/LT1940L
Dual Monolithic 1.4A,
1.1MHz Step-Down
Switching Regulator
FEATURES
s
DESCRIPTIO
s
s
s
s
s
s
s
Wide Input Voltage Range
LT1940: 3.6V to 25V
LT1940L: 3.6V to 7V
Two 1.4A Output Switching Regulators with
Internal Power Switches
Constant 1.1MHz Switching Frequency
Anti-Phase Switching Reduces Ripple
Independent Shutdown/Soft-Start Pins
Independent Power Good Indicators Ease
Supply Sequencing
Uses Small Inductors and Ceramic Capacitors
Small 16-Lead Thermally Enhanced
TSSOP Surface Mount Package
The LT
®
1940 is a dual current mode PWM step-down
DC/DC converter with internal 2A power switches. Both con-
verters are synchronized to a single 1.1MHz oscillator and
run with opposite phases, 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 se-
quencing and interfacing with microcontrollers and DSPs.
The LT1940’s 1.1MHz switching frequency allows the use
of tiny inductors and capacitors, resulting in a very small
dual 1.4A output solution. Constant frequency and ceramic
capacitors combine to produce low, predictable output
ripple voltage. With its wide input range of 3.6V to 25V, the
LT1940 regulates a wide variety of power sources, from
4-cell batteries and 5V logic rails to unregulated wall trans-
formers, lead acid batteries and distributed-power supplies.
The LT1940L is intended to operate from regulated 5V
supplies. A current mode PWM architecture provides fast
transient response with simple compensation components
and cycle-by-cycle current limiting. Frequency foldback and
thermal shutdown provide additional protection.
APPLICATIO S
s
s
s
s
s
s
Disk Drives
DSP Power Supplies
Wall Transformer Regulation
Distributed Power Regulation
DSL Modems
Cable Modems
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
V
IN
7V TO 25V
4.7µF
CMDSH-3
V
IN
BOOST1
BOOST2
0.1µF
CMDSH-3
OUT2
5V
1.4A
0.1µF
OUT1
3.3V
1.4A
3.3µH
SW1
UPS140
16.5k
FB1
V
C1
10.0k
10µF
15k
330pF
1nF
LT1940
SW2
4.7µH
UPS140
30.1k
FB2
V
C2
PG1
PG2
330pF
100k
POWER
GOOD
15k
10.0k
10µF
EFFICIENCY (%)
RUN/SS1
RUN/SS2
GND
1940 F01
Figure 1. 3.3V and 5V Dual Output Step-Down Converter with Output Sequencing (LT1940)
1940fa
U
Efficiency vs Load Current
100
V
IN
= 12V
90
V
OUT
= 5V
80
V
OUT
= 3.3V
70
60
0
0.5
1.0
LOAD CURRENT (A)
1.5
1940 F01b
U
U
1
LT1940/LT1940L
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
BOOST1
SW1
V
IN
V
IN
V
IN
V
IN
SW2
BOOST2
1
2
3
4
5
6
7
8
17
16 FB1
15 V
C1
14 PG1
13 RUN/SS1
12 RUN/SS2
11 PG2
10 V
C2
9
FB2
V
IN
Voltage
LT1940 .................................................... (–0.3), 25V
LT1940L ................................................... (–0.3), 7V
BOOST Pin Voltage
LT1940 ................................................................ 35V
LT1940L ............................................................. 16V
BOOST Pin Above SW Pin
LT1940 ................................................................ 25V
LT1940L ............................................................. 16V
PG Pin Voltage
LT1940 ................................................................ 25V
LT1940L ............................................................... 7V
SW Voltage ................................................................V
IN
RUN/SS, FB Pins ...................................................... 6V
Maximum Junction Temperature .......................... 125°C
