LINEAR TECHNOLOGY LT3800 Technical data

LT3800

High-Voltage Synchronous

Current Mode Step-Down

FEATURES

■

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

U

APPLICATIO S

■

12V and 42V Automotive and Heavy Equipment

■

48V Telecom Power Supplies

■

Avionics and Industrial Control Systems

■

Distributed Power Converters

U

Controller

DESCRIPTIO

The LT®3800 is a 200kHz fixed frequency high voltage
synchronous current mode step-down switching regula­tor 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

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
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 program­mable 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.

, 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 5481178, 6611131, 6304066, 6498466, 6580258.

®

opera-

IN

TYPICAL APPLICATIO

12V 75W DC/DC Converter with Reverse Current Inhibit and Input UVLO

V

IN

20V TO 55V

+

82.5k

20k
1%

100pF

56µF
×2

1M

1.5nF

82.5k

680pF

174k
1%

200k

V

IN

LT3800

SHDN

C

SS

BURST_EN

V

FB

V

C

–

SENSE

SGND

BOOST

PGND

SENSE

TG

SW

V

CC

BG

+

U

1µF

BAS19

1µF

1µF
×3

1N4148

Si7850DP

Si7370DP

15µH

B160

10µF

0.015Ω

Efficiency and Power Loss

3800 TA01b

6

5

4

3

2

1

0

10

3800f

POWER LOSS (W)

100

VIN = 36V

95

90

85

EFFICIENCY (%)

80

75

70

V

OUT

12V AT 75W

+

270µF

3800 TA01a

0.1 1

VIN = 24V

VIN = 48V

LOSS (48V)

I

(A)

LOAD

VIN = 60V

1

LT3800

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

Supply Voltages

Input Supply Pin (V

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 (VCC) ............................. –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, CSS, VFB, VC) .......... –0.3V to 5.0V

Input Currents

SHDN, CSS............................................... –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

) .............................. –0.3V to 65V

IN

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

1

V

IN

2

NC

3

SHDN

4

C

SS

BURST_EN

V

FB

V

C
–

SENSE

16-LEAD PLASTIC TSSOP

T

= 125°C, θJA = 40°C/W, θJC = 10°C/W

JMAX

EXPOSED PAD (PIN 17) IS SGND

MUST BE SOLDERED TO PCB

Consult LTC Marketing for parts specified with wider operating temperature ranges.

5

6

7

8

17

FE PACKAGE

BOOST

16

TG

15

SW

14

NC

13

V

12

CC

BG

11

PGND

10

9

SENSE

+

ORDER PART

NUMBER

LT3800EFE
LT3800IFE

FE PART

MARKING

3800EFE
3800IFE

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V,
SENSE– = SENSE+ = 10V, SGND = PGND = SW = 0V, CTG = CBG = 3300pF, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

IN

I

VIN

V

BOOST

I

BOOST

V

CC

Operating Voltage Range (Note 4) ● 460V
Minimum Start Voltage
UVLO Threshold (Falling)
UVLO Hysteresis 670 mV

VIN Supply Current VCC > 9V 20 µA

Burst Mode Current V

V

IN

V

Shutdown Current V

IN

Operating Voltage ● 75 V
Operating Voltage Range (Note 5) V
UVLO Threshold (Rising) V
UVLO Hysteresis V

BOOST Supply Current (Note 6) 1.4 mA
BOOST Burst Mode Current V
BOOST Shutdown Current V

Operating Voltage (Note 5) ● 20 V
Output Voltage
UVLO Threshold (Rising) 6.25 V
UVLO Hysteresis 500 mV

BURST_EN
SHDN

BOOST
BOOST
BOOST

BURST_EN
SHDN

= 0V, VFB = 1.35V 20 µA

= 0V ● 815 µA

– V

SW

– V

SW

– V

SW

= 0V 0.1 µA

= 0V 0.1 µA

● 7.5 V

● 3.65 3.80 3.95 V

● 20 V

5V

0.4 V

● 8.0 8.3 V

2

3800f

LT3800

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

–

SENSE

= SENSE+ = 10V, SGND = PGND = SW = 0V, CTG = CBG = 3300pF, unless otherwise noted.

