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LM5035C

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SNVS631D – JANUARY 2010 – REVISED OCTOBER 2016

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9.2.1 Design Requirements

Table 1 lists the parameters for this example.

Table 1. Design Parameters

PARAMETER

MIN

NOM

MAX

UNIT

Input voltage (VIN)

36

72

V

Output voltage (VOUT)

3.3

V

Output current (IOUT)

0

30

A

Oscillator frequency

400

kHz

Efficiency (full load)

89%

Efficiency (half load)

92%

Load regulation

0.2%

Line regulation

0.1%

Undervoltage lockout (ON)

33.9

V

Undervoltage lockout (OFF)

31.9

V

Line overvoltage protection (ON)

79.4

V

Line overvoltage protection (OFF)

78.3

V

9.2.2 Detailed Design Procedure

The Device Comparison Table lists the differences between the LM5035, LM5035A, LM5035B, and LM5035C.

9.2.2.1 VIN

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

transformer’s primary (VPWR), can vary in the range of 13 to 105 V. 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. TI recommends using the filter shown in Figure 16 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

LM5035C.

When power is applied to VIN and the UVLO pin voltage is greater than 0.4 V, 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 7.6 V, the voltage reference (REF) is enabled. The reference regulation set point is 5 V.

The HO, LO, SR1 and SR2 outputs are enabled when the two bias regulators reach their setpoint and the UVLO

pin potential is greater than 1.25 V. In typical applications, an auxiliary transformer winding is connected through

a diode to the VCC pin. This winding must raise the VCC voltage above 8.3 V 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.

9.2.2.2 For Applications >100 V

For applications where the system input voltage exceeds 100 V or the IC power dissipation is of concern, the

LM5035C can be powered from an external start-up regulator as shown in Figure 17. In this configuration, the

VIN and the VCC pins should be connected together, which allows the LM5035C to be operated below 13 V. The

voltage at the VCC pin must be greater than 8.3 V yet not exceed 15 V. 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 5 V for some transistors, should be considered when

selecting the transistor.