MIC2177_08 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – 2.5A Synchronous Buck Regulator

Micrel, Inc.
given in the “Applications Information” section. Note that
∆I_L varies as a function of input voltage, and this also
causes I_MIN to vary. In applications 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 operation, 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 hysteresis prevents the
MIC2177 from toggling between modes when I_LOAD is in
the range of 100mA to 300mA.
MIC2177
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 operation in applications that are sensitive to
switching noise.
April 2008
10
M9999-042108