Datasheet LTC3751 Datasheet (LINEAR TECHNOLOGY)

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

DESIGN FEATURES L

P

C FREQUENCY

V V V

AVG

OUT

OUT TRIP RIPPLE

=

• •

•

1

2

2

( )

–

RRIPPLE

W240

(

)

≤

V

CC

R3, 154k

R4, 475k

CHARGE
CLAMP

V

CC

DONE

FAULT

UVLO1

OVLO1

UVLO2

OVLO2

RV

OUT

HVGATE

LVGATE

CSP

CSN

FB

RV

TRANS

T1***
1:3

D1

V

OUT

500V

V

TRANS

100V TO 400V DC

V

CC

10V TO 24V

TO µP

V

CC

LT3751

GND RBG

R6*
625k

OFF ON

C3

100µF

450V

C2

2.2µF
630V
s5

C1
10µF

•

•

R8

137k ×3

R7

88.7k + 7.5k

R10*

208k

R13,20Ω

M1
FQB4N80

R12
68mΩ
1/4W

D2

+

+

C4

220µF

550V

R5

1.11k

V

TRANS

R1**

1.5M

R2**, 9M

DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY

C5

0.47µF
630V

RDCM

F1, 1A

R9

66.5k

R11

14.7k +

17.4k

ALL RESISTORS ARE 0805,

1% RESISTORS UNLESS
OTHERWISE NOTED

C4: HITACHI PS22L221MSBPF

C5: TDK CKG57NX7R2J474M
T1: COILCRAFT HA4060-AL
D1,D2: VISHAY US1M
F1: BUSSMANN PCB-1-R

* USE THREE SERIES 1206, 0.1%

RESISTORS FOR R6 & R10
R6: 249k ×2 + 127k
R10: 66.5k ×2 + 75k

** USE TWO SERIES 1206, 1%

RESISTORS FOR R1 & R2
R1: 750k ×2
R2: 4.53M ×2

*** OUTPUT POWER LIMITED TO

20W FOR 65°C T1 AMBIENT
OPERATION

4.7nF

Y RATED

0

V

OUT

AC RIPPLE

10V/DIV

I

IN(AVG)

20mA/DIV

2s/DIV

CHARGE TIME (ms)

V

OUT,TRIP

(V)

INPUT VOLTAGE (V)

400100

530

490

200 300

500

520

510

1000

400

850

550

700

V

OUT,TRIP

CHARGE TIME

the output capacitor is limited by the
temperature rise in the transformer.
Limiting the transformer surface tem­perature in Figure 2 to 65°C with no
air flow requires the average output
power to be ≤40W given by:

where V
voltage, V

OUT(TRIP)

is the output trip

is the ripple voltage

RIPPLE

on the output node, and frequency is
the charge/discharge frequency. Two
techniques are used to increase the
available output power: increase the
airflow across the transformer, or in­crease the size of the transformer itself.
Figure 3 shows the charging waveform
and average input current for a 100µF
output capacitor charged to 400V in
less than 100ms (R9 = 976Ω).

For output voltages higher than
450V, the transformer in Figure 2 must
be replaced with one having higher
primary inductance and a higher
turns ratio. Consult the LT3751 data

Figure 6. The LT3751 protecting the
output during a no-load condition

sheet for proper transformer design
procedures.

High Voltage Regulated Power
Supply from 10V to 24V Input

The LT3751 can also be used to convert
a low voltage supply to a much higher
voltage. Placing a resistor divider from
the output node to the FB pin and
ground causes the LT3751 to oper­ate as a voltage regulator. Figure 4
shows a 400V regulated power supply
operating from an input supply range
of 10V to 24V.

The LT3751 uses a regulation con­trol scheme that drastically reduces
audible noise in the transformer and
the input and output ceramic bulk

capacitors. This is achieved by using
an internal 26kHz clock to synchronize
the primary winding switch cycles.
Within the clock period, the LT3751
modulates both the peak primary
current and the number of switch­ing cycles. Figures 5a and 5b show
heavy-load and light-load waveforms,
respectively, while Figure 5c shows
efficiency over most of the operating
range for the application in Figure 4.

The clock forces at least one switch
cycle every period which would over­charge the output capacitor during a
no-load condition. The LT3751 han­dles no-load conditions and protects
against over-charging the output node.
Figure 6 shows the LT3751 protecting
during a no-load condition.

