LT3045 datasheet(19/45 Pages) AD

19 / 45 page

Data Sheet

LT83201

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Rev. A 19 of 45

THEORY OF OPERATION

The LT83201 is a constant-frequency, current-mode, monolithic step-down regulator, operating using a current

reference-based architecture to allow 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 the discontinuous conduction mode). If overload conditions result in more than 1.7A flowing through the

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

The LT83201 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, resulting in virtually constant ultra-low output noise

independent of the output voltage.

If the EN/UVLO pin is below 0.2V, the LT83201 shuts down and draws 40μA from the input. When the EN/UVLO pin

rises above 0.75V, the switching regulator becomes active.

To improve efficiency at light loads, connect the LT83201 SYNC/MODE pin to ground for Discontinuous Conduction

Mode (DCM), which allows discontinuous conduction at light loads. The LT83201 provides smooth transitions

between continuous conduction and discontinuous conduction operation in this mode. In discontinuous mode the

LT83201 retains full frequency operation down to light loads for predictable noise.

The SYNC/MODE pin is connected to INTVCC or to a voltage higher than 1.5V or floated to use Forced Continuous Mode

(FCM). If a clock is applied to the SYNC pin, the part synchronizes to an external clock frequency and operates in FCM.

The LT83201 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 LT83201 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.

LT3045 Datasheet(PDF) 19 Page - Analog Devices