Datasheet

0.1 PF

LM5045

VIN

PWR

FILTER

LM5045

RAMP

CLK + LEB

CLK

Current

Sense

FILTER

SLOPE

100 PA

SLOPE

LM5045

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SNVS699G –FEBRUARY 2011–REVISED MARCH 2013

VIN and VCC

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 the 14 to 100V. It is recommended that the filter shown in

Figure 21 be used to suppress the transients that may occur at the input supply. This is particularly important

when VIN is operated close to the maximum operating rating of the LM5045. The current into VIN depends

primarily on the LM5045’s operating current, the switching frequency, and any external loads on the VCC pin,

that typically include the gate capacitances of the power MOSFETs. In typical applications, an auxiliary

transformer winding is connected through a diode to the VCC pin. This pin must raise VCC voltage above 8V 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 the VIN pin drops below 1mA. VIN should remain at a voltage equal to or above the VCC voltage to

avoid reverse current through the internal body diode of the internal VCC regulator.

For Applications with > 100V Input

For applications where the system input voltage exceeds 100V, VIN can be powered from an external start-up

regulator as shown in Figure 22. In this configuration, the VIN and VCC pins should be connected together. The

voltage at the VCC and VIN pins must be greater than 10V (>Max VCC reference voltage) yet not exceed 16V.

To enable operation the VCC voltage must be raised above 10V. The voltage at the VCC pin must not exceed

16V. The voltage source at the right side of Figure 22 is typically derived from the power stage, and becomes

active once the LM5045’s outputs are active.

Figure 20. Current Mode Configuration

Figure 21. Input Transient Protection

Product Folder Links: LM5045