The ● denotes the specifications which apply over the full operating

= 25°C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V,

A

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

V

VCC

SHDN

VCC Supply Current (Note 6) ● 3 3.6 mA
V

Burst Mode Current V

CC

Shutdown Current V

V

CC

Short-Circuit Current

BURST_EN
SHDN

= 0V 80 µA

= 0V 20 µA

● –40 –120 mA

Enable Threshold (Rising) ● 1.30 1.35 1.40 V
Threshold Hysteresis 120 mV

V

SENSE

I

SENSE

f

O

V

FB

I

FB

V

FB(SS)

Common Mode Range ● 036
Current Limit Sense Voltage V
Reverse Protect Sense Voltage V
Reverse Current Offset V

Input Current V
(I

SENSE

+

+ I

–

) 2V < V

SENSE

SENSE
SENSE
BURST_EN

SENSE(CM)

V

SENSE(CM)

+
+

– V
– V

–

SENSE

–

, V

SENSE

= 0V or V

BURST_EN

BURST_EN

= V

= V

● 140 150 175 mV

CC

FB

–150 mV

10 mV

= 0V 0.8 mA

SENSE(CM)

< 3.5V –20 µA

> 4V –0.3 mA

Operating Frequency 190 200 210 kHz

● 175 220 kHz

Error Amp Reference Voltage Measured at VFB Pin 1.224 1.231 1.238 V

● 1.215 1.245 V

Feedback Input Current 25 nA

Soft-Start Disable Voltage VFB Rising 1.185 V
Soft-Start Disable Hysteresis 300 mV

I

g

A

V

I

V

CSS

VC

m

V

C

TG,BG

Soft-Start Capacitor Control Current 2 µA

Error Amp Transconductance ● 275 350 400 µmhos

Error Amp DC Voltage Gain 62 dB

Error Amp Output Range Zero Current to Current Limit 1.2 V

Error Amp Sink/Source Current ±30 µA

Gate Drive Output On Voltage (Note 7) 9.8 V
Gate Drive Output Off Voltage 0.1 V

t

TG,BG

t

TG(OFF)

t

TG(ON)

t

NOL

Gate Drive Rise/Fall Time 10% to 90% or 90% to 10% 50 ns

Minimum Off Time 450 ns

Minimum On Time ● 300 500 ns

Gate Drive Nonoverlap Time TG Fall to BG Rise 200 ns

BG Fall to TG Rise 150 ns

Note 1: Absolute Maximum ratings are those values beyond which the life
of a device may be impaired.

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: VIN voltages below the start-up threshold (7.5V) are only
supported when V

is externally driven above 6.5V.

CC

Note 5: Operating range dictated by MOSFET absolute maximum gate­source 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.

3800f

3

LT3800

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Shutdown Threshold (Rising)
vs Temperature

1.37

1.36

1.35

1.34

SHUTDOWN THRESHOLD, RISING (V)

(V)

CC

V

1.33

8.05

8.00

7.95

7.90

–50 –25

VCC vs I

TA = 25°C

0 50 12525 75

TEMPERATURE (°C)

CC(LOAD)

100

3800 G01

Shutdown Threshold (Falling)
vs Temperature

1.240

1.235

1.230

1.225

SHUTDOWN THRESHOLD, FALLING (V)

1.220
–50 –25 1000 50 12525 75

TEMPERATURE (°C)

(V)

CC

V

VCC vs V

8

7

6

5

4

ICC = 20mA

= 25°C

T

A

IN

3800 G02

VCC vs Temperature

8.1

ICC = 0mA

8.0

(V)

