FEATURES
n
n
n
n
n
n
n
n
n
n
n
n
n
LT3740
Wide Operating Range,
Valley Mode, No R
SENSE
™
Synchronous Step-Down Controller
DESCRIPTION
The LT
®
3740 is a synchronous step-down switching
regulator controller that drives N-channel power MOSFET
stages. The controller uses valley current mode architecture
to achieve very low duty cycles with excellent transient
response without requiring a sense resistor.
The LT3740 includes an internal step-up converter to
provide a bias 7.8V higher than the input voltage for the
drive. This enables the part to work from an input voltage
as low as 2.2V.
The XREF pin is an external reference input that allows the
user to override the internal 0.8V feedback reference with
any lower value, allowing full control of the output voltage
during operation, output voltage tracking or soft-start.
The LT3740 has three current limit levels that can be
chosen by connecting the RANGE pin to ground, open,
and input respectively.
L,
LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
No R
SENSE
is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
Wide V
IN
Range: 2.2V to 22V
Internal Boost Provides 6V Gate Drive
For V
IN
Down to 2.2V
No Sensing Resistor Required
Dual N-Channel MOSFET Synchronous Drive
Valley Current Mode Control
Optimized for High Step-Down Ratio
Power Good Output Voltage Monitor
0.8V Reference
Three Pin-Selected Current Limit Levels
Constant Switching Frequency: 300kHz
Programmable Soft-Start
Output Voltage Tracking
Available in 16-Pin 5mm
×
3mm DFN
APPLICATIONS
n
n
n
Notebook and Palmtop Computers, PDA
Portable Instruments
Distributed Power Systems
TYPICAL APPLICATION
High Efficiency Step-Down Converter
22μH
SWB
V
IN
SHDN
15k
XREF
0.22μF
15k
20k
1nF
LT3740
BGDP
BIAS
TGATE
1Ω
SW
SN
+
1Ω
BGATE
SN
–
PGND
M2
HAT2165H
D1
B320A
M1
HAT2168H
EFFICIENCY (%)
1Ω
0.9μH
V
OUT
1.8V
10A
100μF
3
1μF
V
IN
3V to 12V
10μF
94
92
90
88
86
84
82
V
OUT
= 1.8V
105k
80
0
2
4
6
LOAD CURRENT (A)
8
10
3740 TA01b
Efficiency vs Load Current
V
IN
= 3V
V
IN
= 5V
V
IN
= 12V
39pF
V
C
RANGE GND FB
22pF
80.6k
3740 TA01a
3740fc
1
LT3740
ABSOLUTE MAXIMUM RATINGS
(Note 1)
SN
–
, BGATE, V
C
, FB, XREF PGOOD Voltages .............10V
,
V
IN
,
SHDN,
SW, Range Voltages ...............................22V
BIAS, TGATE, BGDP, SN
+
Voltages ............................32V
PIN CONFIGURATION
TOP VIEW
SN
–
PGND
BGATE
BGDP
SN
+
1
2
3
4
5
6
7
8
17
16 V
C
15 FB
14 XREF
13
SHDN
12 PGOOD
11 RANGE
10 V
IN
9
SWB
SWB Voltage ............................................................36V
Maximum Junction Temperature........................... 125°C
Operating Temperature Range (Note 2)....–40°C to 85°C
Storage Temperature Range...................–65°C to 125°C
SW
TGATE
BIAS
DHC PACKAGE
16-LEAD (5mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W
EXPOSED PAD IS GND (PIN 17), MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LT3740EDHC#PBF
LEAD BASED FINISH
LT3740EDHC
TAPE AND REEL
LT3740EDHC#TRPBF
TAPE AND REEL
LT3740EDHC#TR
PART MARKING
3740
PART MARKING
3740
PACKAGE DESCRIPTION
16-Lead (5mm
×
3mm) Plastic DFN
PACKAGE DESCRIPTION
16-Lead (5mm
×
3mm) Plastic DFN
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/
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C, V