Operating Temperature Range (Note 2) ...–40°C to 85°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT1940EFE
LT1940LEFE
FE PART
MARKING
1940EFE
1940LEFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
EXPOSED PAD (PIN 17) IS GND
MUST BE SOLDERED TO PCB
T
JMAX
= 125°C,
θ
JA
= 45°C/W,
θ
JC
= 10°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 5V, V
BOOST
= 8V unless otherwise noted. (Note 2)
PARAMETER
Minimum Operating Voltage
Quiescent Current
Shutdown Current
Feedback Voltage
0°C to 70°C
–40°C to 85°C
FB Pin Bias Current
Reference Line Regulation
Error Amp GM
Error Amp Voltage Gain
V
C
Source Current
V
C
Sink Current
V
C
Pin to Switch Current Gain
V
C
Switching Threshold
V
C
Clamp Voltage
Switching Frequency
Switching Phase
Maximum Duty Cycle
Frequency Shift Threshold on FB
f
SW
= 1MHz
q
q
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
q
MIN
TYP
3.4
3.8
30
MAX
3.6
4.8
45
1.270
1.270
1.270
1200
UNITS
V
mA
µA
V
V
V
nA
%/V
%/V
uMhos
µA
µA
A/V
V
V
Not Switching
V
RUNSS
= 0V
1.230
1.225
1.215
1.250
1.250
1.250
240
0.005
0.005
330
180
V
FB
= 1.25V, V
C
= 0.4V
LT1940: V
IN
5V to 25V
LT1940L: V
IN
4V to 7V
V
FB
= 1V
V
FB
= 1.5V
42
60
2.4
0.75
1.8
V
FB
= 1.1V
q
1
0.95
150
78
1.1
1.1
180
88
0.5
1.25
1.35
210
1940fa
2
U
MHz
MHz
Deg
%
V
W
U
U
W W
W
LT1940/LT1940L
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 5V, V
BOOST
= 8V unless otherwise noted. (Note 2)
PARAMETER
Foldback Frequency
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Minimum Boost Voltage Above Switch (Note 4)
BOOST Pin Current
RUN/SS Current
RUN/SS Threshold
PG Threshold Offset
PG Voltage Output Low
PG Pin Leakage
V
FB
Rising
V
FB
= 1V, I
PG
= 250µA
V
PG
= 2V
0.3
90
I
SW
= 1A
I
SW
= 1A
1.8
20
2.3
0.6
125
0.22
0.1
160
0.4
1
CONDITIONS
V
FB
= 0V
Note 3
I
SW
= 1A
1.8
MIN
TYP
150
2.4
210
3.2
320
10
2.5
30
MAX
UNITS
kHz
A
mV
µA
V
mA
µA
V
mV
V
µA
ELECTRICAL CHARACTERISTICS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1940E 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 high duty cycle.
Note 4:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency, V
OUT
= 1.8V
90
V
OUT
= 1.8V
L = 2.2µH (SUMIDA CR43-2R2)
T
A
= 25°C
EFFICIENCY (%)
80
EFFICIENCY (%)
V
IN
= 5V
70
80
V
IN
= 12V
70
V
IN
= 18V
60
EFFICIENCY (%)
60
50
0
0.5
1.0
LOAD CURRENT (A)
U W
1.5
1940 G01
Efficiency, V
OUT
= 3.3V
100
V
OUT
= 3.3V
L = 3.3µH (SUMIDA CR43-3R3)
T
A
= 25°C
V
IN
= 5V
90
100
Efficiency, V
OUT
= 5V
V
OUT
= 5V
L = 4.7µH (SUMIDA CR43-4R7)
T
A
= 25°C
V
IN
= 8V
90
80
V
IN
= 18V
70
V
IN
= 12V
50
60
0
0.5
1.0
LOAD CURRENT (A)
1.5
1940 G02
0
0.5
1.0
LOAD CURRENT (A)
1.5
1940 G03
1940fa
3
LT1940/LT1940L
TYPICAL PERFOR A CE CHARACTERISTICS
Maximum Load Current,
V
OUT
= 1.8V
1.8
1.8
SLOPE COMPENSATION REQUIRES
L > 2.2µH FOR V
IN
< 7 WITH V
OUT
= 3.3V
LOAD CURRENT (A)
LOAD CURRENT (A)
L = 4.7µH
L = 2.2µH
1.4
L = 1.5µH
1.4
L = 3.3µH
SWITCH VOLTAGE (mV)
1.6
L = 1µH
1.2
1.0
0
2
4
8
10 12
6
INPUT VOLTAGE (V)*
Boost Pin Current
40
3.0
2.5
BOOST CURRENT (mA)
CURRENT LIMIT (A)
30
20
1.5
1.0
MINIMUM
V
OUT
(V)
10
0.5
0
0
1.0
1.5
0.5
SWITCH CURRENT (A)
Frequency vs Temperature
1.3
SWITCHING FREQUENCY (MHz)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
RUN/SS CURRENT (µA)
1.2
FREQUENCY (MHz)
1.1
1.0
0.9
–50 –25
75
0
25
50
TEMPERATURE (°C)
*Maximum V
IN
is 7V for LT1940L.
