Block Diagram
20034610
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.
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 output voltage (VOUT) can be programmed by 2 external
resistors. It can be calculated as following.
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.
*
VOUT = 1.242 ( R1 + R2 ) / R2
2 external resistors can easily program the output voltage.
The output can be set in a wide range from 1.242V to VIN
.
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.
20034623
FIGURE 1. Hysteretic Window
9
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