Data Sheet

LT8645SA/LT8646SA

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Rev 0

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THEORY OF OPERATION

The LT8645SA/LT8646SA 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 internal 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 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

inductor current falls to zero. If overload conditions result in more than 12A flowing through the bottom switch, the

next clock cycle will be delayed until switch current returns to a safe level.

The S in LT8645SA/LT8646SA refers to the second-generation Silent Switcher technology. This technology allows fast

switching edges for high efficiency at high switching frequencies, while simultaneously achieving good EMI

performance. This includes the integration of ceramic capacitors into the package for VIN, BST, and INTVCC (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 LT8645SA/LT8646SA shuts down and draws approximately 1µA from the input. When the

EN/UV pin is above 1.01V, the switching regulator becomes active.

To optimize efficiency at light loads, the LT8645SA/LT8646SA 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 (LT8645SA) or 230μA (LT8646SA with BIAS = 0). In a typical application, 2.5μA

(LT8645SA) or 120μA (LT8646SA 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 LT8645SA/LT8646SA can operate in forced continuous mode (FCM) for fast transient response and full frequency

operation 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 LT8645SA/LT8646SA 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 LT8645SA/LT8646SA can operate in spread spectrum mode. This feature varies the clock with a

triangular frequency modulation of +20%. For example, if the LT8645SA/LT8646SA’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 (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 LT8645SA/LT8646SA output is programmed at 3.3V to 25V.

The VC pin optimizes the loop compensation of the switching regulator based on the programmed switching

frequency, allowing for a fast transient response. The VCpin also enables current sharing and a CLKOUT pin enables

synchronizing other regulators to the LT8646SA.

Comparators monitoring the FB pin voltage will pull the PG pin low if the output voltage varies more than ±8%

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