FAN5665_08 Datasheet, PDF (12/15 Pages) Fairchild Semiconductor – High-Efficiency, Adaptive Charge Pump 5V Boost

Applications Information
The high switching frequency of 1.2MHz allows the use
of small capacitors, but the material of the capacitor
affects the input and output ripple, so the low-ESR
capacitors are desirable. Another parameter affected by
ESR is the efficiency. For proper operation, two ceramic
bucket capacitors, along with one ceramic input
capacitor and one ceramic output capacitor, are
recommended (as shown in Figure 1).
Input Capacitor Selection
In general, the ripple on the input power rail also affects
the output ripple. The lower the ESR of the input
capacitor, the lower the input and output ripple. The
input capacitor may need to be adjusted, both in its
value and in its physical placement on the PCB,
depending upon the characteristics of the voltage
source proving the input power. In general, a 1 to 2µF
ceramic capacitor placed close to the FAN5665
suffices. X5R and X7R capacitors provide adequate
performance over -40ºC to +85ºC. The following table
represents typical recommended input capacitors.
Description
Part Number
Capacitor
2.2µF, 10%,
6.3V, X5R,
0603
GRM188R60J225KE19D
Capacitor
2.2µF, 20%,
6.3V, X5R,
0402
GRM155R60J225ME15D
Vendor
Murata
Murata
Output Capacitor Selection
In general, multilayer ceramic capacitors are
recommended for low ESR. The value of the output bulk
capacitance in relation to the switching frequency of the
converter also determines the overall output voltage
ripple. A higher value of output capacitance reduces the
voltage droop during load transients. Typically, the
output capacitor can be 5 to 50 times larger than the
bucket capacitor(s), depending on the desired output
ripple tolerance. Both X5R- and X7R-rated capacitors
provide adequate performance over a -40ºC to +85ºC
temperature range. The following table contains typical
recommended output capacitors.
Description
Part Number
Vendor
Capacitor
2.2µF, 10%,
6.3V, X5R,
0603
GRM188R60J225KE19D Murata
Capacitor
2.2µF, 20%,
6.3V, X5R,
0402
GRM155R60J225ME15D Murata
Bucket Capacitor Selection
The bucket (flying) capacitor is usually the smallest
capacitor in a charge pump circuit, but its ESR can play
a significant role in determining the output voltage
tolerance. Only ceramic capacitors are recommended in
this position. The flying capacitance is determined by
the switching frequency. At 1.2MHz, 0.22µF flying
capacitors are suitable for most applications, but can be
increased to improve the output tolerance. Depending
on the material of the multilayer ceramic capacitor, a
substantial amount of capacitance may be lost over a
wide temperature range. At least X5R and X7R
capacitors are recommended for their relatively stable
temperature characteristics. Z5U- or Y5V-type capacitors
are not recommended. The following table shows a
typical recommended bucket capacitor.
Description
Part Number
Capacitor
0.22µF, 10%,
6.3V, X5R,
GRM155R60J224KE01D
0402
Vendor
Murata
Increasing Output Current Capability
Two parallel FAN5665s increase the output current. In
such a configuration, the VIN, VOUT, GND, and EN
pins of each IC should be connected together, but the
respective C1 and C2 pins must be kept separate.
Separate local input and output bypassing / decoupling
may be required to reduce output noise and ripple.
C_OUT
2.2µF
C1
0.22µF
EN
VIN
VOUT GND
C1+
C2-
C1-
C2+
C_IN
2.2µF
C2
0.22µF
2.9 to5.5V
V_BAT
EN
+5V
C_OUT
2.2µF
C1
0.22µF
EN
VIN
VOUT _GND
C1+
C2-
C1-
C2+
C_IN
2.2µF
C2
0.22µF
Figure 40. Two FAN5665s in Parallel
© 2007 Fairchild Semiconductor Corporation
FAN5665 • Rev. 1.0.1
12
www.fairchildsemi.com