MIC2177_08 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – 2.5A Synchronous Buck Regulator

Micrel, Inc.
discharged from the inductor and I_L1 decreases until the
next switching cycle begins. By varying the P-channel
on-time (duty cycle), the average inductor current is
adjusted to whatever value is required to regulate the
output voltage.
The MIC2177 uses current-mode control to adjust the
duty cycle and regulate the output voltage. Current-
mode control has two signal loops that determine the
duty cycle. One is an outer loop that senses the output
voltage, and the other is a faster inner loop that senses
the inductor current. Signals from these two loops
control the duty cycle in the following way: V_OUT is fed
back to the error amplifier which compares the feedback
voltage (V_FB) to an internal reference voltage (V_REF).
When V_OUT is lower than its nominal value, the error
amplifier output voltage increases. This voltage then
intersects the current-sense waveform later in switching
period which increases the duty cycle and 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 MIC2177 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
April 2008
MIC2177
switching cycles that turn on the P-channel.
To begin analyzing MIC2177 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 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, which
begins another switching cycle.
The skip-mode comparator regulates V_OUT by controlling
when the MIC2177 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. Conversely, 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 MIC2177 to
always supply up to 300mA of load current (I_LOAD) when
operating in skip mode.
Changing from PWM to Skip Mode
Refer to “Block Diagram” for circuits described in the
following sections.
The MIC2177 automatically changes from PWM to skip
mode operation when I_LOAD drops below a minimum
value. I_MIN is determined indirectly by detecting when the
peak inductor current (I_L(peak)) is less than 420mA. This is
done by the minimum current comparator which detects
if the output P-Channel current equals 420mA during
each switching cycle. If it does not, the PWM/skip-mode
select logic places the MIC2177 into skip-mode
operation.
The value of I_MIN that corresponds to I_L1(peak) = 420mA is
given by the following equation:
I_MIN
=
420mA −
2
∆I_L1
Where:
∆I_L1 = inductor ripple current
This equation shows I_MIN varies as a function of ∆I_L.
Therefore, the user must select an inductor value that
results in I_MIN = 200mA when I_L(peak) = 420mA. The
formulas for calculating the correct inductor value are
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M9999-042108