PWM

COMPARATOR

5V

COMP

REF

LM5035A

+

_

6k

1V

SOFT-START

FEED-FORWARD RAMP

Voltage

feedback

LM4041

Potential across

Optocoupler detector

is constant (approx. 4.3V)

1 : 1

RES

SS

LO

HO

5V

t1

Current Limit Detected

at CS

2.5V

0V

+110 PA

+1 PA

t2

t3

#1V

LM5035A

www.ti.com

SNVS537F – JANUARY 2008 – REVISED APRIL 2013

Figure 15. Current Limit Restart Timing

Figure 16. Optocoupler to COMP Interface

Soft-Start

The soft-start circuit allows the regulator to gradually reach a steady state operating point, thereby reducing start-

up stresses and current surges. When bias is supplied to the LM5035A, the SS pin capacitor is discharged by an

internal MOSFET. When the UVLO, VCC and REF pins reach their operating thresholds, the SS capacitor is

released and charged with a 110 µA current source. The PWM comparator control voltage is clamped to the SS

pin voltage by an internal amplifier. When the PWM comparator input reaches 1V, output pulses commence with

slowly increasing duty cycle. The voltage at the SS pin eventually increases to 5V, while the voltage at the PWM

comparator increases to the value required for regulation as determined by the voltage feedback loop.

One method to shutdown the regulator is to ground the SS pin. This forces the internal PWM control signal to

ground, reducing the output duty cycle quickly to zero. Releasing the SS pin begins a soft-start cycle and normal

operation resumes. A second shutdown method is discussed in the UVLO section.

Copyright © 2008–2013, Texas Instruments Incorporated

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