CC

V

7.9

7.8
–50 –25

05025

ICC = 20mA

TEMPERATURE (°C)

ICC Current Limit vs Temperature

200

175

150

125

100

CURRENT LIMIT (mA)

CC

I

75

75

100

125

3800 G03

7.85
51525

0

10

I

CC(LOAD)

20

(mA)

VCC UVLO Threshold (Rising)
vs Temperature

6.30

6.25

6.20

UVLO THRESHOLD, RISING (V)

CC

V

6.15

–50 –25 100

0 50 12525 75

TEMPERATURE (°C)

3

4

30

35

3800 G04

40

56

8

79 12

V

IN

10 11

(V)

3800 G05

50

–50 –25 1000 50 12525 75

TEMPERATURE (°C)

3800 G06

Error Amp Transconductance
vs Temperature

380

360

340

ERROR AMP TRANSCONDUCTANCE (µmho)

320

–50 –25 1000 50 12525 75

TEMPERATURE (°C)

3800 G08

3800f

3800 G07

(µA)

CC

I

ICC vs V

25

20

15

10

5

0

0

TA = 25°C

246

(SHDN = 0V)

CC

810

VCC (V)

12 14 18

16

20

3800 G12

4

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LT3800

Operating Frequency

I

(SENSE+ + SENSE–)

800

600

(µA)

400

)

–

200

+ SENSE

+

0

(SENSE

I

–200

–400

0 0.5 1.5 2.5 3.5 4.5

1.0 2.0 3.0 4.0

U

V

SENSE(CM)

vs V

SENSE(CM)

TA = 25°C

(V)

5.0

3800 G09

UU

vs Temperature

220

210

200

190

OPERATING FREQUENCY (kHz)

180

–50

–25

TEMPERATURE (°C)

PI FU CTIO S

VIN (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 incor­porate UVLO functions. If the SHDN pin is pulled below

0.7V, the IC enters a low current shutdown mode with
I

< 10µA. In low-current shutdown, the IC will sink 20µA

VIN

from the VCC pin until that local supply has collapsed.
Typical pin input bias current is <10nA and the pin is
internally clamped to 6V.

CSS (Pin 4): Soft-Start AC Coupling Capacitor Input.
Connect capacitor (CSS) in series with a 200k resistor from
pin to converter output (V
up output voltage slew rate (∆V
sponds to 2µA average current through the soft-start
coupling capacitor. The capacitor value for a desired
output startup slew rate follows the relation:

CSS = 2µA/(∆V

OUT

/∆t)

Shorting this pin to SGND disables the soft-start function.

). Controls converter start-

OUT

/∆t). Slew rate corre-

OUT

Error Amp Reference
vs Temperature

1.232

1.231

1.230

1.229

ERROR AMP REFERENCE (V)

1.228

1.227

1000 50 12525 75

3800 G10

–50 –25 1000 50 12525 75

TEMPERATURE (°C)

3800 G11

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
and reverse-current inhibit functions are enabled. When
the pin voltage is above 0.5V, Burst Mode operation is
disabled, but reverse-current inhibit operation is main­tained. DC/DC converters operating with reverse-current
inhibit operation (BURST_EN = VFB) have a 1mA minimum
load requirement. Reverse-current inhibit is disabled when
the pin voltage is above 2.5V. This pin is typically shorted
to ground to enable Burst Mode operation and reverse­current inhibit, shorted to VFB to disable Burst Mode
operation while enabling reverse-current inhibit, and con­nected to VCC pin to disable both functions. See Applica­tions Information section.

VFB (Pin 6): Error Amplifier Inverting Input. The
noninverting input of the error amplifier is connected to an
internal 1.231V reference. Desired converter output volt­age (V

) is programmed by connecting a resistive

OUT

divider from the converter output to the VFB pin. Values for
the resistor connected from V

to VFB (R2) and the

OUT

resistor connected from VFB to ground (R1) can be calcu­lated via the following relationship:

RR

21

•

V

⎛
⎜

⎝

1 231

.