Resistors can be added to RV

OUT

and
RBG to add a second layer of protec­tion, or they can be omitted to reduce
component count by tying RV

OUT

and
RBG to ground. The trip level for the
V

comparator is typically set 20%

OUT

higher than the nominal regulation
voltage. If the resistor divider were to
fail, the V

comparator would disable

OUT

switching when the output climbed to
20% above nominal.

Linear Technology Magazine • March 2009

Figure 7. A 100V to 400V input, 500V output, isolated capacitor charger

Figure 8. Isolated capacitor charger V
and charge time with respect to input voltage

OUT(TRIP)

11

L DESIGN FEATURES

OUTPUT VOLTAGE (V)

INPUT VOLTAGE (V)

400100

398

395

200 300

396

397

I

OUT

= 10mA

I

OUT

= 25mA

I

OUT

= 50mA

EFFICIENCY (%)

OUTPUT CURRENT (mA)

750

90

40

50

25 50

60

70

80

VIN = 100V
VIN = 250V
VIN = 400V

V

CC

R3, 154k

R4, 475k

CHARGE
CLAMP

V

CC

DONE

FAULT

UVLO1

OVLO1

UVLO2

OVLO2

RV

OUT

HVGATE

LVGATE

CSP

CSN

FB

RV

TRANS

T1***
1:3

D1

V

OUT

400V

V

TRANS

100V TO 400V DC

V

CC

10V TO 24V

TO µP

V

CC

LT3751

GND RBG

R6*
615k

OFF ON

C3
100µF

C2

2.2µF
s5

C1
10µF

C6
10nF

•

•

R8*

411k

R13,20Ω

M1
FQB4N80

R10
68mΩ
¼W

D2

+

+

C4

100µF

R12

1.54k

R11**
499k

V

TRANS

R1**, 1.5M

R2**, 9M

DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY

C5

0.47µF

RDCM

F1, 1A

R9

66.5k

ALL RESISTORS ARE 0805,

1% RESISTORS UNLESS
OTHERWISE NOTED

C4: CDE 380LX101M500J042

C5: TDK CKG57NX7R2J474M
T1: COILCRAFT HA4060-AL
D1,D2: VISHAY US1M
F1: BUSSMANN PCB-1-R

* USE THREE SERIES 1206, 1%

RESISTORS FOR R6 & R8
R6: 205k ×3

R8: 137k ×3

** USE TWO SERIES 1206, 1%

RESISTORS FOR R1, R2 & R11
R1: 750k ×2
R2: 4.53M ×2
R11: 249k ×2

*** OUTPUT POWER LIMITED TO

20W FOR 65°C T1 AMBIENT
OPERATION

R7

95.3k

can also be used for a capacitor
charger. The LT3751 operates as a
capacitor charger until the FB pin
reaches 1.225V, after which the
LT3751 operates as a voltage regulator.
This keeps the capacitor topped-off
until the application needs to use its
energy. The output resistor divider
forms a leakage path from the output
capacitor to ground. When the output
voltage droops, the LT3751 feedback
circuit will keep the capacitor topped-

12

Figure 9. A 100V to 400V input, 400V output, capacitor charger and voltage regulator

Note that the FB pin of the LT3751

a. Overall efficiency b. Line regulation

Figure 10. High voltage input and output regulator performance

off with small, low current bursts of
charge as shown in Figure 6.

High Input Supply Voltage,
Isolated Capacitor Charger

As mentioned above, the LT3751 dif­ferential DCM and V
allow the part to accurately work from
very high input supply voltages. An
offline capacitor charger, shown in
Figure 7, can operate with DC input
voltages from 100V to 400V. The trans­former provides galvanic isolation from

comparators

OUT

the input supply to output node—no
additional magnetics required.

Input voltages greater than 80V
require the use of resistor dividers
on the DCM and V

comparators

OUT

(charger mode only). The accuracy of
the V

trip threshold is heightened

OUT

by increasing current IQ through R10
and R11; however, the ratio of R6/R7
should closely match R10/R11 with
tolerances approaching 0.1%. A trick
is to use resistor arrays to yield the
desired ratio. Achieving 0.1% ratio ac­curacy is not difficult and can reduce
the overall cost compared to using
individual 0.1% surface mount resis­tors. Note that the absolute value of
the individual resistors is not critical,
only the ratio of R6/R7 and R10/R11.
The DCM comparator is less critical
and can tolerate resistance variations
greater than 1%.

The 100V to 400VDC input capaci­tor charger has an overall V
accuracy of better than 6% over the
entire operating range using 0.1% re­sistor dividers. Figure 8 shows a typical
performance for V

OUT(TRIP)

and charge

time for the circuit in Figure 7.

