3724EFE データシートの表示（PDF） - Linear Technology

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3724EFE

High Voltage, Current Mode Switching Regulator Controller

Linear Technology 

LT3724

APPLICATIONS INFORMATION

L1

VFB PIN

VOUT

R2

COUT

R1

3724 F02

Figure 2. Output Voltage Feedback Divider

VSUPPLY

RA

SHDN PIN

RB

3724 F03

Figure 3. Undervoltage Lockout Circuit

The VFB pin input bias current is typically 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:

∆VOUT(BIAS) = 25nA • R2

Supply UVLO and Shutdown

The SHDN pin has a precision voltage threshold with

hysteresis which can be used as an undervoltage lockout

threshold (UVLO) for the power supply. Undervoltage

lockout keeps the LT3724 in shutdown until the supply

input voltage is above a certain voltage programmed by

the user. The hysteresis voltage prevents noise from falsely

tripping UVLO.

Resistors are chosen by first selecting RB. Then:

RA

= RB

⎛

• ⎝⎜

VSUPPLY(ON)

1.35V

⎞

– 1⎠⎟

VSUPPLY(ON) is the input voltage at which the undervoltage

lockout is disabled and the supply turns on.

Example: Select RB = 49.9kΩ, VSUPPLY(ON) = 14.5V (based

on a 15V minimum input voltage)

RA

=

49.9kΩ

•

⎛

⎝⎜

14.5V

1.35V

–

1⎞⎠⎟

= 486.1kΩ (499kΩ resistor is selected)

If low supply current in standby mode is required, select

a higher value of RB.

The supply turn off voltage is 9% below turn on. In the

example the VSUPPLY(OFF) would be 13.2V.

If additional hysteresis is desired for the enable function,

an external positive feedback resistor can be used from

the LT3724 regulator output.

The shutdown function can be disabled by connecting the

SHDN pin to the VIN through a large value pull-up resistor.

This pin contains a low impedance clamp at 6V, so the SHDN

pin will sink current from the pull-up resistor(RPU):

ISHDN=

VIN – 6V

RPU

Because this arrangement will clamp the SHDN pin to the

6V, it will violate the 5V absolute maximum voltage rating of

the pin. This is permitted, however, as long as the absolute

maximum input current rating of 1mA is not exceeded.

Input SHDN pin currents of <100µA are recommended: a

1MΩ or greater pull-up resistor is typically used for this

configuration.

Soft-Start

The soft-start function forces the programmed slew rate

while the converter output rises to 95% of regulation,

which corresponds to 1.185V on the VFB pin. Once 95%

regulation is achieved, the soft-start circuit is disabled.

The soft-start circuit will re-enable when the VFB pin drops

below 70% of regulation, which corresponds to 300mV

of control hysteresis on the VFB pin. This allows for a

controlled recovery from a “brown-out” condition.

VOUT

CSS1

A RSS

LT3724

CSS

3724 F04

Figure 4.Soft-Start Circuit

3724fd

15

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