Block Diagram

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Operation Description

OVERVIEW

The LM3475 is a buck (step-down) DC-DC controller that

uses a hysteretic control architecture, which results in Pulse

Frequency Modulated (PFM) regulation. The hysteretic con-

trol scheme does not utilize an internal oscillator. Switching

frequency depends on external components and operating

conditions. Operating frequency decreases at light loads,

resulting in excellent efficiency compared to PWM architec-

tures. Because switching is directly controlled by the output

conditions, hysteretic control provides exceptional load tran-

sient response.

HYSTERETIC CONTROL CIRCUIT

The LM3475 uses a comparator-based voltage control loop.

The voltage on the feedback pin is compared to a 0.8V

reference with 21mV of hysteresis. When the FB input to the

comparator falls below the reference voltage, the output of

the comparator goes low. 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 C

supplies current to the load through the PFET and the induc-

tor. Current through the inductor ramps up linearly, and the

output voltage increases. As the FB voltage reaches the

upper threshold (reference voltage plus hysteresis) the out-

put of the comparator goes high, and the PGATE turns the

PFET off. When the PFET turns off, the catch diode turns on,

and the current through the inductor ramps down. As the

output voltage falls below the reference voltage, the cycle

repeats. The resulting output, inductor current, and switch

node waveforms are shown in Figure 1.

The LM3475 operates in discontinuous conduction mode at

light load current and 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. The next cycle starts when the FB

voltage reaches the reference voltage. Until then, the induc-

tor current remains zero. Operating frequency is low, as are

switching losses. In continuous conduction mode, current

always flows through the inductor and never ramps down to

zero.

SOFT-START

The LM3475 includes an internal soft-start function to protect

components from excessive inrush current and output volt-

age overshoot. As V

bias circuitry becomes active. When EN goes high, the

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FIGURE 1. Hysteretic Waveforms

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