Linear Technology Magazine • March 2009

OUT(TRIP)

DESIGN FEATURES L

V

TRANS

100V TO 200V DC

V

CC

V

CC

R11, 84.5k

R12, 442k

UVLO1

OVLO1

UVLO2

OVLO2

DONE

FAULT

CHARGE
CLAMP

V

CC

HVGATE

LVGATE

CSP

CSN

FB

RV

TRANS

TO µP

V

CC

LT3751

LT4430

GND RBG

R3
210k

OFF ON

C3
22µF
350V
s2

C2
1µF

C1
100pF

C4
1µF
250V
s2

C7
400µF
330V

C8
22nF

R17

3.16k

R14
249k

V

OUT

282V
225mA

C6

0.1µF
630V

ISOLATION
BOUNDARY

C5

0.01µF
630V

C9

3.3µF

C10

0.47µF

•

M2

M1

D1

R8

2.49k

R7
475Ω

R18

274Ω

R6
40mΩ
1/4W

V

TRANS

R9, 2.7M

R5, 210k

R13

5.11Ω

R16, 1k

R10, 4.3M

DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY

RDCM

RV

OUT

U1

ALL RESISTORS ARE 0805,1% RESISTORS
UNLESS OTHERWISE NOTED

C7: 330FK400M22X38
D1: 12V ZENER
D2: MURS140
D3: P6kE200A
D4, D5: STTH112A
D6: BAT54
D7: BAS516
M1: IRF830
M2: STB11NM60FD
T1: TDK SRW24LQ-UxxH015
(Np:Ns:Npb:Nsb=1:2:0.08:0.08)
U1: PS2801-1
U2: LT4430

R4, 105k

R2, 10ΩD2

D3

D5

D6

D4

Np

Ns

Nsb

U2

VINCOMP

GND

OC FB

OPTO

Npb

T1

D7

R15
221k

R1

49.9k
1/2W

•

•

•

+

+

1

1 1 1

1

11

1

1

1

2

2

2

2

2

2

2

F1, 2A

1 2

4.7nF

Y RATED

0

GND

V

DRAIN

100V/DIV

I

PRIMARY

2A/DIV

20µs/DIV

0

GND

V

DRAIN

100V/DIV

I

PRIMARY

2A/DIV

20µs/DIV

R

V

A

OUT TRIP

11

1 225

50

=

−

( )

.

µ

Figure 11. Fully isolated, high output voltage regulator

High Input Supply Voltage,
Non-Isolated Capacitor
Charger/Regulator

The FB pin of the LT3751 can also
be configured for charging a capaci­tor from a high input supply voltage.
Simply tie a resistor divider from the
output node to the FB pin. The resis­tor dividers on the R
pins can tolerate 5% resistors, and all
the R
removed. This lowers the number and

and RBG pin resistors are

V(OUT)

the tolerance of required components,
reducing board real estate and overall
design costs. With the output voltage
resistor divider, the circuit in Figure
9 is also a fully functional, high-ef
ficiency voltage regulator with load

Linear Technology Magazine • March 2009

VTRANS

a. I

and R

= 225mA b. I

OUT

DCM

-

and line regulation better than 1%.
Efficiency and line regulation for the
circuit in Figure 9 are shown in Figure
10a and Figure 10b, respectively.

Alternatively, a resistor can be tied
from V
pin. This mimics the V

to the OVLO1 pin or OVLO2

OUT

compara-

OUT

tor, stopping charging once the target
voltage is reached. The FB pin is tied
to ground. The CHARGE pin must be
toggled to initiate another charge se­quence, thus the LT3751 operates as
a capacitor charger only. Resistor R12
is omitted from Figure 9 and resistor
R11 is tied from V
or OVLO2. R11 is calculated using the
following equation:

Figure 12. Switching waveforms

OUT

directly to OVLO1

Note that OVLO1 or OVLO2 will
cause the FAULT pin to indicate a
fault when the target outpaut voltage,
V

OUT(TRIP) ,

is reached.

High Voltage Input/Output
Regulator with Isolation

Using a resistor divider from the output
node to the FB pin allows regulation
but does not provide galvanic isolation.
Two auxiliary windings are added to
the transformer in circuit shown in
Figure 11 to drive the FB pin, the

OUT

= 7.1mA

continued on page 42

13

L NEW DEVICE CAMEOS

EFFICIENCY (%)

INPUT DC VOLTAGE (V)

200100

100

70

120 140 160 180

75

80

90

85

95

P

OUT

= 63W

P

OUT

= 48W

P

OUT

= 25W

OUTPUT VOLTAGE ERROR (V)

I

OUT

(mA)

2500

–0.5

–0.25

0

0.25

0.5

10050 150 200

battery whether external or internal.
Programming the charge current only
requires a single external resistor.

