March 2005 11 M9999-031805
MIC2178 Micrel, Inc.
Application Information
Feedback Resistor Selection (Adjustable Version)
The output voltage is programmed by connecting an external
resistive divider to the FB pin as shown in “MIC2178 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 with the following formula:
R1 = R2
V
1.245V
OUT
– 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 =
I
RMS(max)
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 VIN pin
(within 0.2in, 5mm). Also, place a 0.1µF ceramic bypass
capacitor as close as possible to VIN.
Inductor Selection
The MIC2178 is a current-mode controller with internal slope
compensation. As a result, the inductor must be at least a
minimum value to prevent subharmonic oscillations. This
minimum value is calculated by the following formula:
L = V 3.0 H/V
MIN
OUT
×µ
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 increase significantly. Both of these
effects will degrade efficiency. The formula for peak inductor
current is:
I = I
I
2
L(peak)
LOAD(max)
L(max)
+
∆
Where:
∆I = V 1
V
V
5s
L
L(max)
OUT
OUT
IN(max)
−
×
µ
To maximize efficiency, the inductor’s resistance must be
less than the output switch on-resistance (preferably,
50mΩ or 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
x ESR. Therefore, ESR
must be equal or less than a maximum value calculated for a
specified V
RIPPLE
(typically less than 1% of the output volt-
age) and ∆I
L(max)
:
ESR =
V
I
MAX
RIPPLE
L(max)
∆
Typically, capacitors in the range of 100 to 220µF have ESR
less than this maximum value. The output capacitor can be
a low ESR electrolytic or tantalum capacitor, but tantalum is
a better choice for compact layout and operation at tempera-
tures 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 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 condi-
tions is a 1A diode with a reverse voltage rating greater than
V
IN
. It must be a Schottky or ultrafast-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, the 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 to 10nF for the capacitor and 5kΩ to 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 the
MIC2178. A good design takes into consideration compo-
nent placement and routing of power traces.
The first thing to consider is the locations of the input
capacitor, inductor, output diode, and output capacitor. The
input capacitor must be placed very close to the VIN pin, the
inductor and output diode very close to the SW pin, and the
output capacitor near the inductor. These components pass
large high-frequency current pulses, so they must use short,
wide power traces. In addition, their ground pins and PGND
are connected to a ground plane that is nearest the power
supply ground bus.
