LINEAR TECHNOLOGY LTC3703 Technical data

FEATURES

LOAD (A)

0

EFFICIENCY (%)

100

95

90

85

80

4

3703 F01b

1

2

3

5

VIN = 25V

VIN = 50V

VIN = 75V

■

High Voltage Operation: Up to 100V

■

Large 1Ω Gate Drivers

■

No Current Sense Resistor Required

■

Step-Up or Step-Down DC/DC Converter

■

Dual N-Channel MOSFET Synchronous Drive

■

Excellent Line and Load Transient Response

■

Programmable Constant Frequency: 100kHz to
600kHz

■

±1% Reference Accuracy

■

Synchronizable up to 600kHz

■

Selectable Pulse Skip Mode Operation

■

Low Shutdown Current: 50µA Typ

■

Programmable Current Limit

■

Undervoltage Lockout

■

Programmable Soft-Start

■

16-Pin Narrow SSOP and 28-Pin SSOP Packages

U

APPLICATIO S

■

48V Telecom and Base Station Power Supplies

■

Networking Equipment, Servers

■

Automotive and Industrial Control

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Protected by U.S. Patents including 5408150, 5055767, 6677210, 5847554, 5481178,
6304066, 6580258.

LTC3703

100V Synchronous

Switching Regulator

Controller

U

DESCRIPTIO

®

The LTC
regulator controller that can directly step-down voltages
from up to 100V, making it ideal for telecom and automo­tive applications. The LTC3703 drives external N-channel
MOSFETs using a constant frequency (up to 600kHz),
voltage mode architecture.

A precise internal reference provides 1% DC accuracy. A
high bandwidth error amplifier and patented line feed
forward compensation provide very fast line and load
transient response. Strong 1Ω gate drivers allow the
LTC3703 to drive multiple MOSFETs for higher current
applications. The operating frequency is user program­mable from 100kHz to 600kHz and can also be synchro­nized to an external clock for noise-sensitive applications.
Current limit is programmable with an external resistor
and utilizes the voltage drop across the synchronous
MOSFET to eliminate the need for a current sense resistor.
For applications requiring up to 60V operation with logic­level MOSFETS, refer to the LTC3703-5 data sheet.

PARAMETER LTC3703-5 LTC3703

Maximum V
MOSFET Gate Drive 4.5V to 15V 9.3V to 15V
VCC UV
VCC UV

3703 is a synchronous step-down switching

IN

+

–

60V 100V

3.7V 8.7V

3.1V 6.2V

TYPICAL APPLICATIO

30k

10k

1000pF

100Ω

2200pF

470pF

15k

1%

0.1µF

113k
1%

8.06k

Figure 1. High Efficiency High Voltage Step-Down Converter

MODE/SYNC

FSET

LTC3703

COMP

FB

I

MAX

INV

RUN/SS

GND

BOOST

V

DRV

BGRTN

U

9.3V TO 15V

V

IN

TG

SW

CC

CC

BG

V

CC

+

10Ω

10µF

22µF
25V

1µF

BAS19

0.1µF

V

IN

15V TO 100V

Si7456DP

Si7456DP

MBR1100

+

8µH

68µF

270µF

16V

Efficiency vs Load Current

V

OUT

12V
5A

+

3703 F01

3703fa

1

LTC3703

1

2

3

4

5

6

7

8

9

10

11

12

13

14

TOP VIEW

G PACKAGE

28-LEAD PLASTIC SSOP

28

27

26

25

24

23

22

21

20

19

18

17

16

15

BOOST

TG

SW

NC

NC

NC

NC

V

CC

DRV

CC

BG

NC

NC

NC

BGRTN

V

IN

NC

NC

NC

NC

MODE/SYNC

f

SET

COMP

FB

I

MAX

INV

NC

RUN/SS

GND

WW

W

ABSOLUTE AXI U RATI GS

U

(Note 1)