The fault management system of the
LTC4012 family suspends charging
immediately for various conditions.
First is battery overvoltage protection,
which can occur with the sudden loss
of battery load during bulk charge.
Second, each IC features internal
over-temperature protection to pre­vent silicon damage during elevated
thermal operation.

The LTC4012 family has a logic-level
shutdown control input and three
open-drain status outputs. First is an
input current limit (ICL) status flag to
tell the system when VIN is running at
over 95% of its current capacity. The
input current limit accuracy is typi­cally ±3% and a maximum of ±4% over
the full operating temperature range.
Next is the AC present status, which
indicates when VIN is within a valid
range for charging under all modes of
operation. The last is a charge status
output can indicate bulk or C/10
charge states. The control input and
status outputs of the LTC4012, along

with the analog current monitor out­put, can be used by the host system
to perform necessary preconditioning,
charge termination and safety timing
functions.

4MHz Synchronous Step­Down DC/DC Converter
Delivers up to 1.25A from a
3mm × 3mm DFN

The LTC3565 is a high efficiency syn­chronous step-down regulator that
can deliver up to 1.25A of continuous
output current from a 3mm × 3mm
DFN (or MSOP-10E) package. Using
a constant frequency of (up to 4MHz)
and current mode architecture, the
LTC3565 operates from an input volt­age range of 2.5V to 5.5V making it
ideal for single cell Li-Ion, or multicell
Alkaline/NiCad/NiMH applications.
It can generate output voltages as
low as 0.6V, enabling it to power the
latest generation of low voltage DSPs
and microcontrollers. An independent
RUN pin enables simple turn-on and
shutdown. Its switching frequency
is user programmable from 400kHz
to 4MHz, enabling the designer to

optimize efficiency while avoiding criti­cal noise-sensitive frequency bands.
The combination of its 3mm × 3mm
DFN-10 (or MSOP-10) package and
high switching frequency keeps ex­ternal inductors and capacitors small,
providing a very compact, thermally
efficient footprint.

The LTC3565 uses internal switches

with an R

of only 0.13Ω (N-Chan-

DS(ON)

nel lower FET) and 0.15Ω (P-Channel
upper FET) to deliver efficiencies
as high as 95%. It also utilizes low
dropout 100% duty cycle operation
to allow output voltages equal to VIN,
further extending battery run time.
The LTC3565 utilizes Automatic Low
Ripple ( < 25mV

) Burst Mode®

P–P

operation to offer only 40µA no load
quiescent current. If the application is
noise sensitive, Burst Mode operation
can be disabled using a lower noise
pulse-skipping mode, which still offers
only 330µA of quiescent current. The
LTC3565 can be synchronized to an
external clock throughout its entire
frequency range. Other features in­clude ±2% output voltage accuracy and

over-temperature protection.

L

LT3751, continued from page 13

LT3751 controller, and the optocoupler
on the feedback resistor divider. The
auxiliary windings provide the desired
galvanic isolation boundary while
maintaining an isolated feedback path
from the output node to the LT3751
FB pin. Figures 12 and 13 show the
regulator’s performance.

The fully isolated, high voltage in­put/output regulator yields over 90%
efficiency. Load regulation is excellent
as shown in Figure 13b, due mainly
to the added gain of the optocoupler
circuit.

Conclusion

The ability to run from any input
supply voltage ranging from 4.75V
to greater than 400V and the abun­dance of safety features make the
LT3751 an excellent choice for high
voltage capacitor chargers or high
voltage regulated power supplies. In
fact, the LT3751 is, for now, the only

42

42

a. Efficiency b. Load regulation

Figure 13. Fully isolated, high voltage regulator performance

boundary-mode capacitor charger
controller that can accurately operate
from extremely high input voltages.
The LT3751 simplifies design by in­tegrating many functions that—due
to cost and board real-estate—would
otherwise not be realizable. Although

LT3751 includes many more features
than we can show in one article. We
recommended consulting the data
sheet or calling the Linear Technology
applications engineering department
for more in-depth coverage of all avail­able features.

L

several designs are shown here, the

Linear Technology Magazine • March 2009