LINEAR TECHNOLOGY LTC3751 Technical data

DESIGN FEATURES L

GND

0

V

OUT

100V/DIV

I

IN(AVG)

2A/DIV

20ms/DIV

VIN = 24V
C

OUT

= 100µF

2V/DIV

250ns/DIV

GND

CHARGE
CLAMP

V

CC

DONE

FAULT

UVLO1

OVLO1

UVLO2

OVLO2

RDCM

RV

OUT

HVGATE

LVGATE

CSP

CSN

FB

RV

TRANS

T1*

1:10

D1

V

OUT

50V TO 450V

V

TRANS

10V TO 24V

V

CC

TO µP

V

CC

LT3751

GND RBG

R6

40.2k

OFF ON

C3
680µF

C2

2.2µF
s5

C1
10µF

•

•

R7

18.2k

R8

40.2k

M1

R5

6mΩ
1W

D2

+

+

C4

100µF

R9

V

TRANS

R1, 154k

R2, 475k

DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY

C5

0.47µF

ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS
OTHERWISE NOTED

D1,D2: VISHAY MURS260
M1: IRF3710Z
T1: WURTH 750310349

LIMIT OUTPUT POWER TO
40W FOR 65°C T1 MAX
AMBIENT OPERATION

*

4.7nF

Y RATED

DC/DC Converter, Capacitor Charger
Takes Inputs from 4.75V to 400V

Introduction

High voltage power supplies and ca­pacitor chargers are readily found in
a number of applications, including
professional photoflashes, security
control systems, pulsed radar systems,
satellite communication systems, and
explosive detonators. The LT3751
makes it possible for a designer to
meet the demanding requirements
of these applications, including high
reliability, relatively low cost, safe
operation, minimal board space and
high performance.

The LT3751 is a general purpose
flyback controller that can be used as
either a voltage regulator or as a capac­itor charger. The LT3751 operates in
boundary-mode, between continuous
conduction mode and discontinuous
conduction mode. Boundary-mode
operation allows for a relatively small
transformer and an overall reduced
PCB footprint. Boundary-mode also
reduces large signal stability issues
that could arise from using voltage­mode or PWM techniques. Regulation
is achieved with a new dual, overlap­ping modulation technique using both

by Robert Milliken and Peter Liu

Figure 1. Gate driver waveform
in a typical application

peak primary current modulation and
duty-cycle modulation, drastically re­ducing audible transformer noise.

The LT3751 features many safety
and reliability functions, including
two sets of undervoltage lockouts
(UVLO), two sets of overvoltage
lockouts (OVLO), no-load operation,
over-temperature lockout (OTLO), in­ternal Zener clamps on all high voltage
pins, and a selectable 5.6V or 10.5V
internal gate driver voltage clamp (no
external components needed). The
LT3751 also adds a start-up/short­circuit protection circuit to protect
against transformer or external FET

damage. When used as a regulator, the
LT3751’s feedback loop is internally
compensated to ensure stability. The
LT3751 is available in two packages,
either a 20-pin exposed pad QFN or a
20-lead exposed pad TSSOP.

New Gate Driver with Internal
Clamp Requires No External
Components

There are four main concerns when
using a gate driver: output current
drive capability, peak output voltage,
power consumption and propagation
delay. The LT3751 is equipped with a

1.5A push-pull main driver, enough to
drive +80nC gates. An auxiliary 0.5A
PMOS pull-up only driver is also inte­grated into the LT3751 and is used in
parallel with the main driver for VCC
voltages of 8V and below. This PMOS
driver allows for rail-to-rail operation.
Above 8V, the PMOS driver must be
deactivated by tying its drain to VCC.

Most discrete FETs have a VGS limit
of 20V. Driving the FET higher than
20V could cause a short in the inter­nal gate oxide, causing permanent

Figure 2. Isolated high voltage capacitor charger from 10V to 24V input

Linear Technology Magazine • March 2009

Figure 3. Isolated high voltage capacitor

charger charging waveform

9

L DESIGN FEATURES

R

N

V V

R

OUT TRIP DIODE

9 8

0 98=•

+













•

.