Supply Voltages

, DRVCC.......................................... –0.3V to 15V

V

CC

(DRV

– BGRTN), (BOOST – SW) ...... –0.3V to 15V

CC

BOOST................................................ –0.3V to 115V

BGRTN ...................................................... –5V to 0V

Voltage ............................................. –0.3V to 100V

V

IN

SW Voltage ................................................ –1V to 100V

Run/SS Voltage .......................................... –0.3V to 5V

MODE/SYNC, INV Voltages....................... –0.3V to 15V

f

SET

, FB, I

Voltages ............................... – 0.3V to 3V

MAX

UUW

PACKAGE/ORDER I FOR ATIO

ORDER PART

TOP VIEW

MODE/SYNC

1

2

f

SET

3

COMP

4

FB

5

I

MAX

6

INV

7

RUN/SS

8

GND

GN PACKAGE

16-LEAD NARROW PLASTIC SSOP

T

= 150°C, θJA = 110°C/W

JMAX

V

16

IN

15

B00ST

14

TG

13

SW

12

V

CC

11

DRV

CC

10

BG

9

BGRTN

NUMBER

LTC3703EGN
LTC3703IGN
LTC3703HGN

GN PART

MARKING

3703
3703I
3703H

Peak Output Current <10µs BG,TG ............................ 5A

Operating Temperature Range (Note 2)

LTC3703E ............................................–40°C to 85°C

LTC3703I........................................... –40°C to 125°C

LTC3703H (Note 9) ...........................– 40°C to 150°C

Junction Temperature (Notes 3, 7)

LTC3703E, LTC3703I ....................................... 125°C

LTC3703H (Note 9) .......................................... 150°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec.)................. 300°C

ORDER PART

NUMBER

LTC3703EG

T

= 125°C, θJA = 100°C/W

JMAX

Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/

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

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T
BGRTN = 0V, RUN/SS = I

= open, R

MAX

= 25k, unless otherwise specified.

SET

The ● denotes the specifications which apply over the full operating

= 25°C. VCC = DRVCC = V

A

= VIN = 10V, V

BOOST

MODE/SYNC

= V

INV

= VSW =

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

VCC, DRV

V

IN

I

CC

CC

VCC, DRVCC Supply Voltage

V

Pin Voltage

IN

VCC Supply Current VFB = 0V

●

9.3 15 V

●

●

1.7 2.5 mA

RUN/SS = 0V 50 µA

I

DRVCC

I

BOOST

Main Control Loop

V

FB

2

DRVCC Supply Current (Note 5) 0 5 µA

BOOST Supply Current (Note 5), TJ ≤ 125°C

Feedback Voltage (Note 4) 0.792 0.800 0.808 V

RUN/SS = 0V 0 5 µA

> 125°C

T

J

●

●

360 500 µA
360 800 µA

RUN/SS = 0V 0 5 µA

●

0.788 0.812 V

100 V

3703fa

LTC3703

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T
BGRTN = 0V, RUN/SS = I

= open, R

MAX

= 25k, unless otherwise specified.

SET

The ● denotes the specifications which apply over the full operating

= 25°C. VCC = DRVCC = V

A

= VIN = 10V, V

BOOST

MODE/SYNC

= V

= VSW =

INV

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

∆V

FB, LINE

∆V

FB, LOAD

V

MODE/SYNC

∆V

MODE/SYNC

I

MODE/SYNC

V

INV

I

INV

I

VIN

I

MAX

V

OS, IMAX

V

RUN/SS

I

RUN/SS

V

UV

Feedback Voltage Line Regulation 9V < VCC < 15V (Note 4)

Feedback Voltage Load Regulation 1V < V

< 2V (Note 4)

