LT8228 datasheet(29/69 Pages) AD

29 / 69 page

LT8228

Rev. A

For more information www.analog.com

Figure 4. Automatic Mode of Operation

OPERATION

regulation mode based on the UV1, UV2, FB1 and FB2 pin

voltages. When external logic is used, include the pull-up

resistor for cases where the external logic is accidently

disconnected for increased system reliability. This allows

the LT8228’s auto-selection of the operation mode to take

over. In buck mode, the DRXN pin goes high impedance

which externally pulls the pin voltage high through the

pull-up resistor. In boost mode, a typically 100μA pull-

down is enabled which pulls the DRXN pin low.

When the LT8228 is enabled, and the DRXN pin is con-

figured for auto-selection for the mode of operation, the

DRXN pin is high impedance until the internal regula-

tors are functional. The LT8228 then selects the mode of

operation based on the logic shown in Figure 4. If the UV1

pin voltage is higher than 1.2V, the controller is in buck

mode operation. If the UV1 pin voltage is lower than 1.2V,

the controller goes into boost mode. During buck mode

operation, if the UV1 pin voltage drops lower than 1.2V or

FB2 pin voltage stays higher than 1.3V for 1024 switch-

ing cycles, the LT8228 changes mode of operation from

buck to boost. Additional time requirement for the FB2

overvoltage ensures no mode hopping during transients

at the load. In boost mode operation if the UV2 pin volt-

age drops lower than 1.2V or FB1 pin voltage stays higher

than 1.3V for 1024 switching cycles, the LT8228 changes

mode of operation from boost to buck. Anytime both UV1

and UV2 pin voltage drops below 1.2V or both FB1 and

FB2 pin voltage stays higher than 1.3V for 1024 switching

cycles, the controller goes to buck mode operation, stops

switching, pulls down on the FAULT pin and report the

fault at the REPORT pin. For input undervoltage fault, the

SS pin is also pulled low. Anytime DRVCC or INTVCC pin

voltages fall below their respective undervoltage thresh-

old, the part goes to buck mode, the FAULT and SS pin

pulls low and the LT8228 does not switch.

During startup or fast transient, output overshoot higher

than 10% is possible at light load. If the overshoot con-

dition last more than 1024-clock cycle, the DRXN will

change state. Ensure minimum loading to avoid unin-

tended DRXN change.

When the DRXN pin is driven high with external logic

for buck mode operation, and the UV1 pin voltage

drops lower than 1.2V or FB2 pin voltage stays higher

than 1.3V for 1024 switching cycles, the LT8228 stops

switching, pulls down on the FAULT and report the fault

at the REPORT pin. When the DRXN pin is driven low with

external logic for boost mode operation, and the UV2 pin

voltage drops lower than 1.2V or FB1 pin voltage stays

higher than 1.3V for 1024 switching cycles, the LT8228

stops switching, pulls down on the FAULT and report the

fault at the REPORT pin. For input undervoltage fault, the

SS pin is also pulled low.

When multiple LT8228s are in parallel, tie all the DRXN

pins together to operate all LT8228s in the same regu-

lation mode. In the parallel configuration, the common

DRXN node must be pulled up to an external voltage

source through a pull-up resistor. If an external voltage

source is not available, each LT8228 needs its own pull-up

resistor in series with a diode whose anode is connected

to its own INTVCC pin. This diode prevents unintentional

boost mode selection when one or more channels are

disabled. Refer to the Paralleling Multiple LT8228s in

Applications Information section for more information.

ENABLE AND SOFT-START (ENABLE AND SS)

The LT8228 enters shutdown through the ENABLE pin.

Pulling this pin below 1.2V typically disables the control-

ler and most of the internal circuitry. Pulling the ENABLE

pin below 0.5V transitions the LT8228 into complete

shutdown where the controller only consumes 2µA of

BUCK

MODE

BOOST

MODE

UV1 < 1.2V

FB2 > 1.3 FOR 1024 CYCLES

START-UP

(BUCK MODE)

UV1 > 1.2V

UV1 < 1.2V

FAULT

(BUCK MODE)

UV2 < 1.2V

FB1 > 1.3 FOR 1024 CYCLES

UV1 > 1.2V

OR FB2 < 1.3V

UV1 AND UV2 < 1.2V

FB1 AND FB2 > 1.3 FOR 1024 CYCLES

UV2 > 1.2V

OR FB1 < 1.3V

8228 F04

LT8228 Datasheet(PDF) 29 Page - Analog Devices

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