June 1998

7

MIC2178

MIC2178

Micrel

Functional Description

Micrel’s MIC2178 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 2.5A load current and

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

operation. To optimize efficiency, the MIC2178 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. PWM or skip-mode operation

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 MIC2178 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 synchro-

nized 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 follow-

ing sections for more detail.

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 100m

Ω

when the MIC2178 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 MIC2178 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 (V

the output MOSFETs. It prevents the output from turning on

until V

shut off until V

Thermal Shutdown

Thermal shutdown turns off the output when the MIC2178

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 MIC2178 has a low-current shutdown mode that is

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

to EN, the MIC2178 is in shutdown mode, and its quiescent

current drops to less than 5

µA.

Internal Bias Regulator

An internal 3.3V regulator provides power to the MIC2178

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

pin for bypassing by an external 0.01

µF capacitor. Do not

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

provide an external supply voltage.

Frequency Synchronization

The MIC2178 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.

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,

signaling that V

that can sink 1mA from a pull-up resistor connected to V

Low-Dropout Operation

Output regulation is maintained in PWM or skip mode even

when the difference between V

1V. As V

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 V

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 MIC2178 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

N-channel. During this time, inductor current (I

and energy is stored in the inductor. The current sense

amplifier (I

source voltage and outputs a voltage proportional to I

output of I

(corrective ramp) generated by the oscillator, creating a

composite waveform labeled I

When I

PWM comparator will set the RS latch which turns off the P-

channel and turns on the N-channel. Energy is then dis-

charged from the inductor and I

switching cycle begins. 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 MIC2178 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

way: V

the feedback voltage (V