LT3742
Dual, 2-Phase Step-Down
Switching Controller
FEATURES
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DESCRIPTION
The LT
®
3742 is a dual step-down DC/DC switching regulator
controller that drives high side N-channel power MOSFETs.
A 500kHz fixed frequency current mode architecture pro-
vides fast transient response with simple loop compensa-
tion components and cycle-by-cycle current limiting. The
output stages of the two controllers operate 180° out of
phase to reduce the input ripple current, minimizing the
noise induced on the input supply, and allowing less input
capacitance.
An internal boost regulator generates a bias rail of V
IN
+ 7V
to provide gate drive for the N-channel MOSFETs allowing
low dropout and 100% duty cycle operation. The LT3742
can be used for applications where both controllers need
to operate independently, or where both controllers are
used to provide a single higher current output.
The device is available in a thermally enhanced 4mm
×
4mm QFN package.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Wide Input Voltage Range: 4V to 30V
Wide Output Voltage Range: 0.8V to V
IN
Low Shutdown I
Q
: 20μA
Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
0.8V ±1.5% Voltage Reference
500kHz Current Mode Fixed Frequency Operation
Internal Boost Converter Provides Bias Rail for
N-Channel MOSFET Gate Drive
Power Good Voltage Monitor for Each Output
Programmable Soft-Start
24-Lead 4mm
×
4mm
×
0.75mm Package
APPLICATIONS
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Satellite and Cable TV Set-Top Boxes
Distributed Power Regulation
Automotive Systems
Super Capacitor Charger
TYPICAL APPLICATION
8V and 5V Dual Step-Down Converter
V
IN
14V
45.3k
1μF
20.0k
90
LT3742
10μF
V
OUT1
8V
4A
47μF
200Ω
PG1
30k
PG1
RUN/SS1
V
C1
PG2
RUN/SS2
V
C2
GND
PG2
20k
0.010Ω
1.8k
6.5μH
SW1
SENSE1
+
SENSE1
–
FB1
SW2
SENSE2
+
SENSE2
–
FB2
G1
G2
6.5μH
0.010Ω
1.05k
200Ω
10μF
V
OUT2
5V
4A
47μF
50
40
0
RUN2
0.5
1
EFFICIENCY (%)
V
IN
V
IN
80
70
60
5V
OUT
10μH
4.7μF
V
IN
UVLO
SWB BIAS
100
Efficiency vs Load Current
8V
OUT
2.5 3
1.5 2
LOAD CURRENT (A)
3.5
4
RUN1
1nF 1000pF
1000pF
1nF
3742 TA03b
3742 TA03a
3742f
1
LT3742
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
SENSE1
+
SENSE1
–
SW1
FB1
18 V
C1
17 PG1
25
16 RUN/SS1
15 RUN/SS2
14 PG2
13 V
C2
7
SW2
8
NC
9 10 11 12
+
V
IN
Voltage ................................................................30V
UVLO Voltage ............................................................30V
PG1, PG2 Voltage .....................................................30V
SWB, BIAS Voltage ...................................................40V
SENSE1
+
, SENSE2
+
Voltage ......................................30V
SENSE1
–
, SENSE2
–
Voltage ......................................30V
RUN/SS1, RUN/SS2 Voltage .......................................6V
FB1, FB2 Voltage .........................................................6V
V
C1
, V
C2
Voltage..........................................................6V
Junction Temperature ........................................... 125°C
Operating Junction Temperature Range
(Note 2).................................................. –40°C to 125°C
Storage Temperature Range................... –65°C to 125°C
NC
24 23 22 21 20 19
G1 1
V
IN
2
UVLO 3
BIAS 4
SWB 5
G2 6
NC
SENSE2
–
UF PACKAGE
24-LEAD (4mm
×
4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 36°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3742EUF#PBF
TAPE AND REEL
LT3742EUF#TRPBF
PART MARKING
3742
PACKAGE DESCRIPTION
24-Lead (4mm
×
4mm) Plastic QFN
TEMPERATURE RANGE
–40°C to 125°C (Note 2)
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
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 Input Voltage
Quiescent Current
Shutdown Current
UVLO Pin Threshold
UVLO Pin Hysteresis Current
RUN/SS Pin Threshold
RUN/SS Pin Charge Current
FB Pin Voltage
FB Pin Voltage Line Regulation
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 5V unless otherwise specified.