1940fa
4
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14
16
1940 G04
Maximum Load Current,
V
OUT
= 3.3V
400
Switch V
CESAT
T
A
= 25°C
1.6
300
200
L = 2.2µH
1.2
100
1.0
0
0
5
15
20
10
INPUT VOLTAGE (V)*
25
1940 G05
0
0.5
1.0
SW CURRENT (A)
1.5
2.0
1940 G06
Current Limit vs Duty Cycle
3.40
V
OUT
vs Temperature
CHANNEL 1, FIGURE 1, V
IN
= 12V
TYPICAL
2.0
3.35
3.30
3.25
0
2.0
1940 G07
0
20
60
40
DUTY CYCLE (%)
80
100
1940 G08
3.20
–50
–25
0
25
50
75
100
125
TEMPERATURE (°C)
1940 G09
Frequency Foldback
3.0
2.5
2.0
1.5
1.0
0.5
I
RUN/SS
vs Temperature
100
125
0
0.2
0.4
0.6
0.8
1.0
FEEDBACK VOLTAGE (V)
1.2
1940 G11
0
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
1940 G10
1940 G12
LT1940/LT1940L
TYPICAL PERFOR A CE CHARACTERISTICS
RUN/SS Thresholds vs
Temperature
1.4
1.2
MINIMUM INPUT VOLTAGE (V)
RUNN/SS THRESHOLDS (V)
V
IN
TO START
BOOST DIODE
TIED TO OUTPUT
1.0
TO SWITCH
0.8
0.6
TO RUN
0.4
0.2
0
–50 –25
MINIMUM INPUT VOLTAGE (V)
50
25
75
0
TEMPERATURE (°C)
PI FU CTIO S
BOOST1, BOOST2 (Pins 1, 8):
The BOOST pins are used
to provide drive voltages, higher than the input voltage, to
the internal bipolar NPN power switches.Tie through a
diode from V
OUT
or from V
IN
.
SW1, SW2 (Pins 2, 7):
The SW pins are the outputs of the
internal power switches. Connect these pins to the induc-
tors, catch diodes and boost capacitors.
V
IN
(Pins 3, 4, 5, 6):
The V
IN
pins supply current to the
LT1940’s internal regulator and to the internal power
switches. These pins must be tied to the same source, and
must be locally bypassed.
FB1, FB2 (Pins 9, 16):
The LT1940 regulates each feed-
back pin to 1.25V. Connect the feedback resistor divider
taps to these pins.
V
C1
, V
C2
(Pins 10, 15):
The V
C
pins are the outputs of the
internal error amps. The voltages on these pins control the
peak switch currents. These pins are normally used to
compensate the control loops, but can also be used to
override the loops. Pull these pins to ground with an open
drain to shut down each switching regulator.
PG1, PG2 (Pins 11, 14):
The Power Good pins are the
open collector outputs of an internal comparator. PG
remains low until the FB pin is within 10% of the final
regulation voltage. As well as indicating output regulation,
the PG pins can be used to sequence the two switching
regulators. These pins can be left unconnected. The PG
outputs are valid when V
IN
is greater than 2.4V and either
of the RUN/SS pins is high. The PG comparators are
disabled in shutdown.
RUN/SS1, RUN/SS2 (Pins 12, 13):
The RUN/SS pins are
use to shut down the individual switching regulators and
the internal bias circuits. They also provide a soft-start
function. To shut down either regulator, pull the RUN/SS
pin to ground with an open drain or collector. Tie a
capacitor from these pins to ground to limit switch current
during start-up. If neither feature is used, leave these pins
unconnected.
GND (Pin 17):
The Exposed Pad of the package provides
both electrical contact to ground and good thermal con-
tact to the printed circuit board. The Exposed Pad must be
soldered to the circuit board for proper operation.
U W
100
125
1940 G13
Minimum Input Voltage,
V
OUT
= 3.3V
6.0
5.5
5.0
4.5
4.0
V
IN
TO RUN
3.5
3.0
T
A
= 25°C
D
BOOST
= BAT54
7.5
Minimum Input Voltage,
V
OUT
= 5V
7.0 V
IN
TO START
6.5
6.0
5.5
5.0
4.5
V
IN
TO RUN
T
A
= 25°C
D
BOOST
= BAT54
BOOST DIODE
TIED TO OUTPUT
BOOST DIODE
TIED TO INPUT
BOOST DIODE
TIED TO INPUT
1
10
100
LOAD CURRENT (mA)
1000
1940 G14
1
10
100
LOAD CURRENT (mA)
1000
1940 G14
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1940fa
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