LM5155 datasheet(12/48 Pages) TI2

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µs (see Section 9.3.5 for more details), the device starts up and an internal configuration starts. The device

typically requires a 65-µs internal start-up delay before entering standby mode. In standby mode, the VCC

regulator and RT regulator are operational, SS pin is grounded, and there is no switching at the GATE output.

IUVLO

UVLO/

SYNC

VEN

VCC_EN

VSUPPLY

VUVLO

RUVLOT

RUVLOB

RUN

Figure 9-1. Line UVLO and Enable

When the UVLO pin voltage is above the UVLO threshold, the device enters run mode. In run mode, a soft-start

sequence starts if the VCC voltage is greater than 4.5 V, or 50 µs after the VCC voltage exceeds the 2.85-V

VCC UV threshold (VVCC-UVLO), whichever comes first. UVLO hysteresis is accomplished with an internal 50-mV

voltage hysteresis and an additional 5-μA current source that is switched on or off. When the UVLO pin voltage

exceeds the UVLO threshold, the current source is enabled to quickly raise the voltage at the UVLO pin. When

the UVLO pin voltage falls below the UVLO threshold, the current source is disabled, causing the voltage at the

UVLO pin to fall quickly. When the UVLO pin voltage is less than the enable threshold (VEN), the device enters

shutdown mode after a 35-µs (typical) delay with all functions disabled.

UVLO

VCC

VLOAD

GATE

2.7 V

0.55 V

2.85 V

50-µs

VCC UV delay

1.5 V

1 V

VLOAD(TARGET)

VLOAD

Shutdown

Standby

TSS

4.5 V

UVLO should be greater than

0.55 V more than 1.5 µs to start-up

1.5 µs

> 3 cycles

SS is grounded

with 2 cycles

delay

65-µs (typical)

internal start-up delay

BIAS

= VSUPPLY

Figure 9-2. Boost Start-Up Waveforms Case 1: Start-Up by 2.85-V VCC UVLO, UVLO Toggle After Start-Up

LM5155, LM51551

SNVSB75E – DECEMBER 2018 – REVISED AUGUST 2023

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