LT8640 datasheet(13/28 Pages) AD

13 / 28 page

LT8640A

Rev. 0

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OPERATION

The LT8640A is a monolithic, constant frequency, cur-

rent mode step-down DC/DC converter. An oscillator, with

frequency set using a resistor on the RT pin, turns on

the internal 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 current 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 VFB pin with an inter-

nal 0.97V 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. The LT8640A-3.3 fixed output part uses the

VOUT pin and an internal resistor divider to generate an

internal FB node. 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 more than 9A flowing through the

bottom switch, the next clock cycle will be delayed until

switch current returns to a safe level.

If the EN/UV pin is low, the LT8640A is shut down and

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

1V, the switching regulator will become active.

To optimize efficiency at light loads, the LT8640A operates

in Burst Mode operation in light load situations. 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, 2.5µA will be consumed

from the input supply when regulating with no load. The

SYNC/MODE pin is tied low to use Burst Mode operation

and can be floated to use forced continuous mode (FCM).

If a clock is applied to the SYNC/MODE pin, the part will

synchronize to an external clock frequency and operate

in FCM.

The LT8640A can operate in forced continuous mode

(FCM) for fast transient response and full frequency oper-

ation over a wide load range. When in FCM the oscillator

operates continuously and positive SW transitions are

aligned to the clock. Negative inductor current is allowed.

The LT8640A can sink current from the output and return

this charge to the input in this mode, improving load step

transient response.

To improve EMI, the LT8640A can operate in spread

spectrum mode. This feature varies the clock with a tri-

angular frequency modulation of +20%. For example, if

the LT8640A’s frequency is programmed to switch at

2MHz, spread spectrum mode will modulate the oscillator

between 2MHz and 2.4MHz. The SYNC/MODE pin should

be tied high to INTVCC (~3.4V) or an external supply of 3V

to 4V to enable spread spectrum modulation with forced

continuous mode.

To improve efficiency across all loads, supply current to

internal circuitry can be sourced from the BIAS pin when

biased at 3.1V or above. Else, the internal circuitry will

draw current from VIN. The BIAS pin should be connected

to VOUT if the LT8640A output is programmed at 3.3V

to 25V.

Comparators monitoring the FB pin voltage will pull the PG

pin low if the output voltage varies more than ±8% (typi-

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

The oscillator reduces the LT8640A’s operating frequency

when the voltage at the FB pin is low, or the voltage at the

VOUT pin is low on the LT8640A-3.3 fixed voltage option.

This frequency foldback helps to control the inductor cur-

rent when the output voltage is lower than the programmed

value which occurs during start-up or overcurrent condi-

tions. When a clock is applied to the SYNC/MODE pin, the

SYNC/MODE pin is floated, or held DC high, the frequency

foldback is disabled and the switching frequency will slow

down only during overcurrent conditions.

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