0.1 PF

50

LM5033

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Voltage

LM5033

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SNVS181B – APRIL 2004 – REVISED APRIL 2013

Figure 13. Input Transient Protection

be supplied at Vin and through the internal regulator, the required input current (Iin) is shown in the Typical

to flow. The current into Vin will then drop to a nominal 150µA (Pin 10 = open or grounded).

start-up. The time delay allows the internal circuitry of the LM5033, and associated external circuitry, to stabilize

external circuitry connected to the REF output (Pin 2) and Softstart (Pin 10) should be designed to stabilize

during the time delay.

a 1µF capacitor will provide approximately 300 µs of delay, with larger capacitors providing proportionately longer

capacitor.

SOFTSTART (PIN 10)

The capacitor at pin 10 determines the time required for the output duty cycle to increase from zero to the final

value for regulation. The minimum acceptable time is dependent on the response of the feedback loops to the

COMP pin, as well as the characteristics of the magnetic components. If the Softstart time is too quick, the

system output could significantly overshoot its intended voltage before the loop has a chance to establish

regulation, possibly adversely affecting the load. Experimentation with the final design is usually necessary to

determine the minimum value for the SS capacitor.

CURRENT SENSE (PIN 8)

This pin typically receives an input representative of the primary current from the current sense elements of the

external circuitry. The peak amplitude at this pin must be less than 0.5V for normal operation. Filtering at this pin

should be sufficient to prevent false triggering of the Current Sense comparator, but not significantly delay

detection of an overcurrent condition. The filter’s capacitor at pin 8 should not be larger than 2200 pF.

OSCILLATOR, SYNC INPUT (PIN 9)

The internal oscillator frequency is generally selected in conjunction with the system magnetic components, and

frequency according to the formula in the Functional Description. Each output (OUT1 and OUT2) switches at half

the oscillator frequency. If the required frequency value is critical in a particular application, the tolerance of the

external resistor, and the frequency tolerance indicated in the Electrical Characteristics, must be taken into

account when selecting the resistor.

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