COMP

●

●

0.007 0.05 %/V

0.01 0.1 %

MODE/SYNC Threshold MODE/SYNC Rising 0.75 0.8 0.87 V

MODE/SYNC Hysteresis 20 mV
MODE/SYNC Current 0 ≤ V

MODE/SYNC

≤ 15V 0 1 µA

Invert Threshold 1 1.5 2 V
Invert Current 0 ≤ V

≤ 15V 0 1 µA

INV

VIN Sense Input Current VIN = 100V 100 140 µA

I

Source Current V

MAX

V

Offset Voltage |VSW| – V

IMAX

RUN/SS = 0V, V

= 0V 10.5 12 13.5 µA

IMAX

T

IMAX

> 125°C –25 10 65 mV

J

= 10V 0 1 µA

IN

at I

= 0µA, TJ ≤ 125°C –25 10 55 mV

RUN/SS

Shutdown Threshold 0.7 0.9 1.2 V

RUN/SS Source Current RUN/SS = 0V 2.5 4 5.5 µA

Maximum RUN/SS Sink Current |VSW| – V

Undervoltage Lockout VCC Rising

V

Falling

CC

> 100mV 9 17 25 µA

IMAX

●

8.0 8.7 9.3 V

●

5.7 6.2 6.8 V

Oscillator

f

OSC

f

SYNC

t

ON, MIN

DC

MAX

Oscillator Frequency R

= 25kΩ 270 300 330 kHz

SET

External Sync Frequency Range 100 600 kHz

Minimum On-Time 200 ns

Maximum Duty Cycle f < 200kHz 89 93 96 %

Driver

I

BG, PEAK

R

BG, SINK

I

TG, PEAK

R

TG, SINK

BG Driver Peak Source Current 1.5 2 A

BG Driver Pull-Down R

DS, ON

(Note 8) 1 1.5 Ω

TG Driver Peak Source Current 1.5 2 A

TG Driver Pull-Down R

DS, ON

(Note 8) 1 1.5 Ω

Feedback Amplifier

A

VOL

f

U

I

FB

I

COMP

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: The LTC3703E 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. The LTC3703I is guaranteed over the full –40°C to 125°C
operating junction temperature range. The LTC3703H is guaranteed over the
full –40°C to 150°C operating junction temperature range.

Note 3: T

is calculated from the ambient temperature TA and power

J

dissipation PD according to the following formula:

LTC3703: T

Op Amp DC Open Loop Gain (Note 4) 74 85 dB

Op Amp Unity Gain Crossover Frequency (Note 6) 25 MHz
FB Input Current 0 ≤ VFB ≤ 3V 0 1 µA

COMP Sink/Source Current ±5 ±10 mA

Note 4: The LTC3703 is tested in a feedback loop that servos V
reference voltage with the COMP pin forced to a voltage between 1V and 2V.

Note 5: The dynamic input supply current is higher due to the power MOSFET
gate charge being delivered at the switching frequency (Q

• f

G

OSC

Note 6: Guaranteed by design. Not subject to test.
Note 7: 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.

guaranteed by correlation to wafer level measurement.

DS(ON)

= TA + (PD • 100 °C/W) G Package

J

Note 8: R
Note 9: High junction temperatures degrade operating lifetimes. Operating

FB

).

to the

lifetime at junction temperatures greater than 125°C is derated to 1000 hours.

3703fa

3

LTC3703

UW

TYPICAL PERFOR A CE CHARACTERISTICS

TA = 25°C (unless otherwise noted).

Efficiency vs Input Voltage

100

95

90

85

EFFICIENCY (%)

80

V

= 12V

75

OUT

f = 300kHz
PULSE SKIP DISABLED

70

0

10 30

V

Current vs V

CC

3.5

3.0

2.5

2.0

1.5

CURRENT (mA)

CC

V

1.0

0.5

0

6

20

810 1416

40

INPUT VOLTAGE (V)

CC

COMP = 1.5V

VCC VOLTAGE (V)

I

= 5A

OUT

50

Voltage

12

I

= 0.5A

OUT

60

VFB = 0V

70

3703 G01

3703 G04

Efficiency vs Load Current

100

95

90

85

EFFICIENCY (%)

