LM3475 Datasheet, PDF (7/14 Pages) National Semiconductor (TI) – Hysteretic PFET Buck Controller

Block Diagram
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_OUT and
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.
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FIGURE 1. Hysteretic Waveforms
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_IN rises above 2.7V (typical), the internal
bias circuitry becomes active. When EN goes high, the
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