MIC2177-5.0BWM Micrel Inc, MIC2177-5.0BWM Datasheet - Page 9

• Manufacturer Part Number: MIC2177-5.0BWM
• Description: IC REG SYNCBUCK 2.5A 5.0V 20SOIC
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck)r

Specifications of MIC2177-5.0BWM

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 5V
• 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: -

discharged from the inductor and I
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 MIC2177 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
back to the error amplifier which compares the feedback
voltage (V
When V
amplifier output voltage increases. This voltage then
intersects the current-sense waveform later in switching
period which increases the duty cycle and average
inductor current. If V
amplifier 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 MIC2177 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
and the output inductor value. While in skip mode, the N-
channel is kept off to optimize efficiency by reducing
gate charge dissipation. V
Micrel, Inc.
April 2008
OUT
FB
) to an internal reference voltage (V
is lower than its nominal value, the error
OUT
is higher than nominal, the error
OUT
is regulated by skipping
L1
decreases until the
OUT
IN
, V
is fed
REF
OUT
).
,
9
switching cycles that turn on the P-channel.
To begin analyzing MIC2177 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
reaches a current limit of 600mA. When I
value, the current limit comparator sets the RS latch
output to logic 0, turning off the P-channel. The output
switch voltage (V
ground, and I
discharges its energy to the output and I
zero. When I
this triggers a one-shot that resets the RS latch.
Resetting the RS latch turns on the P-channel, which
begins another switching cycle.
The skip-mode comparator regulates V
when the MIC2177 skips cycles. It compares V
and has 10mV of hysteresis to prevent oscillations in the
control loop. When V
comparator output is logic 1, allowing the P-channel to
turn on. Conversely, when V
5mV, the P-channel 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
unique feature (for a pulse-skipping regulator) of
supplying the same value of maximum load current for
any value of V
always supply up to 300mA of load current (I
operating in skip mode.
Changing from PWM to Skip Mode
Refer to “Block Diagram” for circuits described in the
following sections.
The MIC2177 automatically changes from PWM to skip
mode operation when I
value. I
peak inductor current (I
done by the minimum current comparator which detects
if the output P-Channel current equals 420mA during
each switching cycle. If it does not, the PWM/skip-mode
select logic places the MIC2177 into skip-mode
operation.
The value of I
given by the following equation:
Where:
This equation shows I
Therefore, the user must select an inductor value that
results in I
formulas for calculating the correct inductor value are
MIN
∆I
I
MIN
L1
is determined indirectly by detecting when the
MIN
= inductor ripple current
=
L1
IN
MIN
IN
L1
, V
420mA
= 0, V
, V
= 200mA when I
SW
flows through the Schottky diode. L1
that corresponds to I
OUT
OUT
) then swings from V
2
, and the value of L1. It has the
SW
, or L1. This allows the MIC2177 to
FB
−
L(peak)
MIN
swings from –0.4V to V
∆I
LOAD
is less than V
L1
varies as a function of ∆I
L1
) is less than 420mA. This is
FB
to increase linearly until it
drops below a minimum
is greater than V
L(peak)
L1(peak)
OUT
IN
L1
REF
= 420mA. The
M9999-042108
L1
to 0.4V below
de-creases to
by controlling
reaches this
– 5mV, the
= 420mA is
LOAD
MIC2177
FB
OUT
to V
) when
REF
, and
REF
L
+
.