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LT3751

Capacitor Charger Controller with Regulation

Linear Technology 

LT3751

PIN FUNCTIONS

CSN (Pin 9): Negative Current Sense Pin. Senses external

NMOS source current. Connect to local RSENSE ground

connection for proper Kelvin sensing. The current limit

is set by 106mV/RSENSE.

CSP (Pin 10): Positive Current Sense Pin. Senses NMOS

source current. Connect the NMOS source terminal and

the current sense resistor to this pin. The current limit is

fixed at 106mV/RSENSE in charge mode. The current limit

can be reduced to a minimum 11mV/RSENSE in regulation

mode.

VCC (Pin 11): Input Supply Pin. Must be locally bypassed

with high-grade (X5R or better) ceramic capacitor. The

minimum operating voltage for VCC is 4.75V.

LVGATE (Pin 12): Low Voltage Gate Pin. Connect the

NMOS gate terminal to this pin when operating VCC below

8V. The internal gate driver will drive the voltage to the

VCC rail. When operating VCC higher than 8V, tie this pin

directly to VCC.

HVGATE (Pin 13): High Voltage Gate Pin. Connect NMOS

gate terminal to this pin for all VCC operating voltages.

Internal gate driver will drive the voltage to within VCC

– 2V during each switch cycle.

RBG (Pin 14): Bias Generation Pin. Generates a bias

current set by 0.98V/RBG. Select RBG to achieve desired

resistance for RDCM, RVOUT, and RVTRANS.

RVOUT (Pin 16): Output Voltage Sense Pin. Develops a

current proportional to output capacitor voltage. Connect a

resistor between this pin and drain of NMOS such that:

VOUT

=

0.98

•

N

•

⎛

⎝⎜

RVOUT

RBG

⎞

⎠⎟

−

VDIODE

when RVOUT is set equal to RVTRANS, otherwise:

VOUT

=N•

⎡

⎢0.98 •

⎣⎢

RVOUT

RBG

+

VTRANS

⎛

⎝⎜

RVOUT

RVTRANS

− 1⎞⎠⎟

⎤

⎥

⎦⎥

− VDIODE

where VDIODE = forward voltage drop of diode D1 (refer

to Block Diagram).

RDCM (Pin 17): Discontinuous Mode Sense Pin. Senses

when the external NMOS drain is equal to 20μA • RDCM +

VTRANS and initiates the next charge cycle. Place a resis-

tor equal to 0.45 times the resistor on the RVTRANS pin

between this pin and VDRAIN.

RVTRANS (Pin 19): Transformer Supply Sense Pin. Connect

a resistor between the RVTRANS pin and the VTRANS supply.

Refer to Figure 11 for proper sizing of the RVTRANS resistor.

The minimum operation voltage for VTRANS is 4.75V.

UVLO1 (Pin 20): VTRANS Undervoltage Lockout Pin. Senses

when VTRANS drops below:

VUVLO1 = 1.225 + 50μA •RUVLO1

and trips the FAULT latch low, disabling switching. After

VTRANS rises above VUVLO1, toggling the CHARGE pin

reactivates switching.

GND (Pin 21): Ground. Tie directly to local ground plane.

3751f

8

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