Datasheet

I
RMSCout
+ Vout
1 *
Vout
Vin
L ƒ
1
2 3
Ǹ
DVout + Vout
1 *
Vout
Vin
L ƒ
ǒ
1
8 Cout ƒ
) ESR
Ǔ
TPS62420, TPS62421
www.ti.com SLVS676C –JUNE 2006–REVISED JUNE 2010
Table 7. List of Inductors
DIMENSIONS [mm
3
] INDUCTOR TYPE SUPPLIER
3.2×2.6×1.0 MIPW3226 FDK
3×3×0.9 LPS3010 Coilcraft
2.8×2.6×1.0 VLF3010 TDK
2.8x2.6×1.4 VLF3014 TDK
3×3×1.4 LPS3015 Coilcraft
3.9×3.9×1.7 LPS4018 Coilcraft
Output Capacitor Selection
The advanced fast response voltage mode control scheme of the two converters allows the use of small ceramic
capacitors with a typical value of 10mF, without having large output voltage under and overshoots during heavy
load transients. Ceramic X7R/X5R capacitors having low ESR values result in lowest output voltage ripple and
are therefore recommended.
If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application
requirements. The RMS ripple current is calculated as:
(8)
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:
(9)
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. Higher output capacitors like 22mF values minimize the voltage ripple in PFM Mode and tighten DC
output accuracy in PFM Mode.
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 10mF. The input capacitor can be increased
without any limit for better input voltage filtering.
LAYOUT CONSIDERATIONS
As for all switching power supplies, the layout is an important step in the design. Proper function of the device
demands careful attention to PCB layout. Care must be taken in board layout to get the specified performance. If
the layout is not carefully done, the regulator could show poor line and/or load regulation, stability issues as well
as EMI problems. It is critical to provide a low inductance, impedance ground path. Therefore, use wide and
short traces for the main current paths as indicated in bold in Figure 36.
The input capacitor should be placed as close as possible to the IC pins as well as the inductor and output
capacitor.
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