LT8604 datasheet(10/20 Pages) AD

10 / 20 page

LT8604

Rev. 0

For more information www.analog.com

The LT8604 is a monolithic constant-frequency current

mode step-down DC/DC converter. Operation is best

understood by referring to the Block Diagram. An internal

oscillator turns on the integrated top power switch at the

beginning of each clock cycle. Current in the inductor then

increases until the top switch current comparator trips

and turns off the top power switch. The peak inductor cur-

rent at which the top switch turns off is controlled by the

voltage on the internal VC node. The error amplifier servos

the VC node by comparing the voltage on the FB pin with

an internal reference. When the load current increases, it

causes a reduction in the feedback voltage relative to the

reference leading the error amplifier to raise the VC volt-

age until the average inductor current matches the new

load current. When the top power switch turns off, the

synchronous power switch turns on until the next clock

cycle begins or inductor current falls to zero. If overload

conditions result in excess current flowing through the

bottom switch, the next clock cycle will be delayed until

switch current returns to a safe level.

To optimize efficiency, the LT8604 enters Burst Mode

operation at light loads. Between bursts, all circuitry

associated with controlling the output switch is shut down

reducing the input supply current to 1.7µA. In a typical

application with a 24V input, 2.5µA will be consumed from

the input supply when regulating with no load.

To improve efficiency across all loads, supply current to

internal circuitry can be sourced from the BIAS pin when

biased at 3.2V or above. Else, the internal circuitry will

draw current from VIN. The BIAS pin should be connected

to VOUT if the LT8604 output is programmed to a voltage

between 3.3V and 25V.

Comparators monitoring the FB pin voltage will pull the PG

pin low if the output voltage varies more than ±7.5% (typi-

cal) from the set point, or if a fault condition is present.

In the LT8604, the oscillator reduces its operating fre-

quency when the voltage at the FB pin is low. This fre-

quency foldback helps to control the inductor current

when the output voltage is lower than the programmed

value which occurs during start-up.

If the EN/UV pin is low, the LT8604 is shut down and

draws 1µA from the input. When the EN/UV pin is above

1.05V, the switching regulator becomes active.

OPERATION

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