IN
= 5V unless otherwise noted. (Note 2)
PARAMETER
Minimum Operation Voltage
Maximum Operation Voltage
Input Supply Current
Feedback Reference Voltage
Feedback Voltage Line Regulation
FB Pin Input Current
Error Amplifier Transconductance
Controller Switching Frequency
Minimum BGATE On Time (Note 3)
Current Limit
RANGE = 0V
RANGE = Open
RANGE = V
IN
25
55
80
SHDN
= 0V
SHDN
= 5V, BIAS = 14V, FB = 1.5V
XREF=1V
V
IN
= 2.5V to 22V
FB = 800mV
V
C
= 1.2V
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
●
●
MIN
2.2
TYP
MAX
22
UNITS
V
V
μA
mA
0.6
2.5
794
800
0.006
230
380
260
300
500
50
80
105
35
330
700
85
115
140
808
mV
%
nA
μS
kHz
ns
mV
mV
mV
mV
3740fc
Reverse Current Limit
2
LT3740
ELECTRICAL CHARACTERISTICS
PARAMETER
SHDN
Voltage to Enable Device
SHDN
Voltage to Disable Device
TGATE On Voltage
TGATE Off Voltage
BGATE On Voltage
BGATE Off Voltage
TGATE Rise Time
TGATE Fall Time
BGATE Rise Time
BGATE Fall Time
PGOOD Threshold
PGOOD Low Voltage
PGOOD Current Capacity
Internal Boost Switching Frequency
Internal Boost Switch Current Limit
(BIAS – V
IN
) in Operation
(BIAS – V
IN
) to Start Controller
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.
I
PGOOD
= 100μA
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF
●
●
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C, V
IN
= 5V unless otherwise noted. (Note 2)
CONDITIONS
●
●
MIN
1.1
TYP
MAX
0.5
UNITS
V
V
V
V
V
V
ns
ns
ns
ns
5.5
0.2
5.5
0.2
30
30
50
50
720
500
0.8
360
1
440
7.8
7.2
1.2
520
740
765
0.2
mV
V
μA
MHz
mA
V
V
Note 2:
The LT3740E is guaranteed to meet performance specifications
from 0°C to 85°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:
The minimum off time of the LT3740 application consists of the
minimum BGATE on time, the delay from TGATE OFF to BGATE ON (80ns)
and the delay from BGATE OFF to TGATE ON (80ns).
TYPICAL PERFORMANCE CHARACTERISTICS
Transient Response
V
OUT
50mV/DIV
LOAD STEP 0A TO 10A
V
IN
= 10V, V
OUT
= 2.5V
FIGURE 4 CIRCUIT
(T
A
= 25°C, unless otherwise noted)
Transient Response
LOAD STEP 0A TO 10A
V
IN
= 5V, V
OUT
= 1.8V
PAGE 17 CIRCUIT
V
OUT
50mV/DIV
Shutdown Pin Start-Up
SHDN
2V/DIV
I
L
2A/DIV
V
O
2V/DIV
I
L
5A/DIV
I
L
5A/DIV
V
IN
= 10V, V
OUT
= 2.5V
I
LOAD
= 3A
3740 G02
3740 G03
20μs/DIV
3740 G01
10μs/DIV
2ms/DIV
3740fc
3
LT3740
TYPICAL PERFORMANCE CHARACTERISTICS
XREF Pin Start-Up
0
CURRENT SENSING LIMIT (mV)
SS
0.5V/DIV
I
L
2A/DIV
DV
OUT
(%)
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
V
IN
= 10V, V
OUT
= 2.5V
I
LOAD
= 3A
3740 G04
(T
A
= 25°C, unless otherwise noted)
Current Limit vs Bottom Gate
On-Time
200
Load Regulation
V
IN
= 10V
V
OUT
= 2.5V
R
S
= 12mΩ
180
160
140
120
100
80
60
40
20
10
3740 G05
RANGE = V
IN
RANGE = OPEN
RANGE = V
IN
RANGE = GND
RANGE = GND
V
O
2V/DIV
–0.7
–0.8
0
2
RANGE = OPEN
6
4
LOAD CURRENT (A)
8
0
2ms/DIV
30
40
50
60
80
90
70
BOTTOM GATE ON DUTY CYCLE (%)
100
3740 G06
Shutdown vs Maximum V
C
2.0
CURRENT SENSING THRESHOLD (mV)
1.8
1.6
MAXIMUM V
C
(V)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
SHUTDOWN (V)
2.5
3
3740 G07
V
C
vs Current Sensing Threshold
300
250
RANGE = V
IN
200
150
100
50
0
RANGE = GND
V
FB
(mV)
810
808
806
804
802
800
798
796
794
792
1
1.1 1.2
1.3
1.4 1.5
V
C
(V)
1.6
1.7
1.8