80

75

80

70

0 0.5 1.5 2.5 3.5 4.5

Current vs Temperature

V

CC

4

3

2

CURRENT (mA)

CC

V

1

0

–50 –25 0 25

VIN = 15V

VIN = 45V

VIN = 75V

V

= 5V

OUT

f = 250kHz
PULSE SKIP ENABLED

1.0 2.0 3.0 4.0
LOAD CURRENT (A)

COMP = 1.5V

VFB = 0V

50 75 125

TEMPERATURE (°C)

100

5.0

3703 G02

150

3703 G05

Load Transient Response

V

OUT

50mV/DIV

I

OUT

2A/DIV

= 50V

V

IN

= 12V

V

OUT

1A TO 5A LOAD STEP

V

Shut-Down Current vs V

CC

Voltage

100

90

80

70

60

50

40

CURRENT (µA)

CC

V

30

20

10

0

8

6

50µs/DIV

10

V

VOLTAGE (V)

CC

3703 G03

CC

12

14

16

3703 G06

V
Temperature

70

65

60

55

50

CURRENT (µA)

45

CC

V

40

35

30

–50 –25 0 25

4

Shut-Down Current vs

CC

50 75 125

TEMPERATURE (°C)

100

3703 G07

0.803

0.802

0.801

0.800

REFERENCE VOLTAGE (V)

0.799

150

0.798

Reference Voltage vs
Temperature

–50 –25 0 25

50 75 125

TEMPERATURE (°C)

100

3703 G08

150

Normalized Frequency vs
Temperature

1.20

1.15

1.10

1.05

1.00

0.95

0.90

NORMALIZED FREQUENCY

0.85

0.80
–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

3703 G09

3703fa

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LTC3703

Driver Peak Source Current vs
Temperature

3.0
VCC = 10V

2.8

2.6

2.4

2.2

2.0

1.8

1.6

PEAK SOURCE CURRENT (A)

1.4

1.2

1.0

–50 –25 0 25

Driver Pull-Down R

50 75 125

TEMPERATURE (°C)

DS(ON)

Supply Voltage

1.1

1.0

0.9

(Ω)

DS(ON)

0.8

R

0.7

0.6

79

8

6

DRVCC/BOOST VOLTAGE (V)

11 15

10

Driver Pull-Down R

DS(ON)

vs

Temperature

1.6
VCC = 10V

1.4

1.2

1.0

(Ω)

0.8

DS(ON)

R

0.6

0.4

0.2

100

vs

150

3703 G10

0

–50 –25 0 25

50 75 125

TEMPERATURE (°C)

Rise/Fall Time vs Gate

100

150

3703 G11

Capacitance

70

DRVCC, BOOST = 10V

60

50

40

30

RISE/FALL TIME (ns)

20

10

0

0

13

14

12

3703 G13

2000 4000 8000 10000

GATE CAPACITANCE (pF)

RISE

FALL

6000

3703 G14

Driver Peak Source Current vs
Supply Voltage

3.0

2.5

2.0

1.5

1.0

PEAK SOURCE CURRENT (A)

0.5

0

56 8 10 12 14

7 9 11 13

DRVCC/BOOST VOLTAGE (V)

RUN/SS Pull-Up Current vs
Temperature

8

7

6

5

4

3

RUN/SS CURRENT (µA)

2

1

0

–50 –25 0 25

50 75 125

TEMPERATURE (°C)

100

15

3703 G12

150

1573 G06

RUN/SS Pull-Up Current vs
V

Voltage

CC

6

5

4

3

2

RUN/SS PULLUP CURRENT (µA)

1

0

6

8101214

VCC VOLTAGE (V)

3703 G16

RUN/SS Sink Current vs
SW Voltage

25

I

= 0.3V

MAX

20

15

10

5

0

RUN/SS SINK CURRENT (µA)

–5

16

–10

0

0.1 0.2

0.4 0.6 0.7

0.3 0.5

|SW| VOLTAGE (V)

