Datasheet

ADP5020
Rev. 0 | Page 22 of 28
LDO INPUT FILTER
To improve the LDO input-to-output ripple suppression in the
critical switching frequency range of the buck converters, it may be
necessary to add an LC filter tuned to 1 MHz, as shown in
Figure 28. Additional tests and simulation must be performed
to assess if this filter is necessary.
The filter resonance frequency is determined by the following
equation:
C8L3
f
LC
××π×
=
2
1
(3)
where L3 = 250 nH, assuming that f
LC
= 1 MHz and C8 = 100 nF.
The inductor must be able to withstand the LDO load current,
including the overload condition, which is limited to 400 mA.
VBATT
SW1
VOUT1
L1
2.2µH
3.3V
2.8V
LDO
INPUT FILTER
L3
C6
10µF
C1
10µF
C8
0.1µF
C3
1µF
VDD3
VOUT1
VOUT3
BUCK 1
LDO
ADP5020
07774-026
Figure 28. Optional LDO Input Filter