MIC2169B Datasheet, PDF (8/24 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

Micrel, Inc.
Functional Diagram
MIC2169B
MIC2169B Block Diagram
Functional Description
The MIC2169B is a voltage mode, synchronous step-
down switching regulator controller designed for high
power. Current limit is implemented without the use of an
external sense resistor. It includes an internal soft-start
function which reduces the power supply input surge
current at start-up by controlling the output voltage rise
time, a PWM generator, a reference voltage, two
MOSFET drivers, and short-circuit current limiting
circuitry to form a complete 500kHz switching regulator.
MIC2169B is identical to the MIC2169A except it
supports pre-bias loads.
Theory of Operation
The MIC2169B is a voltage mode step-down regulator.
The figure above illustrates the block diagram for the
voltage control loop. The output voltage variation due to
load or line changes will be sensed by the inverting input
of the transconductance error amplifier via the feedback
resistors R3, and R2 and compared to a reference
voltage at the non-inverting input. This will cause a small
change in the DC voltage level at the output of the error
amplifier which is the input to the PWM comparator. The
other input to the comparator is a 0.95V to 1.45V
triangular waveform. The comparator generates a
rectangular waveform whose width t_ON is equal to the
time from the start of the clock cycle t₀ until t₁, the time
the triangle crosses the output waveform of the error
amplifier. To illustrate the control loop, let us assume the
output voltage drops due to sudden load turn-on, this
would cause the inverting input of the error amplifier,
which is divided down version of V_OUT, to be slightly less
than the reference voltage, causing the output voltage of
the error amplifier to go high. This will cause the PWM
comparator to increase t_ON time of the top side
June 2009
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M9999- 060309-A
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