3703 G17

Max % DC vs RUN/SS Voltage

100

90

80

70

60

50

40

30

MAX DUTY CYCLE (%)

20

10

0

–10

0.5

1.0
RUN VOLTAGE (V)

1.5

2.0

2.5

3.0

3703 G18

3703fa

5

LTC3703

UW

TYPICAL PERFOR A CE CHARACTERISTICS

I

Current vs Temperature

MAX

13

% Duty Cycle vs COMP Voltage

100

Max % DC vs Frequency and
Temperature

100

12

SOURCE CURRENT (µA)

MAX

I

11

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

Shutdown Threshold vs
Temperature

1.4

1.2

1.0

0.8

0.6

0.4

SHUTDOWN THRESHOLD (V)

0.2

0

–50 –25 0 25 50 75 100 125 150 –50 –25 0 25 50 75 100 125 150

3703 G19

TEMPERATURE (°C)

VIN = 10V

80

60

VIN = 75V

40

DUTY CYCLE (%)

20

0

0.5

3703 G22 3703 G23

1.00 1.25 1.50

0.75

VIN = 25V

COMP (V)

VIN = 50V

(ns)

ON(MIN)

t

1.75 2.00

t

ON(MIN)

200

180

160

140

120

100

80

60

40

3703 G20

95

90

85

80

MAX DUTY CYCLE (%)

75

70

0 200 400 500100 300 600 700

vs Temperature

TEMPERATURE (°C)

–45°C

25°C

90°C

150°C

125°C

FREQUENCY (kHz)

3703 G21

6

3703fa

LTC3703

U

PI FU CTIO S

UU

(GN16/G28)

MODE/SYNC (Pin 1/Pin 6): Pulse Skip Mode Enable/Sync

Pin. This multifunction pin provides Pulse Skip Mode
enable/disable control and an external clock input for
synchronization of the internal oscillator. Pulling this pin
below 0.8V or to an external logic-level synchronization
signal disables Pulse Skip Mode operation and forces
continuous operation. Pulling the pin above 0.8V enables
Pulse Skip Mode operation. This pin can also be connected
to a feedback resistor divider from a secondary winding on
the inductor to regulate a second output voltage.

f

(Pin 2/Pin 7): Frequency Set. A resistor connected to

SET

this pin sets the free running frequency of the internal
oscillator. See applications section for resistor value se­lection details.

COMP (Pin 3/Pin 8): Loop Compensation. This pin is
connected directly to the output of the internal error ampli­fier. An RC network is used at the COMP pin to compensate
the feedback loop for optimal transient response.

FB (Pin 4/Pin 9): Feedback Input. Connect FB through a
resistor divider network to V

to set the output voltage.

OUT

Also connect the loop compensation network from COMP
to FB.

I

(Pin 5/Pin 10): Current Limit Set. The I

MAX

MAX

pin sets
the current limit comparator threshold. If the voltage drop
across the bottom MOSFET exceeds the magnitude of the
voltage at I
I

pin has an internal 12µA current source, allowing the

MAX

, the controller goes into current limit. The

MAX

current threshold to be set with a single external resistor
to ground. See the Current Limit Programming section for
more information on choosing R

IMAX

.

INV (Pin 6/Pin 11): Top/Bottom Gate Invert. Pulling this
pin above 2V sets the controller to operate in step-up
(boost) mode with the TG output driving the synchronous
MOSFET and the BG output driving the main switch. Below
1V, the controller will operate in step-down (buck) mode.

RUN/SS (Pin 7/Pin 13): Run/Soft-Start. Pulling RUN/SS
below 0.9V will shut down the LTC3703, turn off both of
the external MOSFET switches and reduce the quiescent
supply current to 50µA. A capacitor from RUN/SS to
ground will control the turn-on time and rate of rise of the
output voltage at power-up. An internal 4µA current source
pull-up at the RUN/SS pin sets the turn-on time at approxi­mately 750ms/µF.

