Datasheet

RMSC out
Vout
1
1
Vin
I = Vout
L ×
2 3
f
-
´ ´
´
Vout
1
1
Vin
ΔVout = Vout + ESR
L × 8 Coutf f
-
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´ ´
ç ÷
´ ´
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TPS65708
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SLVSAE1A OCTOBER 2010 REVISED FEBRUARY 2011
Refer to Table 1 and the typical applications for possible inductors.
Table 1. Tested Inductors
Inductor type Inductor Value Supplier
BRC1608 1.5µH Taiyo Yuden
MLP2012 2.2µH TDK
MIPSA2520 2.2µH FDK
GLCR1608T1R5M-HC 1.5µH TDK
LQM21P 2.2µH Murata
Output Capacitor Selection
The advanced Fast Response voltage mode control scheme of the step-down converter allows the use of small
ceramic capacitors with a typical value of 10µF, without having large output voltage under and overshoots during
heavy load transients. Ceramic capacitors having low ESR values result in lowest output voltage ripple and are
therefore recommended. For an inductor value of 1.5µH or 2.2µH, an output capacitor with 10µF can be used.
See the recommended components.
If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application
requirements. Just for completeness the RMS ripple current is calculated as:
(3)
At nominal load currents, the inductive converters operate in PWM mode and the overall output voltage ripple is
the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and
discharging the output capacitor:
(4)
Where the highest output voltage ripple occurs at the highest input voltage Vin.
At light load currents, the converter operates in Power Save Mode and the output voltage ripple is dependent on
the output capacitor value. The output voltage ripple is set by the internal comparator delay and the external
capacitor. The typical output voltage ripple is less than 1% of the nominal output voltage.
Input Capacitor Selection
Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is
required for best input voltage filtering and minimizing the interference with other circuits caused by high input
voltage spikes. The converters need a ceramic input capacitor of 10μF. The input capacitor can be increased
without any limit for better input voltage filtering.
Table 2. Tested Capacitors
Type value Voltage rating Size Supplier Material
GRM155R60G475ME47D 4.7µF 4V 0402 Murata Ceramic X5R
GRM155R60J225ME15D 2.2µF 6.3V 0402 Murata Ceramic X5R
GRM185R60J225 2.2µF 6.3V 0603 Murata Ceramic X5R
GRM188R60J475K 4.7µF 6.3V 0603 Murata Ceramic X5R
GRM188R60J106ME47D 10µF 6.3V 0603 Murata Ceramic X5R
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