MIC2177

Micrel

MIC2177

8

April 1999

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

the output inductor value. While in skip mode, the N-channel

is kept off to optimize efficiency by reducing gate charge

dissipation. V

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

of 600mA. When I

comparator sets the RS latch output to logic 0, turning off the

P-channel. The output switch voltage (V

V

diode. L1 discharges its energy to the output and I

creases to zero. When I

V

Resetting the RS latch turns on the P-channel, which begins

another switching cycle.

The skip-mode comparator regulates V

when the MIC2177 skips cycles. It compares V

has 10mV of hysteresis to prevent oscillations in the control

loop. When V

output is logic 1, allowing the P-channel to turn on. Con-

versely, when V

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

a pulse-skipping regulator) of supplying the same value of

maximum load current for any value of V

allows the MIC2177 to always supply up to 300mA of load

current (I

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

determined indirectly by detecting when the peak inductor

current (I

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

given by the following equation:

I

420mA

I

MIN

L1

=

−∆

2

Where:

∆I

This equation shows I

fore, the user must select an inductor value that results in

I

culating the correct inductor value are given in the “Applica-

tions Information” section. Note that

∆I

of input voltage, and this also causes I

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

operation. This compensates for

∆I

zero when the difference between V

Changing from Skip to PWM Mode

The MIC2177 will automatically change from skip to PWM

mode when I

tion, it can supply up to 300mA, and when I

limit, V

MIC2177 begins operating in PWM mode. Note that the

maximum value of I

minimum value required for PWM mode. This current hyster-

esis prevents the MIC2177 from toggling between modes

when I

The low output comparator determines when V

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 10

µA 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.