GND (Pin 8/Pin 14): Ground Pin.

BGRTN (Pin 9/Pin 15): Bottom Gate Return. This pin

connects to the source of the pull-down MOSFET in the BG
driver and is normally connected to ground. Connecting a
negative supply to this pin allows the synchronous
MOSFET’s gate to be pulled below ground to help prevent
false turn-on during high dV/dt transitions on the SW
node. See the Applications Information section for more
details.

BG (Pin 10/Pin 19): Bottom Gate Drive. The BG pin drives
the gate of the bottom N-channel synchronous switch
MOSFET. This pin swings from BGRTN to DRV

DRV

(Pin 11/Pin 20): Driver Power Supply Pin. DRV

CC

CC

.

CC

provides power to the BG output driver. This pin should be
connected to a voltage high enough to fully turn on the
external MOSFETs, normally 10V to 15V for standard
threshold MOSFETs. DRV

should be bypassed to BGRTN

CC

with a 10µF, low ESR (X5R or better) ceramic capacitor.

(Pin 12/Pin 21) : Main Supply Pin. All internal circuits

V

CC

except the output drivers are powered from this pin. V

CC

should be connected to a low noise power supply voltage
between 9V and 15V and should be bypassed to GND (pin

8) with at least a 0.1µF capacitor in close proximity to the
LTC3703.

SW (Pin 13/Pin 26): Switch Node Connection to Inductor and
Bootstrap Capacitor. Voltage swing at this pin is from a
Schottky diode (external) voltage drop below ground to V

IN

.

TG (Pin 14/Pin 27): Top Gate Drive. The TG pin drives the
gate of the top N-channel synchronous switch MOSFET.
The TG driver draws power from the BOOST pin and
returns to the SW pin, providing true floating drive to the
top MOSFET.

BOOST (Pin 15/Pin 28): Top Gate Driver Supply. The
BOOST pin supplies power to the floating TG driver. The
BOOST pin should be bypassed to SW with a low ESR (X5R
or better) 0.1µF ceramic capacitor. An additional fast
recovery Schottky diode from DRV

to BOOST will create

CC

a complete floating charge-pumped supply at BOOST.

V

(Pin 16/Pin 1): Input Voltage Sense Pin. This pin is

IN

connected to the high voltage input of the regulator and is
used by the internal feedforward compensation circuitry
to improve line regulation. This is not a supply pin.

3703fa

7

LTC3703

U

U

W

FU CTIO AL DIAGRA

4µA

R2

RUN/SS

C

SS

MODE/SYNC

COMP

V

R1

V

(<15V)

5

3.2V

1

3

+

0.8V

+

FB

4

16

IN

CC

12

FB

–

–

+

1V

UVSD OTSD

SYNC

DETECT

% DC

÷

LIMIT

FSET

CHIP
SD

EXT SYNC

FORCED CONTINUOUS

–
PWM

+

RSET

2

OSC

MIN MAX

–

+

0.76V

+

0.84V

OVERCURRENT

12µA

I

INV

MAX

5

R

MAX

V

CC

D

B

BOOST

15

TG

14

SW

13

DRV

CC

11

BG

10

BGRTN

9

INV

6

V

IN

C

B

M1

M2

L1

–

+

50mV

–

±

+

±

+

–

DRIVE
LOGIC

REVERSE

CURRENT

INV

–

OVER

TEMP

OT SD 0.8V

V

CC

C

VCC

BANDGAP

REFERENCE

V

CC

INTERNAL

3.2V V

CC

U

OPERATIO

The LTC3703 is a constant frequency, voltage mode
controller for DC/DC step-down converters. It is designed
to be used in a synchronous switching architecture with
two external N-channel MOSFETs. Its high operating volt­age capability allows it to directly step down input voltages
up to 100V without the need for a step-down transformer.
For circuit operation, please refer to the Functional Dia­gram of the IC and Figure 1. The LTC3703 uses voltage

