MCP1640/B/C/D
5.3
Input Capacitor Selection
5.0
5.1
APPLICATION INFORMATION
Typical Applications
The boost input current is smoothed by the boost
inductor reducing the amount of filtering necessary at
the input. Some capacitance is recommended to
provide decoupling from the source. Low ESR X5R or
X7R are well suited since they have a low temperature
coefficient and small size. For most applications,
4.7 µF of capacitance is sufficient at the input. For high
power applications that have high source impedance or
long leads, connecting the battery to the input 10 µF of
capacitance is recommended. Additional input
capacitance can be added to provide a stable input
voltage.
The MCP1640/B/C/D synchronous boost regulator
operates over a wide input voltage and output voltage
range. The power efficiency is high for several decades
of load range. Output current capability increases with
input voltage and decreases with increasing output
voltage. The maximum output current is based on the
N-Channel peak current limit. Typical characterization
curves in this data sheet are presented to display the
typical output current capability.
Table 5-1 contains the recommended range for the
input capacitor value.
5.2
Adjustable Output Voltage
Calculations
To calculate the resistor divider values for the
MCP1640/B/C/D, the following equation can be used.
Where RTOP is connected to VOUT, RBOT is connected
to GND and both are connected to the FB input pin.
5.4
Output Capacitor Selection
The output capacitor helps provide a stable output
voltage during sudden load transients and reduces the
output voltage ripple. As with the input capacitor, X5R
and X7R ceramic capacitors are well suited for this
application.
EQUATION 5-1:
VOUT
The MCP1640/B/C/D is internally compensated so
output capacitance range is limited. See Table 5-1 for
the recommended output capacitor range.
– 1
------------
RTOP = RBOT
VFB
While the N-Channel switch is on, the output current is
supplied by the output capacitor COUT. The amount of
output capacitance and equivalent series resistance
will have a significant effect on the output ripple
voltage. While COUT provides load current, a voltage
drop also appears across its internal ESR that results
in ripple voltage.
Example A:
VOUT = 3.3V
VFB = 1.21V
RBOT = 309 k
RTOP = 533.7 k (Standard Value = 536 k)
Example B:
EQUATION 5-2:
VOUT = 5.0V
VFB = 1.21V
dV
dt
IOUT = COUT ------
RBOT = 309 k
RTOP = 967.9 k (Standard Value = 976 k)
Where dV represents the ripple voltage and dt
represents the ON time of the N-Channel switch (D * 1/
FSW).
There are some potential issues with higher value
resistors. For small surface mount resistors,
environment contamination can create leakage paths
that significantly change the resistor divider that effect
the output voltage. The FB input leakage current can
also impact the divider and change the output voltage
tolerance.
Table 5-1 contains the recommended range for the
input and output capacitor value.
TABLE 5-1:
CAPACITOR VALUE RANGE
CIN
COUT
Min
4.7 µF
none
10 µF
Max
100 µF
2010 Microchip Technology Inc.
DS22234A-page 15
MICROCHIP [ MICROCHIP ]
