LT8636 datasheet(15/32 Pages) AD

15 / 32 page

LT8636/LT8637

Rev. C

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OPERATION

The LT8636/LT8637 is a monolithic, constant frequency,

current 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. 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 10A 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 LT8636/LT8637 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 LT8636/LT8637

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 (LT8636) or 230µA (LT8637 with BIAS

= 0). In a typical application, 2.5µA (LT8636) or 120µA

(LT8637 with BIAS = 5VOUT) 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 LT8636/LT8637 can operate in forced continuous

mode (FCM) for fast transient response and full fre-

quency operation over a wide load range. When in FCM

the oscillator operates continuously and positive SW tran-

sitions are aligned to the clock. Negative inductor current

is allowed. The LT8636/LT8637 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 LT8636/LT8637 can operate in spread

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

gular frequency modulation of +20%. For example, if the

LT8636/LT8637’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 3.3V or above. Else, the internal circuitry will draw

current from VIN. The BIAS pin should be connected to

VOUT if the LT8636/LT8637 output is programmed at 3.3V

to 25V.

The VC pin optimizes the loop compensation of the

switching regulator based on the programmed switch-

ing frequency, allowing for a fast transient response. The

VC pin also enables current sharing and a CLKOUT pin

enables synchronizing other regulators to the LT8637.

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 LT8636/LT8637’s operating

frequency when the voltage at the FB pin is low. This

frequency foldback helps to control the inductor current

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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