LT3800
High-Voltage Synchronous
Current Mode Step-Down
Controller
DESCRIPTIO
The LT
®
3800 is a 200kHz fixed frequency high voltage
synchronous current mode step-down switching regula-
tor controller. The IC drives standard gate N-channel power
MOSFETs and can operate with input voltages from 4V to
60V. An onboard regulator provides IC power directly from
V
IN
and provides for output-derived power to minimize V
IN
quiescent current. MOSFET drivers employ an internal
dynamic bootstrap feature, maximizing gate-source “ON”
voltages during normal operation for improved operating
efficiencies. The LT3800 incorporates Burst Mode
®
opera-
tion, which reduces no load quiescent current to under
100µA. Light load efficiencies are also improved through
a reverse inductor current inhibit, allowing the controller
to support discontinuous operation. Both Burst Mode
operation and the reverse-current inhibit features can be
disabled if desired. The LT3800 incorporates a program-
mable soft-start that directly controls the voltage slew rate
of the converter output for reduced startup surge currents
and overshoot errors. The LT3800 is available in a 16-lead
thermally enhanced TSSOP package.
, LT, LTC and LTM 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 5481178, 6611131, 6304066, 6498466, 6580258.
FEATURES
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■
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■
■
■
■
■
■
■
■
■
■
Wide 4V to 60V Input Voltage Range
Output Voltages up to 36V
Adaptive Nonoverlap Circuitry Prevents Switch
Shoot-Through
Reverse Inductor Current Inhibit for Discontinuous
Operation Improves Efficiency with Light Loads
Output Slew Rate Controlled Soft-Start with
Auto-Reset
100µA No Load Quiescent Current
Low 10µA Current Shutdown
1% Regulation Accuracy
200kHz Operating Frequency
Standard Gate N-Channel Power MOSFETs
Current Limit Unaffected by Duty Cycle
Reverse Overcurrent Protection
16-Lead Thermally Enhanced TSSOP Package
APPLICATIO S
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■
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12V and 42V Automotive and Heavy Equipment
48V Telecom Power Supplies
Avionics and Industrial Control Systems
Distributed Power Converters
TYPICAL APPLICATIO
V
IN
20V TO 55V
12V 75W DC/DC Converter with Reverse Current Inhibit and Input UVLO
+
56µF
×2
1M
82.5k
1.5nF
SHDN
200k
C
SS
20k
1%
174k
1%
BURST_EN
V
FB
V
C
100pF
82.5k
680pF
V
CC
1µF
BG
PGND
Si7370DP
B160
1N4148
LT3800
SW
BAS19
15µH
1µF
×3
1µF
TG
Si7850DP
95
90
85
80
LOSS (48V)
75
70
0.1
1
0
V
IN
= 36V
V
IN
= 60V
V
IN
= 48V
100
V
IN
= 24V
5
V
IN
BOOST
EFFICIENCY (%)
SENSE
–
SENSE
+
SGND
0.015Ω
+
10µF
V
OUT
12V AT 75W
270µF
3800 TA01a
U
U
U
Efficiency and Power Loss
6
POWER LOSS (W)
4
3
2
1
I
LOAD
(A)
10
3800 TA01b
3800fb
1
LT3800
ABSOLUTE
AXI U RATI GS
(Note 1)
PI CO FIGURATIO
TOP VIEW
V
IN
NC
SHDN
C
SS
BURST_EN
V
FB
V
C
SENSE
–
Supply Voltages
Input Supply Pin (V
IN
) .............................. –0.3V to 65V
Boosted Supply Pin (BOOST) ................... –0.3V to 80V
Boosted Supply Voltage (BOOST – SW) .. –0.3V to 24V
Boosted Supply Reference Pin (SW) ........... –2V to 65V
Local Supply Pin (V
CC
) ............................. –0.3V to 24V
Input Voltages
SENSE
+
, SENSE
–
...................................... –0.3V to 40V
SENSE
+
– SENSE
–
......................................... –1V to 1V
BURST_EN Pin ......................................... –0.3V to 24V
Other Inputs (SHDN, C
SS
, V
FB
, V
C
) .......... –0.3V to 5.0V
Input Currents
SHDN, C
SS
............................................... –1mA to 1mA
Maximum Temperatures
Operating Junction Temperature Range (Note 2)
LT3800E (Note 3) ............................. –40°C to 125°C
LT3800I ............................................ –40°C to 125°C
Storage Temperature Range ................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
1
2
3
4
5
6
7
8
17
16 BOOST
15 TG
14 SW
13 NC
12 V
CC
11 BG
10 PGND
9
SENSE
+
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
EXPOSED PAD (PIN 17) IS SGND
MUST BE SOLDERED TO PCB
ORDER I FOR ATIO
LEAD FREE FINISH
LT3800EFE#PBF
LT3800IFE#PBF
TAPE AND REEL
LT3800EFE#TRPBF
LT3800IFE#TRPBF
PART MARKING
3800EFE
3800IFE
PACKAGE DESCRIPTION
16-Lead Plastic TSSOP
16-Lead Plastic TSSOP
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard 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/
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, CTG = CBG = 3300pF, unless otherwise noted.
