Datasheet

PWM

COMPARATOR

COMP

REF

LM5035

SOFT-START

FEED-FORWARD RAMP

Voltage

feedback

LM4041

Potential across

Optocoupler detector

is constant (approx. 4.3V)

1 : 1

RES

Current Limit Detected

at CS

2.5V

+55 PA

+1 PA

t2 t3

#1V

LM5035

www.ti.com

SNVS428G –JANUARY 2006–REVISED MARCH 2013

Figure 13. Current Limit Restart Timing

Figure 14. 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 LM5035, 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 55µA 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.

Soft-Stop

If the UVLO pin voltage falls below the 1.25V standby threshold but above the 0.4V shutdown threshold, the

55µA SS pin source current is disabled and a 55µA sink current discharges the soft-start capacitor. As SS

voltage falls and clamps the PWM comparator input, the PWM duty cycle will gradually fall to zero. The soft-stop

feature produces a gradual reduction of the power converter output voltage. This soft-stop method of turning off

the converter reduces energy in the output capacitor before control of the main and synchronous rectification

MOSFETs is disabled. The PWM pulses may cease before the SS voltage reduces the duty cycle if the VCC or

REF voltage drops below the respective under-voltage thresholds during the soft-stop process.

Product Folder Links: LM5035