OUT

⎞

1=

–

⎟
⎠

3800f

5

LT3800

U

UU

PI FU CTIO S

The VFB pin input bias current is 25nA, so use of extremely
high value feedback resistors could cause a converter
output that is slightly higher than expected. Bias current
error at the output can be estimated as:

∆V

OUT(BIAS)

(Pin 7): Error Amplifier Output. The voltage on the V

V

C

pin corresponds to the maximum (peak) switch current
per oscillator cycle. The error amplifier is typically config­ured as an integrator by connecting an RC network from
this pin to ground. This network creates the dominant pole
for the converter voltage regulation feedback loop. Spe­cific integrator characteristics can be configured to opti­mize transient response. Connecting a 100pF or greater
high frequency bypass capacitor from this pin to ground
is also recommended. When Burst Mode operation is
enabled (see Pin 5 description), an internal low impedance
clamp on the VC pin is set at 100mV below the burst
threshold, which limits the negative excursion of the pin
voltage. Therefore, this pin cannot be pulled low with a
low-impedance source. If the VC pin must be externally
manipulated, do so through a 1kΩ series resistance.

SENSE– (Pin 8): Negative Input for Current Sense Ampli­fier. Sensed inductor current limit set at ±150mV across
SENSE inputs.

SENSE+ (Pin 9): Positive Input for Current Sense Ampli­fier. Sensed inductor current limit set at ±150mV across
SENSE inputs.

= 25nA • R2

C

PGND (Pin 10): High Current Ground Reference for Syn­chronous Switch. Current path from pin to negative termi­nal of V

BG (Pin 11): Synchronous Switch Gate Drive Output.

VCC (Pin 12): Internal Regulator Output. Most IC func-

tions are powered from this pin. Driving this pin from an
external source reduces VIN pin current to 20µA. This pin
is decoupled with a low ESR 1µF capacitor to PGND.
In shutdown mode, this pin sinks 20µA until the pin
voltage is discharged to 0V. See Typical Performance
Characteristics.

NC (Pin 13): No Connection.

SW (Pin 14): Reference for V

Current Return for Bootstrapped Switch.

TG (Pin 15): Bootstrapped Switch Gate Drive Output.

BOOST (Pin 16): Bootstrapped Supply – Maximum Oper-

ating Voltage (Ground Referred) to 75V. This pin is
decoupled with a low ESR 1µF capacitor to pin SW. The
voltage on the decoupling capacitor is refreshed through
a rectifier from either VCC or an external source.

Exposed Package Backside (SGND) (Pin 17): Low Noise
Ground Reference. SGND connection is made through the
exposed lead frame on back of TSSOP package which
must be soldered to the PCB ground.

decoupling capacitor must not corrupt SGND.

CC

Supply and High

BOOST

6

3800f

LT3800

U

U

W

FU CTIO AL DIAGRA

V

IN

UVLO

(<4V)

1

V

IN

8V

REG

+

–

3

SHDN

BURST_EN

5

FEEDBACK

REFERENCE

1.231V

+

–

+

3.8V
REG

V

CC

UVLO
(<6V)

INTERNAL
SUPPLY RAIL

BST

UVLO

DRIVE

CONTROL

NOL

SWITCH

DRIVE
LOGIC

CONTROL

DRIVE

CONTROL

SYNCHRONOUS

SWITCH DRIVER

BOOSTED

SWITCH

DRIVER

BOOST

16

TG

15

SW

14

V

12

CC

BG

11

PGND

10

V

FB

V

C

SS

GND

–

SQ

+

OSCILLATOR

SLOPE COMP

GENERATOR

REVERSE

CURRENT

INHIBIT

160mV

–

10mV

Q

R

S

+

+

SENSE

9

–

SENSE

8

+

6

–+

g

SOFT-START

ENABLE

+

m

0.5V

1V

ERROR

AMP

7

C

DISABLE/BURST

–

R

CURRENT

–

+

SENSE

COMPARATOR

–

–

Burst Mode

1.185V
2µA

4

OPERATION

–

+

17

3800 FD

3800f

7

LT3800

U

WUU

APPLICATIO S I FOR ATIO

Overview

The LT3800 is a high input voltage range step-down
synchronous DC/DC converter controller IC that uses a
200kHz constant frequency, current mode architecture
with external N-channel MOSFET switches.