SYMBOL
V
IN
PARAMETER
Operating Voltage Range (Note 4)
Minimum Start Voltage
UVLO Threshold (Falling)
UVLO Hysteresis
V
IN
Supply Current
V
IN
Burst Mode Current
V
IN
Shutdown Current
Operating Voltage
Operating Voltage Range (Note 5)
UVLO Threshold (Rising)
UVLO Hysteresis
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
CONDITIONS
●
●
●
ELECTRICAL CHARACTERISTICS
MIN
4
7.5
3.65
I
VIN
●
●
●
V
BOOST
2
U
TEMPERATURE RANGE
–40°C to 125°C
–40°C to 125°C
TYP
MAX
60
3.80
670
20
20
8
3.95
UNITS
V
V
V
mV
µA
µA
µA
V
V
V
V
3800fb
U
U
U
U
W W
W
W
U
15
75
20
5
0.4
LT3800
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, CTG = CBG = 3300pF, unless otherwise noted.
SYMBOL
I
BOOST
PARAMETER
BOOST Supply Current (Note 6)
BOOST Burst Mode Current
BOOST Shutdown Current
Operating Voltage (Note 5)
Output Voltage
UVLO Threshold (Rising)
UVLO Hysteresis
V
CC
Supply Current (Note 6)
V
CC
Burst Mode Current
V
CC
Shutdown Current
Short-Circuit Current
Enable Threshold (Rising)
Threshold Hysteresis
Common Mode Range
Current Limit Sense Voltage
Reverse Protect Sense Voltage
Reverse Current Offset
Input Current
(I
SENSE+
+ I
SENSE–
)
Operating Frequency
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
V
BURST_EN
= 0V
V
SHDN
= 0V
●
●
MIN
TYP
1.4
0.1
0.1
8.0
6.25
500
3
80
20
–120
1.35
120
150
–150
10
0.8
–20
–0.3
MAX
UNITS
mA
µA
µA
V
CC
20
8.3
V
V
V
mV
mA
µA
µA
mA
V
mV
mV
mV
mV
mA
µA
mA
I
VCC
●
3.6
V
BURST_EN
= 0V
V
SHDN
= 0V
●
●
●
●
–40
1.30
0
140
V
SHDN
V
SENSE
1.40
36
175
V
SENSE+
– V
SENSE–
V
SENSE+
– V
SENSE–
, V
BURST_EN
= V
CC
V
BURST_EN
= 0V or V
BURST_EN
= V
FB
V
SENSE(CM)
= 0V
V
SENSE(CM)
= 2.75V
V
SENSE(CM)
> 4V
I
SENSE
f
O
V
FB
I
FB
V
FB(SS)
I
CSS
g
m
A
V
V
C
I
VC
V
TG,BG
t
TG,BG
t
TG(OFF)
t
TG(ON)
t
NOL
190
175
1.224
1.215
200
1.231
25
210
220
1.238
1.245
kHz
kHz
V
V
nA
V
mV
µA
Error Amp Reference Voltage
Feedback Input Current
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
Gate Drive Output On Voltage (Note 7)
Gate Drive Output Off Voltage
Gate Drive Rise/Fall Time
Minimum Off Time
Minimum On Time
Gate Drive Nonoverlap Time
Measured at V
FB
Pin
●
V
FB
Rising
1.185
300
2
●
275
350
62
1.2
±30
9.8
0.1
400
µmhos
dB
V
µA
V
V
ns
ns
Zero Current to Current Limit
10% to 90% or 90% to 10%
●
50
450
300
200
150
500
ns
ns
ns
TG Fall to BG Rise
BG Fall to TG Rise
3800fb
3
LT3800
ELECTRICAL CHARACTERISTICS
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:
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 3:
The LT3800E 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 LT3800I 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 V
CC
is externally driven above 6.5V.
