LT3724
High Voltage, Current Mode
Switching Regulator Controller
FEATURES
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DESCRIPTIO
Wide Input Range: 4V to 60V
Output Voltages up to 36V (Step-Down)
Burst Mode
®
Operation: <100µA Supply Current
<10µA Shutdown Supply Current
±1.3%
Reference Accuracy
200kHz Fixed Frequency
Drives N-Channel MOSFET
Programmable Soft-Start
Programmable Undervoltage Lockout
Internal High Voltage Regulator for Gate Drive
Thermal Shutdown
Current Limit Unaffected by Duty Cycle
16-Pin Thermally Enhanced TSSOP Package
The LT
®
3724 is a DC/DC controller used for medium
power, low part count, low cost, high efficiency supplies.
It offers a wide 4V-60V input range (7.5V minimum startup
voltage) and can implement step-down, step-up, inverting
and SEPIC topologies.
The LT3724 includes Burst Mode operation, which re-
duces quiescent current below 100µA and maintains high
efficiency at light loads. An internal high voltage bias
regulator allows for simple biasing and can be back driven
to increase efficiency.
Additional features include fixed frequency current mode
control for fast line and load transient response; a gate
driver capable of driving large N-channel MOSFETs; a
precision undervoltage lockout function; 10µA shutdown
current; short-circuit protection; and a programmable
soft-start function that directly controls output voltage
slew rates at startup which limits inrush current, mini-
mizes overshoot and facilitates supply sequencing.
The LT3724 is available in a 16-lead thermally enhanced
TSSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5731694, 6498466, 6611131.
APPLICATIO S
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Industrial Power Distribution
12V and 42V Automotive and Heavy Equipment
High Voltage Single Board Systems
Distributed Power Systems
Avionics
Telecom Power
TYPICAL APPLICATIO
V
IN
30V TO
60V
C
IN
68µF
1M
High Voltage Step-Down Regulator
95
V
IN
BOOST
TG
LT3724
SHDN
68.1k
200k
C
SS
Burst_EN
V
FB
40.2k
V
C
680pF
120pF
4.99k
93.1k
3724 TA01a
0.22µF
Si7852
0.025Ω
47µH
SS3H9
SW
+
C
OUT
330µF
EFFICIENCY (%)
V
OUT
24V
75W
V
CC
1µF
PGND
SENSE
+
SENSE
–
65
0.1
1
LOAD CURRENT (A)
1000pF
SGND
U
Efficiency and Power Loss
vs Load Current
12
10
EFFICIENCY
90
85
80
75
LOSS
70
V
IN
= 48V
0
10
3724 TA01b
U
U
POWER LOSS (W)
8
6
4
2
3724f
1
LT3724
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
IN
NC
SHDN
C
SS
BURST_EN
V
FB
V
C
SGND
1
2
3
4
5
6
7
8
17
16 BOOST
15 TG
14 SW
13 NC
12 V
CC
11 PGND
10 SENSE
+
9
SENSE
–
Input Supply Voltage (V
IN
)......................... 65V to –0.3V
Boosted Supply Voltage (BOOST) .............. 80V to –0.3V
Switch Voltage (SW) .................................... 65V to –1V
Differential Boost Voltage
(BOOST to SW) ..................................... 24V to –0.3V
Bias Supply Voltage (V
CC
) ......................... 24V to –0.3V
SENSE
+
and SENSE
–
Voltages ................... 40V to –0.3V
Differential Sense Voltage
(SENSE
+
to SENSE
–
) .................................. 1V to –1V
BURST_EN Voltage .................................... 24V to –0.3V
V
C
, V
FB
, C
SS
, and SHDN Voltages ................ 5V to –0.3V
C
SS
and SHDN Pin Currents .................... 1mA to –1mA
Operating Junction Temperature Range (Note 2)
LT3724E (Note 3) ..............................–40°C to 125°C
LT3724I .............................................–40°C to 125°C
Storage Temperature .............................–65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LT3724EFE
LT3724IFE
FE PART
MARKING
3724EFE
3724IFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
EXPOSED PAD IS SGND (PIN 17)
MUST BE SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
CC
= BOOST = BURST_EN = 10V, SHDN = 2V,
SENSE
–
= SENSE
+
= 10V, SGND = PGND = SW = 0V, unless otherwise noted.
