LT8331 datasheet(10/34 Pages) AD

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LT8357

Rev. 0

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OPERATION

Shutdown and Power-On-Reset

The LT8357 enters shutdown mode and drains less than

3μA quiescent current when the EN/UVLO pin is below its

shutdown threshold (0.3V minimum). Once the EN/UVLO

pin is above its shutdown threshold (0.9V maximum),

the LT8357 wakes up the startup circuitry, generates the

bandgap reference, and powers up the internal INTVCC

LDO. The INTVCC LDO supplies the internal control cir-

cuitry and gate driver. The LT8357 then enters undervolt-

age lockout (UVLO) mode. When in UVLO mode, the part

is in a power-on-reset (POR) state, waking up the entire

internal control circuitry and settling to the right initial

conditions. When the INTVCC pin is charged above its

rising UVLO threshold (2.60V typical), the EN/UVLO pin

passes its rising enable threshold (1.220V typical), and

the junction temperature is less than the thermal shut-

down limit (165°C typical), the LT8357 enters enable

mode, and the part is ready and waiting for the control

signals to start switching.

The LT8357 has an accurate enable threshold (typically

1.220V rising and 1.178V falling) on the EN/UVLO pin

to enable the part. The accurate enable threshold allows

the user to program an undervoltage lockout (UVLO)

threshold with a resistor divider from the input supply

VIN to ground. When operating in Burst Mode, the current

flowing through the resistor divider can easily exceed the

quiescent current consumed by the part. Therefore, the

resistor divider values should be large enough to minimize

its effect on the efficiency at light load conditions.

Start-Up and Fault Protection

When the LT8357 is in the initial POR state, the SS pin

is hard pulled down to ground with a 55Ω internal resis-

tor. Once the part enters enable mode, the POR signal is

reset, but the SS pin is still hard pulled down to ground

and the part waits for 10μs so that all the internal circuits

and logic can settle to the correct states before initiating

the start-up sequence. After the 10μs, the SS pin is then

charged up by a 15μA pull-up current while the switch-

ing is disabled. When the SS pin is charged above 0.25V,

switching of the power switches is enabled and the start-

up of the output voltage VOUT is controlled by the voltage

on the SS pin. When the SS pin voltage is less than 1V,

the LT8357 regulates the FB pin voltage to the SS pin

voltage instead of the 1V reference. This allows the SS

pin to be used to program soft-start by connecting an

external capacitor from the SS pin to ground. The internal

15μA pull-up current charges up the capacitor, creating a

voltage ramp on the SS pin. As the SS pin voltage rises

linearly from 0.25V to 1V (and beyond), the output volt-

age VOUT rises smoothly to its final regulation voltage.

After passing 1V, the SS pin continues to be charged up

until exceeding 1.55V, then the start-up period is finished.

Before the SS pin rising above 1.55V, the control logic

forbids synchronizing from external clock and disables

the Burst Mode operation and spread spectrum frequency

modulation. After the SS pin rising above 1.55V, both

external frequency synchronization and operation mode

selection is controlled by the SYNC/MODE pin setting.

Once the switch over current fault is detected on the

SENSE pin, the LT8357 disables the switching immedi-

ately with the over current fault flag being latched, and

the SS pin is discharged by a 1.5μA pull-down current.

During the period when the over current fault is detected

and being latched, the part forbids the Burst Mode opera-

tion. Once the SS pin is discharged below 0.2V, the over

current fault flag is reset and the SS pin is charged up

by the 15μA pull-up current again. Once the SS pin rises

above 0.25V, switching is enabled and the soft start-up

is re-initiated again.

Once the FB pin voltage exceeds 8% (typical) above its

1V regulation voltage, the output overvoltage fault will be

triggered, which disables the switching immediately. After

the FB pin voltage drops below the overvoltage threshold,

the switching will be enabled again. No soft start-up will

be re-initiated in this fault case.

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