MIC2178-5.0BWM datasheet(8/16 Pages) MICREL

8 / 16 page

MIC2178

Micrel

MIC2178

June 1998

REF). When VOUT is lower than its nominal value, the error

amplifier output voltage increases. This voltage then inter-

sects the current sense waveform later in switching period

which increases the duty cycle and the 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 MIC2178 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, VOUT, 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 MIC2178 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

IN to 0.4V below ground, and IL1 flows through the Schottky

diode. L1 discharges its energy to the output and I

L1 de-

creases to zero. When I

L1 = 0, VSW swings from –0.4V to

OUT, and this triggers a one-shot that resets the RS latch.

Resetting the RS latch turns on the P-channel, and this

begins another switching cycle.

The skip-mode comparator regulates V

OUT by controlling

when the MIC2178 skips cycles. It compares V

FB to VREF and

has 10mV of hysteresis to prevent oscillations in the control

loop. When V

FB is less than VREF –5mV, the comparator

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

versely, when V

FB is greater than VREF + 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,

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, VOUT, or L1. This

allows the MIC2178 to always supply up to 300mA of load

current when operating in skip mode.

Selecting PWM- or Skip-Mode Operation

PWM or skip mode operation is selected by an external logic

signal applied to the PWM pin. A logic low places the

MIC2178 into PWM mode, and logic high places it into skip

mode. Skip mode operation provides the best efficiency

when load current is less than 200mA, and PWM operation is

more efficient at higher currents.

The MIC2178 was designed to be used in intelligent systems

that determine when it should operate in PWM or skip mode.

This makes the MIC2178 ideal for applications where a

regulator must guarantee low noise operation when supply-

ing light load currents, such as cellular telephone, audio, and

multimedia circuits.

There are two important items to be aware of when selecting

PWM or skip mode. First, the MIC2178 can start-up only in

PWM mode, and therefore requires a logic low at PWM during

start-up. Second, in skip mode, the MIC2178 will supply a

maximum load current of approximately 300mA, so the

output will drop out of regulation when load current exceeds

this limit. To prevent this from occurring, the MIC2178 should

change from skip to PWM mode when load current exceeds

200mA.

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