Datasheet

Output Capacitor Selection
I
RMSCout
+ Vout
1 *
Vout
Vin
L ƒ
1
2 3
Ǹ
(7)
DVout + Vout
1 *
Vout
Vin
L ƒ
ǒ
1
8 Cout ƒ
) ESR
Ǔ
(8)
Input Capacitor Selection
Low Drop Out Voltage Regulators (LDOs)
V
OUT
+ V
REF
ǒ
1 )
R5
R6
Ǔ
(9)
V
OUT
+ V
FB_LDOx
R5 ) R6
R6
R5 + R6
ǒ
V
OUT
V
FB_LDOx
Ǔ
* R6
(10)
TPS65053
TPS650531
TPS650532
SLVS754C – MARCH 2007 – REVISED SEPTEMBER 2009 ............................................................................................................................................. www.ti.com
The advanced Fast Response voltage mode control scheme of the two converters allow the use of small ceramic
capacitors with a typical value of 10 µ F, 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. See the recommended components in Table 4 .
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:
At nominal load current, 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:
Where the highest output voltage ripple occurs at the highest input voltage Vin.
At light load currents, the converters operate 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.
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 10 µ F. The input capacitor can be increased
without any limit for better input voltage filtering.
Table 3. Possible Capacitors For DCDC Converters And LDOs
Capacitor Value Size Supplier Type
2.2 µ F 0805 TDK C2012X5R0J226MT Ceramic
2.2 µ F 0805 Taiyo Yuden JMK212BJ226MG Ceramic
10 µ F 0805 Taiyo Yuden JMK212BJ106M Ceramic
10 µ F 0805 TDK C2012X5R0J106M Ceramic
The output voltage of LDO1 and LDO2 can be set by an external resistor network and can be calculated to:
with an internal reference voltage, V
REF
, typical 1 V.
It is recommended to set the total resistance of R5 + R6 to less than 1M Ω . Typically, there is no feedforward
capacitor needed at the voltage dividers for the LDOs.
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