MIC2177 Datasheet, PDF (8/16 Pages) Micrel Semiconductor – 2.5A Synchronous Buck Regulator

MIC2177
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 simpli-
fied 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 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. 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 MIC2177 to always supply up to 300mA of load
current (I_LOAD) when operating in skip mode.
Micrel
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 − ∆I_L1
2
Where:
∆I_L1 = inductor ripple current
This equation shows I_MIN varies as a function of ∆I_L . There-
fore, the user must select an inductor value that results in
I_MIN = 200mA when I_L(peak) = 420mA. The formulas for cal-
culating the correct inductor value are given in the “Applica-
tions Information” section. Note that ∆I_L varies as a function
of input voltage, and this also causes I_MIN to vary. In applica-
tions where the input voltage changes by a factor of two, I_MIN
will typically vary from 130mA to 250mA.
During low-dropout operation, the minimum current thresh-
old circuit reduces the minimum value of I_L1(peak) for PWM
operation. This compensates for ∆I_L1 decreasing to almost
zero when the difference between V_IN and V_OUT is very low.
Changing from Skip to PWM Mode
The MIC2177 will automatically change from skip to PWM
mode when I_LOAD exceeds 300mA. During skip-mode opera-
tion, it can supply up to 300mA, and when I_LOAD exceeds this
limit, V_OUT will fall below its nominal value. At this point, the
MIC2177 begins operating in PWM mode. Note that the
maximum value of I_LOAD for skip mode is greater than the
minimum value required for PWM mode. This current hyster-
esis prevents the MIC2177 from toggling between modes
when I_LOAD is in the range of 100mA to 300mA.
The low output comparator determines when V_OUT is low
enough for the regulator to change operating modes. It
detects when the feedback voltage is 3% below nominal, and
pulls the AUTO pin to ground. When AUTO is less than 1.6V,
the PWM/skip-mode select logic places the MIC2177 into
PWM operation. The external 2.2nF capacitor connected to
AUTO is charged by a 10µA current source after the regulator
begins operating in PWM mode. As a result, AUTO stays
below 1.6V for several switching cycles after PWM operation
begins, forcing the MIC2177 to remain in PWM mode during
this transition.
External PWM-Mode Selection
The MIC2177 can be forced to operate in only PWM mode by
connecting AUTO to ground. This prevents skip-mode opera-
tion in applications that are sensitive to switching noise.
MIC2177
8
April 1999