CONDITIONS
V
UVLO
= 1.5V
V
RUN/SS1
= V
RUN/SS2
= V
FB1
= V
FB2
= 1V
V
RUN/SS1
= V
RUN/SS2
= 0V
UVLO Pin Voltage Rising
V
UVLO
= 1V, Current Flows Into Pin
V
RUN/SS
= 0V
●
●
●
MIN
SENSE2
FB2
NC
TYP
3.5
5.0
20
MAX
4.0
7.0
35
1.28
4
1.5
0.812
UNITS
V
mA
μA
V
μA
V
μA
V
%/V
1.20
1.8
0.2
0.5
0.788
1.24
2.7
0.6
0.9
0.800
0.01
V
IN
= 5V to 30V
3742f
2
LT3742
ELECTRICAL CHARACTERISTICS
PARAMETER
FB Pin Bias Current
FB Pin Voltage Matching
Error Amplifier Transconductance
Error Amplifier Voltage Gain
V
C
Pin Source Current
V
C
Pin Sink Current
Controller Switching Frequency
Switching Phase
Maximum Current Sense Voltage
Current Sense Matching
Current SENSE Pins Total Current
Gate Rise Time
Gate Fall Time
Gate On Voltage (V
G
– V
SW
)
Gate Off Voltage (V
G
– V
SW
)
PG Pin Voltage Low
Lower PG Trip Level (Relative to V
FB
)
Lower PG Trip Level (Relative to V
FB
)
Upper PG Trip Level (Relative to V
FB
)
Upper PG Trip Level (Relative to V
FB
)
PG Pin Leakage Current
PG Pin Sink Current
Bias Pin Voltage
SWB Pin Current Limit
SWB Pin Leakage Current
Bias Supply Switching Frequency
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.
V
SWB
= 12V
0.88
V
SENSE–
= 3.3V
Between Controllers
SENSE
–
, SENSE
+
= 0V
SENSE
–
, SENSE
+
= 3.3V
C
LOAD
= 3300pF
C
LOAD
= 3300pF
V
IN
= 5V, V
BIAS
= 12V
V
IN
= 5V, V
BIAS
= 12V
I
PG
= 100μA
V
FB
Increasing
V
FB
Decreasing
V
FB
Increasing
V
FB
Decreasing
V
PG
= 2V, V
FB
= 1V
V
PG
= 0.5V
200
V
IN
+ 6.6
250
–7
–10
7
4
6.0
●
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 5V unless otherwise specified.
CONDITIONS
V
FB
= 0.8V, V
C
= 0.4V
–4
MIN
TYP
50
0
250
500
V
FB
= 0.6V
V
FB
= 1V
440
50
15
15
500
180
60
±5
–1.0
40
40
60
6.7
0.4
0.20
–10
–13
10
7
0.1
500
V
IN
+ 7
340
0.01
1.0
V
IN
+ 7.7
500
1
1.12
7.0
0.75
0.5
–13
–16
13
10
70
560
MAX
200
4
UNITS
nA
mV
μmho
V/V
μA
μA
kHz
Deg
mV
%
mA
μA
ns
ns
V
V
V
%
%
%
%
μA
μA
V
mA
μA
MHz
Note 2:
The LT3742E is guaranteed to meet performance specifications
from 0°C to 125°C operating junction temperature range. Specifications
over the – 40°C to 125°C operating junction temperature range are
assured by design, characterization and correlation with statistical process
controls.
3742f
3
LT3742
TYPICAL PERFORMANCE CHARACTERISTICS
I
Q
-SHDN vs Temperature
50
10
9
8
CURRENT (μA)
CURRENT (mA)
7
6
5
4
10
–50
3
–50 –25
50
–50
SENSE VOLTAGE (mV)
40
65
I
Q
-Running vs Temperature
70
Controller Current Sense Voltage
vs Temperature
30
60
20
55
–25
75
0
25
50
TEMPERATURE (°C)
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3742 G01
3742 G02
3742 G03
V
IN
Minimum vs Temperature
5
3.0
2.5
4
2.0
UVLO (V)
3
1.5
1.0
2
0.5
1
–50
UVLO Threshold vs Temperature
830
820
810
V
FB
(mV)
50
25
75
0
TEMPERATURE (°C)
100
125
800
790
780
V
FB
vs Temperature
V
IN
(V)
–25
75
0
25
50
TEMPERATURE (°C)
100
125
0
–50 –25
770
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3742 G04
3742 G05
3742 G06
RUN/SS Current vs Temperature
1.8
3.0
UVLO I
HYST
vs Temperature
2.8
RUN/SS CURRENT (μA)
1.2
I
HYST
(μA)
0.6
2.2
0
–50
2.6
2.4
–25
0
25
50
75
TEMPERATURE (°C)
100
125
2.0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
3742 G07
3742 G08
3742f
4
LT3742
TYPICAL PERFORMANCE CHARACTERISTICS
Controller Frequency vs
Temperature
600
1.0
PG Threshold vs Temperature
10
V
BIAS
– V
IN
vs Temperature
FEEDBACK VOLTS RISING (V)
550
FREQUENCY (kHz)
0.9
(V
BIAS
– V
IN
) (V)
100
125
8
500
0.8
POWER GOOD
6
450
0.7
400
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
0.6
–50
–25
75
0
25
50
TEMPERATURE (°C)
4
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3742 G09
3742 G10
3742 G11
SWB Current Limit vs
Temperature
380
1.0
RUN/SS Threshold vs
Temperature
SWB CURRENT LIMIT (mA)
360
RUN/SS VOLTS (V)
–25
75
0
25
50
TEMPERATURE (°C)
100
125
0.7
340
0.4
320
300
–50
0.1
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3742 G13
3742 G12
3742f
5
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