LM5032MTC datasheet(16/22 Pages) NSC

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Applications Information

VIN

The voltage applied to the VIN pin, normally the same as the

system voltage applied to the power transformer’s primary

PWR), can vary in the range of 13 to 100V with transient

capability to 105V. The current into VIN depends primarily on

the output driver capacitive loads, the switching frequency,

and any external load at VCC. If the power dissipation

associated with the VIN current exceeds the package capa-

bility, an external voltage should be applied to VCC (see

Figures3&4)to reduce power in the internal start-up

regulator. It is recommended the circuit of Figure 20 be used

to suppress transients which may occur at the input supply,

in particular where VIN is operated close to the maximum

operating rating of the LM5032.

When all internal bias currents for the LM5032 and output

driver currents are supplied through VIN and the internal V

regulator, the required input current (I

IN) is shown in Figures

3 & 4. In most applications, upon turn-on, I

IN increases with

as shown in Figure 3 until the UVLO threshold is

reached. After the outputs are enabled and the external VCC

supply voltage is active, the current into VIN then drops to a

nominal 120 µA.

FOR APPLICATIONS >100V

For applications where the system input voltage (V

PWR)

exceeds 100V, VIN can be powered from an external

start-up regulator as shown in Figure 21, or from any other

low voltage source as shown in Figure 22. Connecting VIN

and VCC together allows the LM5032 to be operated with

VIN below 13V. The voltage at VCC must not exceed 15V.

The voltage source at the right side of Figure 21 is typically

derived from the power stage, and becomes active once the

LM5032’s outputs are active.

UVLO

The under-voltage lockout threshold (UVLO) is internally set

at 1.25V at the UVLO pin. With two external resistors as

shown in Figure 23, the LM5032 is enabled when V

PWR

exceeds the programmed threshold voltage. When V

PWR is

above the threshold, the internal 20 µA current source is

enabled to raise the voltage at the UVLO pin, providing

hysteresis. R1 and R2 are determined from the following

equations:

R1=V

HYS/20 µA

where V

HYS is the desired UVLO hysteresis at VPWR, and

PWR in the second equation is the turn-on voltage. For

example, if the LM5032 is to be enabled when V

PWR reaches

20V, and disabled when V

PWR is decreased to 17V, R1

calculates to 150 k

Ω, and R2 calculates to 10 kΩ. The

voltage at UVLO should not exceed 6V at any time.

The LM5032 can be remotely shutdown by taking the UVLO

pin below 1.25V with an external open collector or open

drain device, as shown in Figure 24. The outputs, and the

CC regulator, are disabled, and the LM5032 enters a low

power mode. To shut down one regulator without affecting

the other, see the Soft-start section.

20135027

FIGURE 20. Input Transient Protection

20135047

FIGURE 21. Start-up Regulator for V

PWR >100V

20135048

FIGURE 22. Bypassing the Internal Start-up Regulator

20135028

FIGURE 23. UVLO Circuit

LM5032MTC Datasheet(PDF) 16 Page - National Semiconductor (TI)

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