MIC2179-5.0BSM Micrel Inc, MIC2179-5.0BSM Datasheet - Page 7

• Manufacturer Part Number: MIC2179-5.0BSM
• Description: IC REG SYNCBUCK 1.5A 5.0V 20SSOP
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck)r

Specifications of MIC2179-5.0BSM

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 1.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 4.5 ~ 16.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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-side P-channel and low-side N-channel power MOSFET.
These MOSFETs have a typical on-resistance of 150mΩ when
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
input voltage (V
for the output MOSFETs. It prevents the output from turning
on until V
not shut off until V
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
June 2009
IN
exceeds 4.3V. Once operating, the output will
IN
) is to low to provide sufficient gate drive
IN
drops below 4.2V.
7
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,
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 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
N-channel. During this time, inductor current (I
and energy is stored in the inductor. The current sense ampli-
fier (I
voltage and outputs a voltage proportional to I
of I
ramp) generated by the oscillator, creating a composite wave-
form labeled I
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-
tor and I
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
way: V
the feedback voltage (V
SENSE
SENSE
IN
OUT
IN
L1
– V
Amp is added to a sawtooth waveform (corrective
– V
decreases until the next switching cycle begins.
is fed back to the error amplifier which compares
Amp) measures the P-channel drain-to-source
OUT
SENSE
OUT
OUT
falls below the dropout voltage (dropout
decreases, the duty cycle increases until
is to low. PWRGD is an open-drain output
on the timing diagram. When I
FB
) to an internal reference voltage
IN
and V
OUT
decreases below
L1
M9999-063009
L1
. The output
) increases
SENSE
IN
is
.