Block Diagram

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Functional Description

Overview

The LM3485 is buck (step-down) DC-DC controller that uses

a hysteretic control scheme. The comparator is designed

with approximately 10mV of hysteresis. In response to the

voltage at the FB pin, the gate drive (PGATE pin) turns the

external PFET on or off. When the inductor current is too

high, the current limit protection circuit engages and turns

the PFET off for approximately 9µs.

The hysteretic control does not provide an internal oscillator.

Switching frequency depends on the external components

and operating conditions. Operating frequency reduces at

light loads resulting in excellent efficiency compared to other

architectures.

2 external resistors can easily program the output voltage.

The output can be set in a wide range from 1.242V to V

Hysteretic Control Circuit

The LM3485 uses a comparator based voltage control loop.

The feedback is compared to a 1.242V reference and a

10mV hysteresis is designed into the comparator to ensure

noise free operation.

When the FB input to the comparator falls below the refer-

ence voltage, the output of the comparator moves to a low

state. This results in the driver output, PGATE, pulling the

gate of the PFET low and turning on the PFET. With the

PFET on, the input supply charges Cout and supplies cur-

rent to the load via the series path through the PFET and the

inductor. Current through the Inductor ramps up linearly and

the output voltage increases. As the FB voltage reaches the

upper threshold, which is the internal reference voltage plus

10mV, the output of the comparator changes from low to

high, and the PGATE responds by turning the PFET off. As

the PFET turns off, the inductor voltage reverses, the catch

diode turns on, and the current through the inductor ramps

down. Then, as the output voltage reaches the internal ref-

erence voltage again, the next cycle starts.

The LM3485 operates in discontinuous conduction mode at

light load current or continuous conduction mode at heavy

load current. In discontinuous conduction mode, current

through the inductor starts at zero and ramps up to the peak,

then ramps down to zero. Next cycle starts when the FB

voltage reaches the internal voltage. Until then, the inductor

current remains zero. Operating frequency is lower and

switching losses reduce. In continuous conduction mode,

current always flows through the inductor and never ramps

down to zero.

The output voltage (V

resistors. It can be calculated as following.

V

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FIGURE 1. Hysteretic Window

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