Feedback Reference Voltage
vs Temperature
RANGE =
OPEN
790
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
3740 G09a
3740 G08
PGOOD Threshold vs Temperature
770
760
750
SHDN
(V)
V
FB
(mV)
740
730
720
710
–50
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
–25
0
25
50
TEMPERATURE (°C)
75
100
Shutdown Threshold
vs Temperature
320
CONTROLLER FREQUENCY (KHz)
–25
0
25
50
TEMPERATURE (°C)
315
310
305
300
295
290
285
75
100
3740 G10
Switching Frequency
vs Temperature
0.4
–50
280
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
3740 G11
3740 G09b
3740fc
4
LT3740
TYPICAL PERFORMANCE CHARACTERISTICS
Error Amplifier Transconductance
vs Temperature
400
390
380
ERROR AMP (μs)
370
360
350
340
330
320
310
300
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
3740 G12
(T
A
= 25°C, unless otherwise noted)
Undervoltage Lockout Threshold
vs Temperature
3.0
UNDERVOLTAGE LOCKOUT V
IN
(V)
2.5
2.0
1.5
1.0
0.5
0
–50
BIAS-V
IN
to Enable Controller
vs Temperature
7.50
7.45
7.40
7.35
BIAS-V
IN
(V)
7.30
7.25
7.20
7.15
7.10
7.05
7.00
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
3740 G13
–25
0
25
50
TEMPERATURE (°C)
75
100
3740 G14
PIN FUNCTIONS
SN
–
(Pin 1):
Negative Current Sensing Pin. Connect this
pin to the source of the bottom MOSFET for No R
SENSE
or to a current sense resistor.
PGND (Pin 2):
Power Ground. Connect this pin closely to
the source of the bottom N-channel MOSFET.
BGATE (Pin 3):
Bottom Gate Drive. Drives the gate of the
bottom N-channel MOSFET.
BGDP (Pin 4):
Bottom Gate Drive Power Supply. Connect
this pin to a voltage source higher than 7V (V
IN
or BIAS).
SN
+
(Pin 5):
Positive Current Sensing Pin. Connect this
pin to the drain of the bottom MOSFET for No R
SENSE
or
to a current sense resistor.
SW (Pin 6):
Switch Node. Connect this pin to the source
of the top N-channel MOSFET and the drain of the bottom
N-channel MOSFET.
TGATE (Pin 7):
Top Gate Drive. Drives the gate of the top
N-channel MOSFET to BIAS.
BIAS (Pin 8):
Top Gate Drive Power Supply. Connect a
capacitor between this pin and V
IN
.
SWB (Pin 9):
Switch Pin of the Internal Boost. Connect
the boost inductor here.
V
IN
(Pin 10):
Input Supply Pin. Must be locally bypassed
with a capacitor.
RANGE (Pin 11):
Current Limit Range Select Pin. Ground
this pin for 50mV current sense voltage limit. Leave this
pin open for 80mV current sense voltage limit. Connect
this pin to V
IN
for current sense voltage limit of 105mV.
PGOOD (Pin 12):
Power Good Output. Open collector
logic output that is pulled low when the FB voltage lower
than 720mV.
SHDN
(Pin 13):
Shutdown Pin. Connect to 2.5V or higher to
enable device; 0.5V or less to disable device. Also, this pin
functions as soft-start when a voltage ramp is applied.
XREF (Pin 14):
External Reference Pin. This pin sets the
FB voltage externally between 0V and 0.8V. It can be used
to slave the output voltage during normal operation or the
output start-up behavior to an external signal source. Tie
this pin to 1V or higher to use the internal 0.8V reference.
FB (Pin 15):
Feedback Pin. Pin voltage is regulated to 0.8V
if internal reference is used or to the XREF pin if voltage
is between 0V and 0.8V. Connect the feedback resistor
divider to this pin.
3740fc
5
