MIC2179YSM 데이터 시트보기 (PDF) - Micrel

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MIC2179YSM

1.5A Synchronous Buck Regulator

Micrel 

Micrel, Inc.

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

amplifier output voltage increases. This voltage then intersects

the current sense waveform later in switching period which

increases the duty cycle and the average inductor current. If

VOUT 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 dis-

sipation 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 MIC2179 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 VIN, VOUT, and

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

nel is kept off to optimize efficiency by reducing gate charge

dissipation. VOUT is regulated by skipping switching cycles

that turn on the P-channel.

To begin analyzing MIC2179 skip mode operation, assume

the skip-mode comparator output is high and the latch out-

put has been reset to a logic 1. This turns on the P-channel

and causes IL1 to increase linearly until it reaches a current

limit of 400mA. When IL1 reaches this value, the current limit

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

MIC2179

the P-channel. The output switch voltage (VSW) then swings

from VIN to 0.4V below ground, and IL1 flows through the

Schottky diode. L1 discharges its energy to the output and

IL1 decreases to zero. When IL1 = 0, VSW swings from –0.4V

to VOUT, 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 VOUT by controlling

when the MIC2179 skips cycles. It compares VFB to VREF

and has 10mV of hysteresis to prevent oscillations in the

control loop. When VFB is less than VREF – 5mV, the com-

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

Conversely, when VFB 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 VIN, VOUT, 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 VIN, VOUT, or L1.

This allows the MIC2179 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

MIC2179 into PWM mode, and logic high places it into skip

mode. Skip mode operation provides the best efficiency when

load current is less than 150mA, and PWM operation is more

efficient at higher currents.

The MIC2179 was designed to be used in intelligent sys-

tems that determine when it should operate in PWM or skip

mode. This makes the MIC2179 ideal for applications where

a regulator must guarantee low noise operation when sup-

plying 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 MIC2179 can start-up only in

PWM mode, and therefore requires a logic low at PWM dur-

ing start-up. Second, in skip mode, the MIC2179 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 MIC2179 should

change from skip to PWM mode when load current exceeds

200mA.

June 2009

8

M9999-063009

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