LT8350RVPBF datasheet(14/24 Pages) AD

14 / 24 page

LT8350

Rev. 0

For more information www.analog.com

OPERATION

up from ground. From the time of entering enable mode to

the time of VREF passing its rising UVLO threshold (1.89V

typical), the LT8350 is going through a power-on-reset

(POR), waking up the entire internal control circuitry and

settling to the right initial conditions. After the POR, the

LT8350 is ready and waiting for the signals on the CTRL

and LOADEN pins to start switching.

Start-Up and Fault Protection

Figure 8 shows the start-up and fault sequence for the

LT8350. During the POR state, the SS pin is hard pulled

down with a 130Ω to ground. In a pre-biased condition,

the SS pin has to be pulled below 0.2V to enter the INIT

state, where the LT8350 waits 10µs so that the SS pin can

be fully discharged to ground. After the 10µs, the LT8350

enters the UP/PRE state when the LOADON signal goes

high. The LOADON high signal happens when CTRL pin

is above its CTRL OFF thresholds (0.225V typical) and the

LOADEN is high.

During the UP/PRE state, the SS pin is charged up by a

12.5µA pull-up current while the switching is disabled

and the LOADTG is turned off. Once the SS pin is charged

above 0.25V, the LT8350 enters the UP/TRY state, where

the LOADTG is turned on first while the switching is still

disabled. If an excessive current flowing through the

current sense resistor triggers the ISP/ISN over current

(ISOC) signal, it will reset the LT8350 back into the POR

state. After 10µs in the UP/TRY state without triggering

the ISOC signal, the LT8350 enters the UP/RUN state.

During the UP/RUN state, the switching 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 LT8350 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 GND. The internal

12.5µA pull-up current charges up the capacitor, creating

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

region, switch D is turned off whenever the boost reverse

current threshold is triggered during (A+D) phase, and

both switches B and D are turned off whenever the buck

reverse current threshold is triggered during (B+D) phase.

As the load becomes lower and lower, or when a smaller

value inductor is used and the inductor current ripple

is bigger, the LT8350 may run in pulse-skipping mode,

where the switches are held off for multiple cycles (i.e.,

skipping pulses) to maintain the regulation.

Internal Charge Path

Each of the two high side gate drivers is biased from

its floating bootstrap capacitor CBST1 and CBST2, which

is normally recharged by INTVCC through the integrated

bootstrap diode D1 and D2 when the bottom power

switches are turned on. When the LT8350 operates exclu-

sively in the buck or boost regions, one of the top power

switches is constantly on. Internal charge paths, from

VOUT and BST2 to BST1, or from VIN and BST1 to BST2,

charge the bootstrap capacitors to above 3.3V so that the

top power switch can be kept on.

Shutdown and Power-On-Reset

The LT8350 enters shutdown mode and drains less than

2µA (typical) 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 maxi-

mum), the LT8350 wakes up startup circuitry, generates

bandgap reference, and powers up the internal INTVCC

LDO. The INTVCC LDO supplies the internal control cir-

cuitry and gate drivers. Now the LT8350 enters under-

voltage lockout (UVLO) mode with a hysteresis current

(2.5µA typical) pulled into the EN/UVLO pin. When the

INTVCC pin is charged above its rising UVLO threshold

(2.52V typical), and the EN/UVLO pin passes its rising

enable threshold (1.235V typical), and the junction tem-

perature is less than its thermal shutdown (165°C typical),

the LT8350 enters enable mode, in which the EN/UVLO

hysteresis current is turned off and VREF is being charged

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