PM6685TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
PM6685TR
STMicroelectronics
'PM6685TR' PDF : 52 Pages View PDF
PM6685
Monitoring and protections
8.8
Output capacitor
The selection of the output capacitor is based on the ESR value Rout and the voltage rating
rather than on the capacitor value Cout.
The output capacitor has to satisfy the output voltage ripple requirements. Lower inductor
value can reduce the size of the choke but increases the inductor current ripple ∆IL.
Since the voltage ripple VRIPPLEout is given by:
Equation 14
VRIPPLEout = Rout × ∆IL
A low ESR capacitor is required to reduce the output voltage ripple. Switching sections can
work correctly even with 20mV output ripple.
However, to reduce jitter noise between the two switching sections it’s preferable to work
with an output voltage ripple greater than 30mV. If lower output ripple is required, a further
compensation network is needed (see Closing the integrator loop paragraph).
Finally the output capacitor choice deeply impacts on the load transient response (see Load
transient response paragraph). Below there is a list of some capacitor manufacturers.
Table 12. Output capacitor manufacturer
Manufacturer
Series
Capacitor value
(µF)
Rated voltage (V)
ESR max (mΩ)
SANYO
POSCAP
TPB,TPD
150 to 330
4 to 6.3
35 to 65
PANASONIC
SPCAP UD, UE
150 to 220
4 to 6.3
9 to 18
8.9
Input capacitors selection
In a buck topology converter the current that flows into the input capacitor is a pulsed current
with zero average value. The input RMS current of the two switching sections can be roughly
estimated as follows:
Equation 15
ICinRMS = D1 × I12 × (1− D1) + D2 × I22 × (1− D2 )
Where D1, D2 are the duty cycles and I1, I2 are the maximum load currents of the two
sections.
Input capacitor should be chosen with an RMS rated current higher than the maximum RMS
current given by both sections.
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