PM6670AS Datasheet, PDF (44/54 Pages) STMicroelectronics – Complete DDR2/3 memory power supplies controller

Application information
PM6670AS
8.1.4
If ceramic capacitors are used, the output voltage ripple due to inductor current ripple is
negligible. Then the inductance could be smaller, reducing the size of the choke. In this case
it is important that output capacitor can adsorb the inductor energy without generating an
over-voltage condition when the system changes from a full load to a no load condition.
The minimum output capacitance can be chosen by the following equation:
Equation 38
C _OUT,min
=
L ⋅ I_LOAD,MAX ²
Vf ² − Vi²
where Vf is the output capacitor voltage after the load transient, while V_I is the output
capacitor voltage before the load transient.
In Table 8 are listed some tested polymer capacitors are listed.
Table 8. Evaluated output capacitors
Manufacturer
Series
Capacitance
(µF)
4TPE220MF
220
SANYO
4TPE150MI
220
4TPC220M
220
HITACHI
TNCB OE227MTRYF
220
Rated voltage
(V)
4V
4V
4V
2.5 V
ESR max @100 kHz
(mΩ)
15 to 25
18
40
25
MOSFETs selection
In most of the applications, power management efficiency is a high level requirement. Power
dissipation on the power switches becomes animportant factor in the selection of switches.
Losses of high-side and low-side MOSFETs depend on their working condition.
Considering the high-side MOSFET, the power dissipation is calculated as:
Equation 39
P_DHighSide = P_conduction + P_switching
Maximum conduction losses are approximately given by:
Equation 40
P_conduction
= R_DSon
⋅
V_OUT
V_IN.min
⋅ I_LOAD,MAX2
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