Note 5:
Operating range dictated by MOSFET absolute maximum gate-
source voltage ratings.
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.
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Threshold (Rising)
vs Temperature
1.37
SHUTDOWN THRESHOLD, FALLING (V)
SHUTDOWN THRESHOLD, RISING (V)
1.36
1.35
1.230
V
CC
(V)
1.34
1.33
–50
–25
0
25
50
75
TEMPERATURE (°C)
V
CC
vs I
CC(LOAD)
8.05
8
T
A
= 25°C
7
8.00
6
I
CC
CURRENT LIMIT (mA)
V
CC
(V)
V
CC
(V)
7.95
7.90
7.85
0
5
10
15 20 25
I
CC(LOAD)
(mA)
30
4
U W
100
3800 G01
Shutdown Threshold (Falling)
vs Temperature
1.240
V
CC
vs Temperature
8.1
1.235
I
CC
= 0mA
8.0
I
CC
= 20mA
7.9
1.225
125
1.220
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
7.8
–50
–25
0
50
75
25
TEMPERATURE (°C)
100
125
3800 G02
3800 G03
V
CC
vs V
IN
I
CC
= 20mA
T
A
= 25°C
I
CC
Current Limit vs Temperature
200
175
150
125
100
75
50
–50
5
4
3
35
40
4
5
6
7
8
V
IN
(V)
3800 G04
3800 G05
9
10
11
12
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3800 G06
3800fb
LT3800
TYPICAL PERFOR A CE CHARACTERISTICS
V
CC
UVLO Threshold (Rising)
vs Temperature
6.30
25
20
6.25
ERROR AMP TRANSCONDUCTANCE (µmho)
V
CC
UVLO THRESHOLD, RISING (V)
I
CC
(µA)
6.20
5
6.15
–50
–25
0
25
50
75
TEMPERATURE (°C)
I
(SENSE + SENSE–)
vs V
SENSE(CM)
800
600
I
(SENSE+ + SENSE
–
)
(µA)
+
OPERATING FREQUENCY (kHz)
210
400
200
0
–200
–400
ERROR AMP REFERENCE (V)
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)
3800 G09
PI FU CTIO S
V
IN
(Pin 1):
Converter Input Supply.
NC (Pin 2):
No Connection.
SHDN (Pin 3):
Precision Shutdown Pin. Enable threshold
is 1.35V (rising) with 120mV of input hysteresis. When in
shutdown mode, all internal IC functions are disabled. The
precision threshold allows use of the SHDN pin to incor-
porate UVLO functions. If the SHDN pin is pulled below
0.7V, the IC enters a low current shutdown mode with
I
VIN
< 10µA. In low-current shutdown, the IC will sink 20µA
from the V
CC
pin until that local supply has collapsed.
Typical pin input bias current is <10nA and the pin is
internally clamped to 6V.
C
SS
(Pin 4):
Soft-Start AC Coupling Capacitor Input.
Connect capacitor (C
SS
) in series with a 200k resistor from
pin to converter output (V
OUT
). Controls converter start-
up output voltage slew rate (∆V
OUT
/∆t). Slew rate corre-
sponds to 2µA average current through the soft-start
coupling capacitor. The capacitor value for a desired
output startup slew rate follows the relation:
C
SS
= 2µA/(∆V
OUT
/∆t)
Shorting this pin to SGND disables the soft-start function
BURST_EN (Pin 5):
Burst Mode Operation Enable Pin.
This pin also controls reverse-inhibit mode of operation.
When the pin voltage is below 0.5V, Burst Mode operation
3800fb
U W
100
3800 G07
I
CC
vs V
CC
(SHDN = 0V)
T
A
= 25°C
Error Amp Transconductance
vs Temperature
380
360
15
10
340
125
0
0
2
4
6
8
10 12 14 16 18 20
V
CC
(V)
3800 G12
320
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3800 G08
Operating Frequency
vs Temperature
220
Error Amp Reference
vs Temperature
1.232
T
A
= 25°C
1.231
1.230
200
1.229
190
1.228
180
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
1.227
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
3800 G10
3800 G11
U
U
U
5
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