LM5035 datasheet(17/28 Pages) NSC

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Applications Information

The following information is intended to provide guidelines

for the power supply designer using the LM5035.

VIN

The voltage applied to the VIN pin, which may be the same

as the system voltage applied to the power transformer’s

primary (V

PWR), can vary in the range of 13 to 105V. The

current into VIN depends primarily on the gate charge pro-

vided to the output drivers, the switching frequency, and any

external loads on the VCC and REF pins. It is recommended

the filter shown in Figure 6 be used to suppress transients

which may occur at the input supply. This is particularly

important when VIN is operated close to the maximum op-

erating rating of the LM5035.

When power is applied to VIN and the UVLO pin voltage is

greater than 0.4V, the VCC regulator is enabled and supplies

current into an external capacitor connected to the VCC pin.

When the voltage on the VCC pin reaches the regulation

point of 7.6V, the voltage reference (REF) is enabled. The

reference regulation set point is 5V. The HO, LO, SR1 and

SR2 outputs are enabled when the two bias regulators reach

their set point and the UVLO pin potential is greater than

1.25V. In typical applications, an auxiliary transformer wind-

ing is connected through a diode to the VCC pin. This

winding must raise the VCC voltage above 8.3V to shut off

the internal start-up regulator.

After the outputs are enabled and the external VCC supply

voltage has begun supplying power to the IC, the current into

VIN drops below 1 mA. VIN should remain at a voltage equal

to or above the VCC voltage to avoid reverse current through

protection diodes.

FOR APPLICATIONS >100V

For applications where the system input voltage exceeds

100V or the IC power dissipation is of concern, the LM5035

can be powered from an external start-up regulator as

shown in Figure 7. In this configuration, the VIN and the VCC

pins should be connected together, which allows the

LM5035 to be operated below 13V. The voltage at the VCC

pin must be greater than 8.3V yet not exceed 15V. An

auxiliary winding can be used to reduce the power dissipa-

tion in the external regulator once the power converter is

active. The NPN base-emitter reverse breakdown voltage,

which can be as low as 5V for some transistors, should be

considered when selecting the transistor.

CURRENT SENSE

The CS pin needs to receive an input signal representative

of the transformer’s primary current, either from a current

sense transformer or from a resistor in series with the source

of the LO switch, as shown in Figure 8 and Figure 9. In both

cases, the sensed current creates a ramping voltage across

R1, and the R

F/CF filter suppresses noise and transients.

R1, R

F and CF should be located as close to the LM5035 as

possible, and the ground connection from the current sense

transformer, or R1, should be a dedicated track to the AGND

pin. The current sense components must provide greater

than 0.25V at the CS pin when an over-current condition

exists.

20177522

FIGURE 6. Input Transient Protection

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