Datasheet

 2004-2013 Microchip Technology Inc. DS21921C-page 13
MCP1612
Solving for C:
There will also be some ripple voltage caused by the
ESR of the capacitor. The ripple is defined as follows.
EQUATION 5-7:
For this example:
5.2.4 INPUT CAPACITOR
For the buck topology, the input current is pulled from
the source and the input capacitor in pulses. The size
of the input capacitor will determine the amount of
current pulled from the source. For most applications,
a 10 µF ceramic capacitor connected between the
MCP1612’s V
IN
and P
GND
is recommended to filter the
current pulses. Less capacitance can be used for
applications that have low source impedance. The
ripple current rating for ceramic capacitors are typically
very high due to their low loss characteristics. Low-cost
electrolytic capacitors can be used, but their ripple
current rating should not be exceeded.
5.2.5 V
CC
INPUT
The V
CC
input is used to bias the internal MCP1612
circuitry. A 10 resistor is recommended between the
unregulated inputs V
IN
and V
CC
, along with a 0.1 µF
capacitor to ground to help isolate the V
CC
pin from the
switching noise.
5.2.6 COMPENSATION COMPONENTS
An internal transconductance error amplifier is used to
compensate the buck converter. An external resistor
(R
C
) and capacitor (C
C
), connected between COMP
and GND, are all that is needed to provide a high-
bandwidth loop.
Table 5-1 identifies values for R
C
and C
C
for standard
buck inductor (L) and output capacitor (C
OUT
) values.
5.3 Printed Circuit Board (PCB)
Layout
The MCP1612 is capable of switching over 1A at
1.4 MHz. As with all high-frequency switching power
supplies, good PCB layout techniques are essential to
prevent noise generated by the switching power-train
from interfering with the sensing circuitry.
There are two ground pins (P
GND
and A
GND
) on the
MCP1612 to separate the large-signal ground current
from the small-signal circuit ground. These two
grounds should be kept separate, only connecting near
the input bulk capacitor.
Care must also be taken to minimize the length and
loop area of the large signal connections. Components
connected to this loop consist of the input bulk
capacitor, V
IN
, P
GND
and L
X
pins of the MCP1612, the
buck inductor and the output filter capacitor.
I
C
= 165 mA
C=4.7µF
t = 260 ns
ESR = 8 m
V
C
= (260 ns x 165 mA)/4.7 µF
V
C
=9.13mV
V
ESRRIPPLE
=8m x 165 mA
V
ESRRIPPLE
=1.32mV
V
OUT
= V
C
+ V
ESRRIPPLE
V
OUT
=9.13mV + 1.32mV
V
OUT
= 10.45 mV
CI
C
t
V
C
-----------=
Where:
I
C
= peak-to-peak ripple current
t = on-time of P-channel MOSFET
V
C
= output ripple voltage
V
ESRRIPPLE
ESR I
C
=
TABLE 5-1: R
C
and C
C
VALUES
LC
OUT
R
C
C
C
3.3 µH 10.0 µF 25 k 1000 pF
2.2 µH 4.7 µF 10 k 1000 pF