(Refer to Functional Diagram)

OUT

V

OUT

UVLO

UV SD

GN16

GND

8

C

3703 FD

mode control in which the duty ratio is controlled directly
by the error amplifier output and thus requires no current
sense resistor. The V

pin receives the output voltage

FB

feedback and is compared to the internal 0.8V reference by
the error amplifier, which outputs an error signal at the
COMP pin. When the load current increases, it causes a
drop in the feedback voltage relative to the reference. The
COMP voltage then rises, increasing the duty ratio until the

3703fa

8

U

OPERATIO

output feedback voltage again matches the reference
voltage. In normal operation, the top MOSFET is turned on
when the RS latch is set by the on-chip oscillator and is
turned off when the PWM comparator trips and resets the
latch. The PWM comparator trips at the proper duty ratio
by comparing the error amplifier output (after being “com­pensated” by the line feedforward multiplier) to a sawtooth
waveform generated by the oscillator. When the top
MOSFET is turned off, the bottom MOSFET is turned on
until the next cycle begins or, if Pulse Skip Mode operation
is enabled, until the inductor current reverses as deter­mined by the reverse current comparator. MAX and MIN
comparators ensure that the output never exceed ±5% of
nominal value by monitoring VFB and forcing the output
back into regulation quickly by either keeping the top
MOSFET off or forcing maximum duty cycle. The opera­tion of its other features—fast transient response, out­standing line regulation, strong gate drivers, short-circuit
protection, and shutdown/soft-start—are described be­low.

Fast Transient Response

The LTC3703 uses a fast 25MHz op amp as an error
amplifier. This allows the compensation network to be
optimized for better load transient response. The high
bandwidth of the amplifier, along with high switching
frequencies and low value inductors, allow very high loop
crossover frequencies. The 800mV internal reference al­lows regulated output voltages as low as 800mV without
external level shifting amplifiers.

Line Feedforward Compensation

The LTC3703 achieves outstanding line transient response
using a patented feedforward correction scheme. With
this circuit the duty cycle is adjusted instantaneously to
changes in input voltage, thereby avoiding unacceptable
overshoot or undershoot. It has the added advantage of
making the DC loop gain independent of input voltage.
Figure 2 shows how large transient steps at the input have
little effect on the output voltage.

(Refer to Functional Diagram)

LTC3703

V

OUT

50mV/DIV

V

IN

20V/DIV

I

L

2A/DIV

V

= 12V

OUT

= 1A

I

LOAD

25V TO 60V V

Figure 2. Line Transient Performance

IN

20µs/DIV

STEP

Strong Gate Drivers

The LTC3703 contains very low impedance drivers ca­pable of supplying amps of current to slew large MOSFET
gates quickly. This minimizes transition losses and allows
paralleling MOSFETs for higher current applications. A
100V floating high side driver drives the top side MOSFET
and a low side driver drives the bottom side MOSFET (see
Figure 3). They can be powered from either a separate DC
supply or a voltage derived from the input or output
voltage (see MOSFET Driver Supplies section). The bot­tom side driver is supplied directly from the DRV
The top MOSFET drivers are biased from floating boot­strap capacitor C

, which normally is recharged during

B

each off cycle through an external diode from DRV
the top MOSFET turns off. In Pulse Skip Mode operation,
where it is possible that the bottom MOSFET will be off for
an extended period of time, an internal counter guarantees
that the bottom MOSFET is turned on at least once every
10 cycles for 10% of the period to refresh the bootstrap
capacitor. An undervoltage lockout keeps the LTC3703
shut down unless this voltage is above 9V.

