Data Sheet

LT83401/LT83402

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Rev. 0

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THEORY OF OPERATION

The LT83401/LT83402 is a constant-frequency, current-mode, monolithic step-down regulator that operates using a

current reference-based architecture, allowing the employment of unity gain to minimize output noise across all

output voltages. An oscillator, with the frequency set using a resistor on the RT pin, turns on the internal top power

switch at the beginning of each clock cycle. The current in the inductor 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 VC pin. The error amplifier servos the VC node by comparing the voltage on the OUTS

pin to the reference voltage on the SET pin, which is set by the user with a resistor from the SET pin to the ground.

When the load current increases, it causes a reduction in the OUTS 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 the inductor current falls

to zero (only in pulse-skipping mode). If overload conditions result in more than 1.5A (LT83401)/2.4A (LT83402)

flowing through the bottom switch, the next clock cycle is delayed until the switch current returns to a safe level.

The LT83401/LT83402 features third-generation Silent Switcher technology, which combines an ultra-low-noise

current reference with previous-generation Silent Switcher technology. The output voltage can be programmed with

a single resistor, providing unity-gain operation over the output range, and resulting in virtually constant ultra-low

output noise independent of the output voltage.

If the EN/UVLO pin is below 0.2V, the LT83401/LT83402 shuts down and draws 45μA from the input. When the

EN/UVLO pin rises above 0.75V, the switching regulator becomes active.

To improve efficiency at light loads, the LT83401/LT83402 can operate in pulse-skipping mode in light load

situations. The SYNC/MODE pin is connected to ground to use pulse-skipping operation and connected to INTVCC or

to a voltage higher than 1.5V or floated to use FCM. If a clock is applied to the SYNC pin, the part synchronizes to an

external clock frequency and operates in FCM.

The LT83401/LT83402 can operate in 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. In this mode, the LT83401/LT83402 can sink current from the output and return

this charge to the input, improving load-step transient response.

The VC pin allows the loop compensation of the switching regulator to be optimized based on the programmed

switching frequency, allowing for a fast transient response.