MIC2178-3.3BWM Micrel Inc, MIC2178-3.3BWM Datasheet - Page 8

• Manufacturer Part Number: MIC2178-3.3BWM
• Description: IC REG SYNCBCK 2.5A 3.3V 20SOIC
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck)r

Specifications of MIC2178-3.3BWM

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 3.3V
• Current - Output: 2.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 4.5 ~ 16.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

MIC2178
(V
amplifier output voltage increases. This voltage then inter-
sects the current sense waveform later in switching period
which increases the duty cycle and the average inductor
current . If V
output voltage decreases, reducing the duty cycle.
The PWM control loop is stabilized in two ways. First, the
inner signal loop is compensated by adding a corrective ramp
to the output of the current sense amplifier. This allows the
regulator to remain stable when operating at greater than
50% duty cycle. Second, a series resistor-capacitor load is
connected to the error amplifier output (COMP pin). This
places a pole-zero pair in the regulator control loop.
One more important item is synchronous rectification. As
mentioned earlier, the N-channel output MOSFET is turned
on after the P-channel turns off. When the N-channel turns
on, its on-resistance is low enough to create a short across
the output diode. As a result, inductor current flows through
the N-channel and the voltage drop across it is significantly
lower than a diode forward voltage. This reduces power
dissipation and improves efficiency to greater than 95%
under certain operating conditions.
To prevent shoot through current, the output stage employs
break-before-make circuitry that provides approximately 50ns
of delay from the time one MOSFET turns off and the other
turns on. As a result, inductor current briefly flows through the
output diode during this transition.
Skip-Mode Operation
Refer to “Skip Mode Functional Diagram” which is a simplified
block diagram of the MIC2178 operating in skip mode and its
associated waveforms.
Skip-mode operation turns on the output P-channel at a
frequency and duty cycle that is a function of V
the output inductor value. While in skip mode, the N-channel
is kept off to optimize efficiency by reducing gate charge
dissipation. V
that turn on the P-channel.
To begin analyzing MIC2178 skip mode operation, assume
the skip-mode comparator output is high and the latch output
has been reset to a logic 1. This turns on the P-channel and
causes I
of 600mA. When I
comparator sets the RS latch output to logic 0, turning off the
M9999-031805
REF
). When V
L1
to increase linearly until it reaches a current limit
OUT
OUT
OUT
is higher than nominal, the error amplifier
is regulated by skipping switching cycles
L1
is lower than its nominal value, the error
reaches this value, the current limit
IN
, V
OUT
, and
8
P-channel. The output switch voltage (V
V
diode. L1 discharges its energy to the output and I
creases to zero. When I
V
Resetting the RS latch turns on the P-channel, and this
begins another switching cycle.
The skip-mode comparator regulates V
when the MIC2178 skips cycles. It compares V
has 10mV of hysteresis to prevent oscillations in the control
loop. When V
output is logic 1, allowing the P-channel to turn on. Con-
versely, when V
is turned off.
Note that this is a self oscillating topology which explains why
the switching frequency and duty cycle are a function of V
V
pulse-skipping regulator) of supplying the same value of
maximum load current for any value of V
allows the MIC2178 to always supply up to 300mA of load
current when operating in skip mode.
Selecting PWM- or Skip-Mode Operation
PWM or skip mode operation is selected by an external logic
signal applied to the PWM pin. A logic low places the
MIC2178 into PWM mode, and logic high places it into skip
mode. Skip mode operation provides the best efficiency
when load current is less than 200mA, and PWM operation is
more efficient at higher currents.
The MIC2178 was designed to be used in intelligent systems
that determine when it should operate in PWM or skip mode.
This makes the MIC2178 ideal for applications where a
regulator must guarantee low noise operation when supply-
ing light load currents, such as cellular telephone, audio, and
multimedia circuits.
There are two important items to be aware of when selecting
PWM or skip mode. First, the MIC2178 can start-up only in
PWM mode, and therefore requires a logic low at PWM during
start-up. Second, in skip mode, the MIC2178 will supply a
maximum load current of approximately 300mA, so the
output will drop out of regulation when load current exceeds
this limit. To prevent this from occurring, the MIC2178 should
change from skip to PWM mode when load current exceeds
200mA.
IN
OUT
OUT
to 0.4V below ground, and I
, and this triggers a one-shot that resets the RS latch.
, and the value of L1. It has the unique feature (for a
FB
FB
is less than V
is greater than V
L1
= 0, V
L1
REF
flows through the Schottky
SW
REF
–5mV, the comparator
swings from –0.4V to
+ 5mV, the P-channel
SW
IN
OUT
, V
) then swings from
OUT
FB
by controlling
March 2005
to V
, or L1. This
Micrel, Inc.
REF
L1
and
de-
IN
,