The bottom driver has an additional feature that helps
minimize the possibility of external MOSFET shoot-thru.
When the top MOSFET turns on, the switch node dV/dt
pulls up the bottom MOSFET’s internal gate through the
Miller capacitance, even when the bottom driver is holding
the gate terminal at ground. If the gate is pulled up high
enough, shoot-thru between the top side and bottom side

3703 F02

CC

CC

pin.

when

3703fa

9

LTC3703

OPERATIO

U

(Refer to Functional Diagram)

MOSFETs can occur. To prevent this from occuring, the
bottom driver return is brought out as a separate pin
(BGRTN) so that a negative supply can be used to reduce
the effect of the Miller pull-up. For example, if a –2V supply
is used on BGRTN, the switch node dV/dt could pull the
gate up 2V before the V

of the bottom MOSFET has more

GS

than 0V across it.

DRV

CC

LTC3703

DRV

BGRTN

0V TO –5V

CC

BOOST

TG

SW

BG

Figure 3. Floating TG Driver Supply and Negative BG Return

V

IN

D

B

C

B

MT

MB

+

C

IN

L

3703 F03

V

OUT

+

C

OUT

Constant Frequency

The internal oscillator can be programmed with an exter­nal resistor connected from f

to ground to run between

SET

100kHz and 600kHz, thereby optimizing component size,
efficiency, and noise for the specific application. The
internal oscillator can also be synchronized to an external
clock applied to the MODE/SYNC pin and can lock to a
frequency in the 100kHz to 600kHz range. When locked to
an external clock, Pulse Skip Mode operation is automati­cally disabled. Constant frequency operation brings with it
a number of benefits: Inductor and capacitor values can be
chosen for a precise operating frequency and the feedback
loop can be similarly tightly specified. Noise generated by
the circuit will always be at known frequencies.
Subharmonic oscillation and slope compensation, com­mon headaches with constant frequency current mode
switchers, are absent in voltage mode designs like the
LTC3703.

Shutdown/Soft-Start

The main control loop is shut down by pulling RUN/SS pin
low. Releasing RUN/SS allows an internal 4µA current
source to charge the soft-start capacitor C

. When C

SS

SS

reaches 1V, the main control loop is enabled with the duty

cycle control set to 0%. As C

continues to charge, the

SS

duty cycle is gradually increased, allowing the output
voltage to rise. This soft-start scheme smoothly ramps the
output voltage to its regulated value, with no overshoot.
The RUN/SS voltage will continue ramping until it reaches
an internal 4V clamp. Then the MIN feedback comparator
is enabled and the LTC3703 is in full operation. When the
RUN/SS is low, the supply current is reduced to 50µA.

V

OUT

V

RUN/SS

1.4V

0V

4V

3V

1V

0V

SHUTDOWN

LTC3703

ENABLE

START-UP

MINIMUM

DUTY CYCLE

POWER

DOWN MODE

NORMAL OPERATION

MIN COMPARATOR ENABLED

OUTPUT VOLTAGE
IN REGULATION

RUN/SS SOFT-STARTS
OUTPUT VOLTAGE AND
INDUCTOR CURRENT

CURRENT

LIMIT

3703 F04

Figure 4. Soft-Start Operation in Start Up and Current Limit

Current Limit

The LTC3703 includes an onboard current limit circuit that
limits the maximum output current to a user-programmed
level. It works by sensing the voltage drop across the
bottom MOSFET and comparing that voltage to a user­programmed voltage at the I

pin. Since the bottom

MAX

MOSFET looks like a low value resistor during its on-time,
the voltage drop across it is proportional to the current
flowing in it. In a buck converter, the average current in the
inductor is equal to the output current. This current also
flows through the bottom MOSFET during its on-time.
Thus by watching the drain-to-source voltage when the
bottom MOSFET is on, the LTC3703 can monitor the
output current. The LTC3703 senses this voltage and
inverts it to allow it to compare the sensed voltage (which
becomes more negative as peak current increases) with a
positive voltage at the I
12µA pull-up, enabling the user to set the voltage at I
with a single resistor (R
Limit Programming section for R

pin. The I

MAX

) to ground. See the Current

IMAX

IMAX

pin includes a

MAX

selection.

MAX

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