LT8610AB datasheet(14/28 Pages) AD

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LT8638S

Rev. 0

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OPERATION

The LT8638S 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 internal

0.6V 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 voltage 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 in Burst Mode operation, inductor current falls to zero.

If overload conditions result in more than 15.5A flowing

through the bottom switch, the next clock cycle will be

delayed until switch current returns to a safe level.

The “S” in LT8638S refers to the second generation silent

switcher technology. This technology allows fast switch-

ing edges for high efficiency at high switching frequen-

cies, while simultaneously achieving good EMI perfor-

mance. This includes the integration of ceramic capacitors

into the package for VIN (see Block Diagram). These caps

keep all the fast AC current loops small, which improves

EMI performance.

If the EN/UV pin is low, the LT8638S is shut down and

draws approximately 6µA from the input. When the

EN/UV pin is above 0.98V, the switching regulator will

become active.

To optimize efficiency at light loads, the LT8638S operates

in Burst Mode operation in light load situations. Between

bursts, all circuitry associated with controlling the out-

put switch is shut down, reducing the input supply cur-

rent to 125µA (BIAS = 0). In a typical application, 90µA

(VIN = 12V, 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 LT8638S 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 LT8638S 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 LT8638S can operate in spread spec-

trum mode. This feature varies the clock with a triangu-

lar frequency modulation of +24%. For example, if the

LT8638S’s frequency is programmed to switch at 2MHz,

spread spectrum mode will modulate the oscillator between

2MHz and approximately 2.5MHz. The SYNC/MODE pin

should be tied high to INTVCC (or >3V) 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 LT8638S output is programmed at 3.3V to 25V.

The VC pin allows the loop compensation of the switch-

ing regulator to be optimized based on the programmed

switching frequency, allowing for a fast transient response.

The VC and CLKOUT pins enable multiple LT8638S reg-

ulators to run with interleaving phase shift, reducing the

amount of required input and output capacitors. The

PHMODE pin selects the phasing of CLKOUT for different

multiphase applications.

Comparators monitoring the FB pin voltage will pull

the PG pin low if the output voltage varies more than

±7.75% (typical) from the set point, or if a fault condition

is present.

The oscillator reduces the LT8638S device’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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