2003-2013 Microchip Technology Inc. DS21762B-page 13
MCP1601
5.2.1.1 Input
For all BUCK-derived topologies, the input current is
pulled from the source in pulses, placing some burden
on the input capacitor. For most applications, a 10 µF
ceramic capacitor connected to the MCP1601 input is
recommended to filter the current pulses. Less capaci-
tance can be used for applications that have low source
impedance. The ripple current ratings for ceramic
capacitors are typically very high due to their low loss
characteristics. Lower-cost electrolytic capacitors can
be used, but ripple current ratings should not be
exceeded.
5.2.1.2 Output
For BUCK-derived topologies, the output capacitor fil-
ters the continuous AC inductor ripple current while
operating in the PWM mode. Typical inductor AC ripple
current for the MCP1601 is 120 mA peak-to-peak with
a 3.6V input, 10 µH inductor for a 1.8V output applica-
tion. Using an output capacitor with 0.3 of ESR, the
output ripple will be approximately 36 mV.
The recommended range for the output capacitor is
from 10 µF (±20%) to 47 µF (±20%). Larger value
capacitors can be used, but require evaluation of the
control system stability.
EQUATION
The above equation assumes that the output capaci-
tance is large enough so that the ripple voltage (as a
result of charging and discharging the capacitor) is
negligible and can be used for applications that use
electrolytic capacitors with esr > 0.3
When using a 10 µF ceramic X5R dielectric capacitor,
the output ripple voltage is typically less than 10 mV.
5.2.2 BUCK INDUCTOR
There are many suppliers and choices for selecting the
BUCK inductor. The application, physical size require-
ments (height vs. area), current rating, resistance,
mounting method, temperature range, minimum induc-
tance and cost all need to be considered in making the
best choice.
When choosing an inductor for the MCP1601 Synchro-
nous BUCK, there are two primary electrical
specifications to consider.
1. Current rating of the inductor.
2. Resistance of the inductor.
When selecting a BUCK inductor, many suppliers
specify a maximum peak current.
The maximum peak inductor current is equal to the
maximum DC output current plus 1/2 the peak-to-peak
AC ripple current in the inductor. The AC ripple current
in the inductor can be calculated using the following
relationship.
EQUATION
Solving for I
L
:
EQUATION
Example:
The approximate “on” time is equal to the
Duty Cycle (V
OUT
/ V
IN
) x 1/F
SW
.
Many suppliers of inductors rate the maximum RMS
(Root Mean Square) current. The BUCK inductor RMS
current is dependent on the output current, inductance,
input voltage, output voltage and switching frequency.
For the MCP1601, the inductor RMS current over the
2.7V to 5.5V input range, 0.9V to 5V output voltage
range is no more than 15% higher than the average DC
output current for the minimum recommended induc-
tance of 10 µH ±20%. When selecting an inductor that
has a maximum RMS current rating, use a simple
approximation that the RMS current is 1.2 times the
maximum output current.
Example:
I
OUT(MAX)
= 300 mA, the inductor should have an RMS
rating > 360 mA (1.2 x I
OUT(MAX)
).
V
Ripple
I
LRipple
C
OUTesr
=
V
IN
=3.6V
V
OUT
=1.8V
F
SW
=750kHz
I
OUT(MAX)
=300mA
T
ON
= (1.8V/3.6V) x 1/(750 kHz)
T
ON
= 667 ns
V
L
= 3.6V - 1.8V = 1.8V
I
L
= (1.8V/10 µH) x 667 ns
I
L
=120mA
I
L(PEAK)
=I
OUTMAX
+ 1/2 I
L
I
L(PEAK)
= 300 mA + (120 mA) / 2
I
L(PEAK)
=360mA
I
L
V
L
Lt=
Where: t is equal to the “on” time of the P-Channel
switch and,
V
L
= the voltage across the inductor
(V
IN
- V
OUT
)
