MIC2159_09 Datasheet, PDF (6/16 Pages) Micrel Semiconductor – SYNCHRONOUS-itty Step-Down Converter IC

Micrel, Inc.
Block Diagram
MIC2159
MIC2159 Block Diagram
Functional Description
The MIC2159 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 400kHz switching regulator.
Theory of Operation
The MIC2159 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 1v
triangular waveform. The comparator generates a
rectangular waveform whose width tON 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, assume the
output voltage drops due to sudden load turn-on, this
would cause the inverting input of the error amplifier,
which is the divided down version of VOUT, 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
MOSFET, causing the output voltage to go up and
bringing VOUT back in regulation. If this sudden load
transient was large enough to cause a 3% change in
output voltage, then the output of the Hysteretic
comparator will bypass the PWM comparator and drive
the LSD and HSD outputs at full Duty cycle in order to
recover the nominal output voltage in the fastest manner
possible whilst fixed frequency PWM switching is
maintained during normal loading.
Soft-Start
The COMP/EN pin on the MIC2159 is used for the
following three functions:
1. Disables the part by grounding this pin.
2. External compensation to stabilize the voltage
control loop.
3. Soft-start.
March 2009
6
M9999-030509-B