June 2009

7

M9999-063009

Micrel, Inc.

MIC2179

Functional Description

Micrel’s MIC2179 is a synchronous buck regulator that oper-

ates from an input voltage of 4.5V to 16.5V and provides a

regulated output voltage of 1.25V to 16.5V. Its has internal

power MOSFETs that supply up to 1.5A load current and

operates with up to 100% duty cycle to allow low-dropout

operation. To optimize efficiency, the MIC2179 operates in

PWM and skip mode. Skip mode provides the best efficiency

when load current is less than 150mA, while PWM mode is

more efficient at higher current. PWM or skip-mode opera-

tion is selected externally, allowing an intelligent system (i.e.

microprocessor controlled) to select the correct operating

mode for efficiency and noise requirements.

During PWM operation, the MIC2179 uses current-mode

control which provides superior line regulation and makes

the control loop easier to compensate. The PWM switching

frequency is set internally to 200kHz and can be synchronized

to an external clock frequency up to 300kHz. Other features

include a low-current shutdown mode, current limit, under-

voltage lockout, and thermal shutdown. See the following

sections for more detail.

Switch Output

The switch output (SW) is a half H-bridge consisting of a

high-sideP-channelandlow-sideN-channelpowerMOSFET.

TheseMOSFETshaveatypicalon-resistanceof150mΩwhen

the MIC2179 operates from a 12V supply. Antishoot-through

circuitry prevents the P-channel and N-channel from turning

on at the same time.

Current Limit

The MIC2179 uses pulse-by-pulse current limiting to protect

the output. During each switching period, a current limit com-

parator detects if the P-Channel current exceeds 4.3A. When

it does, the P-channel is turned off until the next switching

period begins.

Undervoltage Lockout

Undervoltage lockout (UVLO) turns off the output when the

for the output MOSFETs. It prevents the output from turning

Thermal Shutdown

Thermal shutdown turns off the output when the MIC2179

junction temperature exceeds the maximum value for safe

operation. After thermal shutdown occurs, the output will not

turn on until the junction temperature drops approximately

10°C.

Shutdown Mode

The MIC2179 has a low-current shutdown mode that is con-

trolled by the enable input (EN). When a logic 0 is applied

to EN, the MIC2179 is in shutdown mode, and its quiescent

current drops to less than 5µA.

Internal Bias Regulator

An internal 3.3V regulator provides power to the MIC2179

control circuits. This internal supply is brought out to the BIAS

pin for bypassing by an external 0.01µF capacitor. Do not

connect an external load to the BIAS pin. It is not designed

to provide an external supply voltage.

Frequency Synchronization

The MIC2179 operates at a preset switching frequency of

200kHz. It can be synchronized to a higher frequency by con-

necting an external clock to the SYNC pin. The SYNC pin is

a logic level input that synchronizes the oscillator to the rising

edge of an external clock signal. It has a frequency range of

220kHz to 300kHz, and can operate with a minimum pulse

width of 500ns. If synchronization is not required, connect

SYNC to ground.

Power Good Flag

The power good flag (PWRGD) is an error flag that alerts a

system when the output is not in regulation. When the output

voltage is 10% below its nominal value, PWRGD is logic low,

Low-Dropout Operation

Output regulation is maintained in PWM or skip mode even

it reaches 100%. At this point, the P-channel is kept on for

several cycles at a time, and the output stays in regulation

voltage = P-channel on-resistance × load current).

PWM-Mode Operation

Refer to “PWM Mode Functional Diagram” which is a simpli-

fied block diagram of the MIC2179 operating in PWM mode

and its associated waveforms.

When operating in PWM mode, the output P-channel and N-

channel MOSFETs are alternately switched on at a constant

frequency and variable duty cycle. A switching period begins

when the oscillator generates a reset pulse. This pulse resets

the RS latch which turns on the P-channel and turns off the

and energy is stored in the inductor. The current sense ampli-

ramp) generated by the oscillator, creating a composite wave-

greater than the error amplifier output, the PWM comparator

will set the RS latch which turns off the P-channel and turns

on the N-channel. Energy is then discharged from the induc-

By varying the P-channel on-time (duty cycle), the average

inductor current is adjusted to whatever value is required to

regulate the output voltage.

The MIC2179 uses current-mode control to adjust the duty

cycle and regulate the output voltage. Current-mode control

has two signal loops that determine the duty cycle. One is an

outer loop that senses the output voltage, and the other is

a faster inner loop that senses the inductor current. Signals

from these two loops control the duty cycle in the following