SYMBOL
V
IN
PARAMETER
Operating Voltage Range (Note 4)
Minimum Start Voltage
UVLO Threshold (Falling)
UVLO Threshold Hysteresis
V
IN
Supply Current
V
IN
Burst Mode Current
V
IN
Shutdown Current
Operating Voltage Range
Operating Voltage Range (Note 5)
UVLO Threshold (Rising)
UVLO Threshold Hysteresis
BOOST Supply Current (Note 6)
BOOST Burst Mode Current
BOOST Shutdown Current
Operating Voltage Range (Note 5)
Output Voltage
UVLO Threshold (Rising)
UVLO Threshold Hysteresis
V
CC
Supply Current (Note 6)
V
CC
Burst Mode Current
V
CC
Shutdown Current
Short-Circuit Current
V
CC
> 9V
V
BURST_EN
= 0V, V
FB
= 1.35V
V
SHDN
= 0V
V
BOOST
- V
SW
V
BOOST
- V
SW
V
BOOST
- V
SW
V
BURST_EN
= 0V
V
SHDN
= 0V
Over Full Line and Load Range
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
●
●
●
MIN
4
7.5
3.65
TYP
MAX
60
UNITS
V
V
V
mV
µA
µA
µA
V
V
V
mV
mA
µA
µA
3.8
670
20
20
9
3.95
I
VIN
●
●
●
15
75
20
V
BOOST
5
400
1.4
0.1
0.1
8
6.25
500
1.7
80
20
–55
20
8.3
I
BOOST
V
CC
I
VCC
●
2.1
V
BURST_EN
= 0V
V
SHDN
= 0V
●
–30
2
U
V
V
V
mV
mA
µA
µA
mA
3724f
W
U
U
W W
W
LT3724
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 20V, V
CC
= BOOST = BURST_EN = 10V, SHDN = 2V,
SENSE
–
= SENSE
+
= 10V, SGND = PGND = SW = 0V, unless otherwise noted.
SYMBOL
V
FB
I
FB
V
SHDN
V
SENSE
I
SENSE
PARAMETER
Error Amp Reference Voltage
Feedback Input Current
Enable Threshold (Rising)
Threshold Hysteresis
Common Mode Range
Current Limit Sense Voltage
Input Current
(I
SENSE+
+ I
SENSE–
)
Operating Frequency
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
Measured at V
FB
Pin
●
MIN
1.224
1.215
1.3
0
140
TYP
1.231
25
1.35
120
150
400
2
–150
MAX
1.238
1.245
1.4
36
175
UNITS
V
V
nA
V
mV
V
mV
µA
µA
µA
V
SENSE+
– V
SENSE–
V
SENSE(CM)
= 0V
2V < V
SENSE(CM)
< 3.5V
V
SENSE(CM)
> 4V
f
SW
V
FB(SS)
I
SS
g
m
A
V
V
C
I
VC
V
TG
t
TG
t
TG(OFF)
t
TG(ON)
I
SW
190
175
200
1.185
300
2
210
220
kHz
kHz
V
mV
µA
Soft-Start Disable Voltage
Soft-Start Disable Hysteresis
Soft-Start Capacitor Control Current
Error Amp Transconductance
Error Amp DC Voltage Gain
Error Amp Output Range
Error Amp Sink/Source Current
V
FB
Rising
●
275
340
62
1.2
±30
9.8
0.1
60
350
400
µmhos
dB
V
µA
V
V
ns
ns
Zero Current to Current Limit
Gate Drive Output On Voltage (Note 7) C
LOAD
= 3300pF
Gate Drive Output Off Voltage
C
LOAD
= 3300pF
Gate Drive Rise/Fall Time
10% to 90% or 90% to 10%, C
LOAD
= 3300pF
Minimum Switch Off Time
Minimum Switch On Time
SW Pin Sink Current
V
SW
= 2V
●
300
300
500
ns
mA
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT3724 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 3:
The LT3724E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the – 40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT3724I is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 4:
V
IN
voltages below the start-up threshold (7.5V) are only
supported when the V
CC
is externally driven above 6.5V.