The LT3800 has provisions for high efficiency, low load
operation for battery-powered applications. Burst Mode
operation reduces total average input quiescent currents
to 100µA during no load conditions. A low current shutdown
mode can also be activated, reducing quiescent current to
<10µA. Burst Mode operation can be disabled if desired.

The LT3800 also employs a reverse-current inhibit fea­ture, allowing increased efficiencies during light loads
through nonsynchronous operation. This feature disables
the synchronous switch if inductor current approaches
zero. If full time synchronous operation is desired, this
feature can be disabled.

Much of the LT3800’s internal circuitry is biased from an
internal linear regulator. The output of this regulator is the
VCC pin, allowing bypassing of the internal regulator. The
associated internal circuitry can be powered from the
output of the converter, increasing overall converter effi­ciency. Using externally derived power also eliminates the
IC’s power dissipation associated with the internal VIN to
VCC regulator.

threshold is not obtained for the entire oscillator cycle, the
switch driver is disabled at the end of the cycle for 450ns.
This minimum off-time mode of operation assures regen­eration of the BOOST bootstrapped supply.

Power Requirements

The LT3800 is biased using a local linear regulator to
generate internal operational voltages from the VIN pin.
Virtually all of the circuitry in the LT3800 is biased via an
internal linear regulator output (V
with a low ESR 1µF capacitor to PGND.

The V
is ample voltage on the VIN pin. The VCC regulator has
approximately 1V of dropout, and will follow the VIN pin
with voltages below the dropout threshold.

The LT3800 has a start-up requirement of VIN > 7.5V. This
assures that the onboard regulator has ample headroom
to bring the VCC pin above its UVLO threshold. The V
regulator can only source current, so forcing the VCC pin
above its 8V regulated voltage allows use of externally
derived power for the IC, minimizing power dissipation in
the IC. Using the onboard regulator for start-up, then
deriving power for VCC from the converter output maxi­mizes conversion efficiencies and is common practice. If
VCC is maintained above 6.5V using an external source, the
LT3800 can continue to operate with VIN as low as 4V.

regulator generates an 8V output provided there

CC

). This pin is decoupled

CC

CC

Theory of Operation (See Block Diagram)

The LT3800 senses converter output voltage via the V
pin. The difference between the voltage on this pin and an
internal 1.231V reference is amplified to generate an error
voltage on the VC pin which is, in turn, used as a threshold
for the current sense comparator.

During normal operation, the LT3800 internal oscillator
runs at 200kHz. At the beginning of each oscillator cycle,
the switch drive is enabled. The switch drive stays enabled
until the sensed switch current exceeds the VC derived
threshold for the current sense comparator and, in turn,
disables the switch driver. If the current comparator

FB

8

The LT3800 operates with 3mA quiescent current from the
VCC supply. This current is a fraction of the actual V
quiescent currents during normal operation. Additional
current is produced from the MOSFET switching currents
for both the boosted and synchronous switches and are
typically derived from the V

Because the LT3800 uses a linear regulator to generate
VCC, power dissipation can become a concern with high
VIN voltages. Gate drive currents are typically in the range
of 5mA to 15mA per MOSFET, so gate drive currents can
create substantial power dissipation. It is advisable to
derive VCC and V
whenever possible.

BOOST

supply.

CC

power from an external source

CC

3800f