PM6685 datasheet(35/52 Pages) STMICROELECTRONICS

35 / 52 page

PM6685

Monitoring and protections

35/52

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

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.

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

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.

Table 12.

Output capacitor manufacturer

Manufacturer

Series

Capacitor value

(

µF)

Rated voltage (V)

ESR max (m

Ω)

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

out

RIPPLEout

∆

)

(

)

(

CinRMS

−

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