MIC2177_08 Datasheet, PDF (13/15 Pages) Micrel Semiconductor – 2.5A Synchronous Buck Regulator

Micrel, Inc.
MIC2177
Application Information
Feedback Resistor Selection (Adjustable Version)
The output voltage is configured by connecting an
external resistive divider to the FB pin as shown in
“MIC2177 Block Diagram.” The ratio of R1 to R2
determines the output voltage. To optimize efficiency
during low output current operation, R2 should not be
less than 20kΩ. However, to prevent feedback error due
to input bias current at the FB pin, R2 should not be
greater than 100kΩ. After selecting R2, calculate R1
using the following formula:
R1
=
R2_{⎢⎣⎡⎜⎜⎝⎛}
V_OUT
1.245V
⎟⎟⎠⎞
−
⎤
1_⎥
⎦
Input Capacitor Selection
The input capacitor is selected for its RMS current and
voltage rating and should be a low ESR (equivalent
series resistance) electrolytic or tantalum capacitor. As a
rule-of-thumb, the voltage rating for a tantalum capacitor
should be twice the value of V_IN, and the voltage rating
for an electrolytic should be 40% higher than V_IN. The
RMS current rating must be equal or greater than the
maximum RMS input ripple current. A simple, worst-case
formula for calculating this RMS current is:
I_RMS(max)
=
I_LOAD(max)
2
Tantalum capacitors are a better choice for applications
that require the most compact layout or operation below
0°C. The input capacitor must be located very close to
the V_IN pin (within 0.2 inches, 5mm). Also place a 0.1µF
ceramic bypass capacitor as close as possible to V_IN.
Inductor Selection
The inductor must be at least a minimum value in order
for the MIC2177 to change from PWM to skip mode at
the correct value of output current. This minimum value
ensures the inductor ripple current never exceeds
600mA, and is calculated using the following formula:
L _MIN
=
V_{OUT ⎜⎜⎝⎛}1 −
V_OUT
V_IN(max)
⎟⎞
⎟⎠
×
8.3µ.3µ
Where:
V_IN(max) = maximum input voltage
In general, a value at least 20% greater than L_MIN should
be selected because inductor values have a tolerance of
±20%.
Two other parameters to consider in selecting an
inductor are winding resistance and peak current rating.
The inductor must have a peak current rating equal or
greater than the peak inductor current. Otherwise, the
inductor may saturate, causing excessive current in the
output switch. Also, the inductor’s core loss may
April 2008
increase significantly. Both of these effects will degrade
efficiency. The formula for peak inducto rcurrent is:
I_L(peak) = I_LOAD(max) + 300mA
To maximize efficiency, the inductor’s resistance must
be less than the output switch on-resistance (preferably
50mΩor less).
Output Capacitor Selection
Select an output capacitor that has a low value of ESR.
This parameter determines a regulator’s output ripple
voltage (V_RIPPLE) which is generated by ∆I_L × ESR. As
mentioned in “Inductor Selection,” the maximum value
for ∆I_L is 600mA.
Therefore, the maximum value of ESR is:
ESR _MAX
=
600mA
V_RIPPLE
Where:
V_RIPPLE < 1% of V_OUT
Typically, capacitors in the range of 100µF to 220µF
have ESR less than this maximum value. The output
capacitor can be either a low ESR electrolytic or
tantalum capacitor, but tantalum is a better choice for
compact layout and operation at temperatures below
0°C. The voltage rating of a tantalum capacitor must be
2 × V_OUT, and the voltage rating of an electrolytic must
be 1.4 × V_OUT.
Output Diode Selection
In PWM operation, inductor current flows through the
output diode approximately 50ns during the dead time
when one output MOSFET turns off and the other turns
on. In skip-mode, the inductor current flows through the
diode during the entire P-channel off time. The correct
diode for both of these conditions is a 1A diode with a
reverse voltage rating greater than V_IN. It must be a
Schottky or ultra fast-recovery diode (t_R<100ns) to
minimize power dissipation from the diode’s reverse-
recovery charge.
Compensation
Compensation is provided by connecting a series RC
load to the COMP pin. This creates a pole-zero pair in
the regulator control loop, allowing the regulator to
remain stable with enough low frequency loop-gain for
good load and line regulation. At higher frequencies
pole-zero reduces loop-gain to a level referred to as the
mid-band gain. The mid-band gain is low enough so that
the loop gain crosses 0dB with sufficient phase margin.
Typical values for the RC load are 4.7nF – 10nF for the
capacitor and 5kΩ – 20kΩ for the resistor.
Printed Circuit Board Layout
A well designed PC board will prevent switching noise
and ground bounce from interfering with the operation of
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M9999-042108