Note 5:
Operating range is dictated by MOSFET absolute maximum V
GS
.
Note 6:
Supply current specification does not include switch drive
currents. Actual supply currents will be higher.
Note 7:
DC measurement of gate drive output “ON” voltage is typically
8.6V. Internal dynamic bootstrap operation yields typical gate “ON”
voltages of 9.8V during standard switching operation. Standard operation
gate “ON” voltage is not tested but guaranteed by design.
3724f
3
LT3724
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Threshold (Rising)
vs Temperature
1.38
SHUTDOWON THRESHOLD, FALLING (V)
SHUTDOWON THRESHOLD, RISING (V)
1.37
1.36
1.35
1.34
1.33
1.32
–50 –25
1.23
1.22
1.21
1.20
–50 –25
V
CC
(V)
0
50
25
75
TEMPERATURE (°C)
V
CC
vs I
CC(LOAD)
8.2
T
A
= 25°C
8.1
8.0
7
V
CC
(V)
V
CC
(V)
I
CC
CURRENT LIMIT (mA)
7.9
7.8
7.7
7.6
7.5
0
5
10
15
20
25
30
35
I
CC (LOAD)
(mA)
3724 G04
V
CC
UVLO Threshold (Rising)
vs Temperature
6.5
V
CC
UVLO THRESHOLD, RISING (V)
T
A
= 25°C
ERROR AMP TRANSCONDUCTANCE (µMhos)
6.4
I
CC
(µA)
6.3
6.2
6.1
6.0
–50 –25
0
25
50
75
TEMPERATURE (°C)
4
U W
100
125
3724 G01
Shutdown Threshold (Falling)
vs Temperature
1.26
1.25
1.24
7.9
7.8
7.7
7.6
8.2
8.1
8.0
V
CC
vs Temperature
I
CC
= 20mA
0
50
25
75
TEMPERATURE (°C)
100
125
7.5
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
3724 G02
3724 G03
V
CC
vs V
IN
9
8
I
CC
= 20mA
T
A
= 25°C
70
I
CC
Current Limit vs Temperature
60
50
6
5
4
3
40
30
4
5
6
7
8
V
IN
(V)
9
10
11
12
20
–50 –25
0
25
50
75
TEMPERATURE (°C)
100
125
3724 G05
3724 G06
I
CC
vs V
CC
(SHDN = 0V)
25
350
345
340
335
330
325
Error Amp Transconductance
vs Temperature
20
15
10
5
100
125
0
0
2
4
6
8
10 12 14 16 18 20
V
CC
(V)
3724 G08
320
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3724 G07
3724 G09
3724f
LT3724
TYPICAL PERFOR A CE CHARACTERISTICS
I
(SENSE+ + SENSE–)
vs
V
SENSE (CM)
400
T
A
= 25°C
300
230
220
210
200
190
180
170
–50 –25
OPERATING FREQUENCY (kHz)
ERROR AMP REFERENCE (V)
I
(SENSE
+
+ SENSE
–
)
(µA)
200
100
0
–100
–200
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
V
SENSE (CM)
(V)
3724 G10
Maximum Current Sense
Threshold vs Temperature
160
4.54
V
IN
UVLO THRESHOLD, RISING (V)
4.52
4.50
4.48
4.46
4.44
4.42
4.40
–50 –25
156
154
152
150
148
146
144
142
140
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
50
25
75
0
TEMPERATURE (°C)
100
125
V
IN
UVLO THRESHOLD, FALLING (V)
CURRENT SENSE THRESHOLD (mV)
158
U W
3724 G13
Operating Frequency
vs Temperature
1.234
1.233
1.232
1.231
1.230
1.229
1.228
Error Amp Reference
vs Temperature
50
25
75
0
TEMPERATURE (°C)
100
125
1.227
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3724 G11
3724 G12
V
IN
UVLO Threshold (Rising)
vs Temperature
3.86
V
IN
UVLO Threshold (Falling)
vs Temperature
3.84
3.82
3.80
3.78
3.76
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3724 G14
3724 G15
3724f
5
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