MIC2177BWM datasheet(10/15 Pages) MICREL

10 / 15 page

Micrel, Inc.

MIC2177

April 2008

M9999-042108

given in the “Applications Information” section. Note that

∆IL varies as a function of input voltage, and this also

causes IMIN to vary. In applications where the input

voltage changes by a factor of two, IMIN will typically vary

from 130mA to 250mA.

During low-dropout operation, the minimum current

thresh-old circuit reduces the minimum value of IL1(peak)

for PWM operation. This compensates for ∆IL1

decreasing to almost zero when the difference between

VIN and VOUT is very low.

Changing from Skip to PWM Mode

The MIC2177 will automatically change from skip to

PWM mode when ILOAD exceeds 300mA. During skip-

mode operation, it can supply up to 300mA, and when

ILOAD exceeds this limit, VOUT will fall below its nominal

value. At this point, the MIC2177 begins operating in

PWM mode. Note that the maximum value of ILOAD for

skip mode is greater than the minimum value required

for PWM mode. This current hysteresis prevents the

MIC2177 from toggling between modes when ILOAD is in

the range of 100mA to 300mA.

The low output comparator determines when VOUT 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.

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