( )

0

GND

V

DRAIN

20V/DIV

I

PRIMARY

5A/DIV

10µs/DIV

V

OUT

(V)

EFFICIENCY (%)

LOAD CURRENT (mA)

1000

90

60

20 40 60 80

65

70

80

75

85

402

399

400

401

LOAD REGULATION

EFFICIENCY

0

GND

V

DRAIN

20V/DIV

I

PRIMARY

5A/DIV

10µs/DIV

CHARGE
CLAMP

V

CC

DONE

FAULT

UVLO1

OVLO1

UVLO2

OVLO2

RDCM

RV

OUT

HVGATE

LVGATE

CSP

CSN

FB

RV

TRANS

T1**
1:10

D1

V

OUT

400V

V

TRANS

10V TO 24V

V

CC

TO µP

V

CC

LT3751

GND RBG

R6

40.2k

OFF ON

C3
680µF

R10*
499k

R11

1.54k

C2

2.2µF
s5

C1
10µF

•

•

R7

18.2k

R8

40.2k

M1

R5
6mΩ
1W

D2

+

+

C4

100µF

R9
787Ω

V

TRANS

R1,154k

R2, 475k

DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY

C5

0.47µF

C6
10nF

ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS
OTHERWISE NOTED

C4: CDE 380LX101M500J042
C5: TDK CKG57NX7R2J474M
D1,D2: VISHAY MURS260
M1: IRF3710Z
T1: WURTH 750310349

USE TWO SERIES 1206,
1% RESISTORS FOR R10
R10: 249k s2

LIMIT OUTPUT POWER TO
40W FOR 65°C T1 MAX
AMBIENT OPERATION

*

**

Figure 4. A 10V to 24V input, 400V regulated power supply

damage. To alleviate this issue, the
LT3751 has an internal, selectable

5.6V or 10.5V gate driver clamp. No
external components are needed, not
even a capacitor. Simply tie the CLAMP
pin to ground for 10.5V operation or
tie to VCC for 5.6V operation. Figure
1 shows the gate driver clamping at

10.5V with a VCC voltage of 24V.
Not only does the internal clamp

protect the FET from damage, it also
reduces the amount of energy injected
into the gate. This increases overall
efficiency and reduces power con­sumption in the gate driver circuit. The
gate driver overshoot is very minimal,
as seen in Figure 1. Placing the external
FET closer to the LT3751 HVGATE pin
reduces overshoot.

a. Switching waveform for I

10

High Voltage, Isolated
Capacitor Charger from
10V to 24V Input

The LT3751 can be configured as
a fully isolated stand-alone capaci­tor charger using a new differential
discont inuous-c on duction- mode
(DCM) comparator—used to sense
the boundary-mode condition—and
a new differential output voltage
(V

) comparator. The differential

OUT

operation of the DCM comparator and
V

comparator allow the LT3751 to

OUT

accurately operate from high voltage
input supplies of greater than 400V.
Likewise, the LT3751’s DCM compara­tor and V
input supplies down to 4.75V. This
accommodates an unmatched range
of power sources.

= 100mA b. Switching waveform for I

OUT

Figure 5. High voltage regulator performance

comparator can work with

OUT

Figure 2 shows a high voltage ca­pacitor charger driven from an input
supply ranging from 10V to 24V. Only
five resistors are needed to operate
the LT3751 as a capacitor charger.
The output voltage trip point can be
continuously adjusted from 50V to
450V by adjusting R9 given by:

The LT3751 stops charging the
output capacitor once the programmed
output voltage trip point (V
reached. The charge cycle is repeated
by toggling the CHARGE pin. The
maximum charge/discharge rate in

= 10mA c. Efficiency and load regulation

OUT

Linear Technology Magazine • March 2009

OUT(TRIP)

) is