LM5155 datasheet(16/51 Pages) TI

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less than VCC UVLO threshold, the UVLO is less than the UVLO threshold, during hiccup mode off-time or

thermal shutdown.

In boost topology, soft-start time (tSS) varies with the input supply voltage. The soft-start time in boost topology is

calculated as shown in Equation 3.

u ¨

SUPPLY

LOAD

(3)

In SEPIC topology, the soft-start time (tSS) is calculated as follows.

(4)

TI recommends choosing a soft-start time long enough so that the converter can start up without going into an

overcurrent state. See Section 9.3.10 for more detailed information.

Figure 9-8 shows an implementation of primary side soft start in flyback topology.

COMP

Figure 9-8. Primary-Side Soft-Start in Flyback

Figure 9-9 shows an implementation of secondary side soft start in flyback topology.

VLOAD

Secondary Side

Soft-start

Figure 9-9. Secondary-Side Soft Start in Flyback

9.3.4 Switching Frequency (RT Pin)

The switching frequency of the device can be set by a single RT resistor connected between the RT and the

AGND pins. The resistor value to set the RT switching frequency (fRT) is calculated as shown in Equation 5.

RT(TYPICAL)

2.21 10

955

(5)

The RT pin is regulated to 0.5 V by the internal RT regulator when the device is enabled.

LM5155-Q1, LM51551-Q1

SNVSAY4E – AUGUST 2018 – REVISED JANUARY 2021

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LM5155 Datasheet(PDF) 16 Page - Texas Instruments