MIC2178_05 Datasheet, PDF (8/15 Pages) Micrel Semiconductor – 2.5A Synchronous Buck Regulator

MIC2178
(V_REF). When V_OUT is lower than its nominal value, the error
amplifier output voltage increases. This voltage then inter-
sects the current sense waveform later in switching period
which increases the duty cycle and the average inductor
current . If V_OUT is higher than nominal, the error amplifier
output voltage decreases, reducing the duty cycle.
The PWM control loop is stabilized in two ways. First, the
inner signal loop is compensated by adding a corrective ramp
to the output of the current sense amplifier. This allows the
regulator to remain stable when operating at greater than
50% duty cycle. Second, a series resistor-capacitor load is
connected to the error amplifier output (COMP pin). This
places a pole-zero pair in the regulator control loop.
One more important item is synchronous rectification. As
mentioned earlier, the N-channel output MOSFET is turned
on after the P-channel turns off. When the N-channel turns
on, its on-resistance is low enough to create a short across
the output diode. As a result, inductor current flows through
the N-channel and the voltage drop across it is significantly
lower than a diode forward voltage. This reduces power
dissipation and improves efficiency to greater than 95%
under certain operating conditions.
To prevent shoot through current, the output stage employs
break-before-make circuitry that provides approximately 50ns
of delay from the time one MOSFET turns off and the other
turns on. As a result, inductor current briefly flows through the
output diode during this transition.
Skip-Mode Operation
Refer to “Skip Mode Functional Diagram” which is a simplified
block diagram of the MIC2178 operating in skip mode and its
associated waveforms.
Skip-mode operation turns on the output P-channel at a
frequency and duty cycle that is a function of V_IN, V_OUT, and
the output inductor value. While in skip mode, the N-channel
is kept off to optimize efficiency by reducing gate charge
dissipation. V_OUT is regulated by skipping switching cycles
that turn on the P-channel.
To begin analyzing MIC2178 skip mode operation, assume
the skip-mode comparator output is high and the latch output
has been reset to a logic 1. This turns on the P-channel and
causes I_L1 to increase linearly until it reaches a current limit
of 600mA. When I_L1 reaches this value, the current limit
comparator sets the RS latch output to logic 0, turning off the
Micrel, Inc.
P-channel. The output switch voltage (V_SW) then swings from
V_IN to 0.4V below ground, and I_L1 flows through the Schottky
diode. L1 discharges its energy to the output and I_L1 de-
creases to zero. When I_L1 = 0, V_SW swings from –0.4V to
V_OUT, and this triggers a one-shot that resets the RS latch.
Resetting the RS latch turns on the P-channel, and this
begins another switching cycle.
The skip-mode comparator regulates V_OUT by controlling
when the MIC2178 skips cycles. It compares V_FB to V_REF and
has 10mV of hysteresis to prevent oscillations in the control
loop. When V_FB is less than V_REF –5mV, the comparator
output is logic 1, allowing the P-channel to turn on. Con-
versely, when V_FB is greater than V_REF + 5mV, the P-channel
is turned off.
Note that this is a self oscillating topology which explains why
the switching frequency and duty cycle are a function of V_IN,
V_OUT, and the value of L1. It has the unique feature (for a
pulse-skipping regulator) of supplying the same value of
maximum load current for any value of V_IN, V_OUT, or L1. This
allows the MIC2178 to always supply up to 300mA of load
current when operating in skip mode.
Selecting PWM- or Skip-Mode Operation
PWM or skip mode operation is selected by an external logic
signal applied to the PWM pin. A logic low places the
MIC2178 into PWM mode, and logic high places it into skip
mode. Skip mode operation provides the best efficiency
when load current is less than 200mA, and PWM operation is
more efficient at higher currents.
The MIC2178 was designed to be used in intelligent systems
that determine when it should operate in PWM or skip mode.
This makes the MIC2178 ideal for applications where a
regulator must guarantee low noise operation when supply-
ing light load currents, such as cellular telephone, audio, and
multimedia circuits.
There are two important items to be aware of when selecting
PWM or skip mode. First, the MIC2178 can start-up only in
PWM mode, and therefore requires a logic low at PWM during
start-up. Second, in skip mode, the MIC2178 will supply a
maximum load current of approximately 300mA, so the
output will drop out of regulation when load current exceeds
this limit. To prevent this from occurring, the MIC2178 should
change from skip to PWM mode when load current exceeds
200mA.
M9999-031805
8
March 2005