LM5035ASQ datasheet(19/30 Pages) NSC

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Applications Information

The following information is intended to provide guidelines for

the power supply designer using the LM5035A.

VIN

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

as the system voltage applied to the power transformer’s pri-

mary (V

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

into VIN depends primarily on the gate charge provided 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 operating rating of the

LM5035A.

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 refer-

ence 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 winding is con-

nected 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.

30034122

FIGURE 6. Input Transient Protection

FOR APPLICATIONS >100V

For applications where the system input voltage exceeds

100V or the IC power dissipation is of concern, the LM5035A

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 LM5035A 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 dissipation 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, RF and

F should be located as close to the LM5035A as possible,

and the ground connection from the current sense trans-

former, 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.

30034123

FIGURE 7. Start-up Regulator for V

PWR >100V

30034124

FIGURE 8. Current Sense Using Current Sense Transformer

LM5035ASQ Datasheet(PDF) 19 Page - National Semiconductor (TI)

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