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MIC2177

2.5A Synchronous Buck Regulator

Micrel 

Micrel, Inc.

Functional Description

Micrel’s MIC2177 is a synchronous buck regulator that

operates from an input voltage of 4.5V to 16.5V and

provides a regulated output voltage of 1.25V to 16.5V. It

has internal power MOSFETs that supply up to 2.5A of

load current and operates with up to 100% duty cycle to

allow low-dropout operation. To optimize efficiency, the

MIC2177 operates in PWM and skip mode. Skip mode

provides the best efficiency when load current is less

than 200mA, while PWM mode is more efficient at higher

current. A patented technique allows the MIC2177 to

automatically select the correct operating mode as the

load current changes.

During PWM operation, the MIC2177 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, undervoltage lockout, and thermal

shutdown. See the following sections for details.

Switch Output

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

high-side P-channel and low-side N-channel power

MOSFET. These MOSFETs have a typical on-resistance

of 100mΩ when the MIC2177 operates from a 12V

supply. Anti-shoot-through circuitry prevents the P-

channel and N-channel from turning on at the same

time.

Current Limit

The MIC2177 uses pulse-by-pulse current limiting to

protect the output. During each switching period, a

current limit comparator detects if the P-channel current

exceeds 4.7A. 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 input voltage (VIN) is too low to provide sufficient gate

drive for the output MOSFETs. It prevents the output

from turning on until VIN exceeds 4.3V. Once operating,

the output will not shut off until VIN drops below 4.2V.

Thermal Shutdown

Thermal shutdown turns off the output when the

MIC2177 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.

MIC2177

Shutdown Mode

The MIC2177 has a low-current shutdown mode that is

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

applied to EN, the MIC2177 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

MIC2177 control circuits. This internal supply is brought

out to the BIAS pin for bypassing by an external 0.01µF

capacitor. Do not connect any external load to the BIAS

pin. It is not designed to provide an external supply

voltage.

Frequency Synchronization

The MIC2177 operates at a preset switching frequency

of 200kHz. It can be synchronized to a higher frequency

by connecting 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–300kHz, and can

operate with a minimum pulse-width of 500ns. If

synchronization is not required, connect SYNC to

ground.

Low-Dropout Operation

Output regulation is maintained in PWM or skip mode

even when the difference between VIN and VOUT

decreases below 1V. As VIN – VOUT decreases, the duty

cycle increases until it reaches 100%. At this point, the

P-channel is kept on for several cycles at a time, and the

output stays in regulation until VIN – VOUT falls below the

dropout voltage (dropout voltage = P-channel on

resistance × load current).

PWM-Mode Operation

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

simplified block diagram of the MIC2177 operating in

PWM mode with 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 N-channel. During

this time, inductor current (IL1) increases and energy is

stored in the inductor. The current sense amplifier (ISENSE

Amp) measures the P-channel drain-to-source voltage

and outputs a voltage proportional to IL1. The output of

ISENSE Amp is added to a saw tooth waveform (corrective

ramp) generated by the oscillator, creating a composite

waveform labeled ISENSE on the timing diagram. When

ISENSE is 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

April 2008

8

M9999-042108

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