Datasheet

HO1, LO2

SR2

HO2, LO1

SR1

LM5045

SNVS699G –FEBRUARY 2011–REVISED MARCH 2013

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Figure 15. Synchronous Rectifier Timing Diagram

Soft-Start of the Synchronous Rectifiers

In addition to the basic soft-start already described, the LM5045 contains a second soft-start function that

gradually turns on the synchronous rectifiers to their steady-state duty cycle. This function keeps the

synchronous rectifiers off during the basic soft-start allowing a linear start-up of the output voltage even into pre-

biased loads. Then the SR output duty cycle is gradually increased to prevent output voltage disturbances due to

the difference in the voltage drop between the body diode and the channel resistance of the synchronous

MOSFETs. Initially, when bias is supplied to the LM5045, the SSSR capacitor is discharged by an internal

MOSFET. When the SS capacitor reaches a 2V threshold and once it is established that COMP is in control of

the duty cycle i.e. I

COMP

< 800µA, the SSSR discharge is released and SSSR capacitor begins charging with a

20µA current source. Once the SSSR cap crosses the internal 1V threshold, the LM5045 begins the soft-start of

the synchronous FETs. The SR soft-start follows a leading edge modulation technique i.e. the leading edge of

the SR pulse is soft-started as opposed trailing edge modulation of the primary FETs. As shown in the

Figure 16(a), SR1 and SR2 are turned-on simultaneously with a narrow pulse-width during the freewheeling

cycle. At the end of the freewheel cycle i.e. at the rising edge of the internal CLK, the SR FET in-phase with the

next power transfer cycle is kept on while the SR FET out of phase with it is turned-off. The in-phase SR FET is

kept on throughout the power transfer cycle and at the end of it, both the primary FETs and the in-phase SR

FETs are turned-off together. The synchronous rectifier outputs can be disabled by grounding the SSSR pin.

Product Folder Links: LM5045