March 2005 7 M9999-031805
MIC2178 Micrel, Inc.
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 100mΩ
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
IN
) is to low to provide sufficient gate drive for
the output MOSFETs. It prevents the output from turning on
until V
IN
exceeds 4.3V. Once operating, the output will not
shut off until V
IN
drops below 4.2V.
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 5µA.
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 0.01µF capacitor. Do not
connect an external load to the BIAS pin. It is not designed to
provide an external supply voltage.
Frequency Synchronization
The 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
OUT
is to low. PWRGD is an open-drain output
that can sink 1mA from a pull-up resistor connected to V
IN
.
Low-Dropout Operation
Output regulation is maintained in PWM or skip mode even
when the difference between V
IN
and V
OUT
decreases below
1V. As V
IN
– V
OUT
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 V
IN
– V
OUT
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 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
L1
) increases
and energy is stored in the inductor. The current sense
amplifier (I
SENSE
Amp) measures the P-channel drain-to-
source voltage and outputs a voltage proportional to I
L1
. The
output of I
SENSE
Amp is added to a sawtooth waveform
(corrective ramp) generated by the oscillator, creating a
composite waveform labeled I
SENSE
on the timing diagram.
When I
SENSE
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 dis-
charged from the inductor and I
L1
decreases until the next
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
OUT
is fed back to the error amplifier which compares
the feedback voltage (V
FB
) to an internal reference voltage
