PM6685 datasheet(39/52 Pages) STMICROELECTRONICS

39 / 52 page

PM6685

Monitoring and protections

39/52

The stability of the system depends firstly on the output capacitor zero frequency. The

following condition should be satisfied:

Equation 21

where k is a design parameter greater than 3 and Rout is the ESR of the output capacitor. It

determinates the minimum integrator capacitor value CINT:

Equation 22

where gm=50us is the integrator transconductance.

In order to ensure stability it must be also verified that:

Equation 23

In order to reduce ground noise due to load transient on the other section, it is

recommended to add a resistor RINT and a capacitor Cfilt that, together with CINT, realize a

low pass filter (see Figure 36). The cutoff frequency fCUT must be much greater (10 or more

times) than the switching frequency of the section:

Equation 24

Due to the capacitive divider (CINT, Cfilt), the ripple voltage at the COMP pin is given by:

Equation 25

out

Zout

OUT

Zout

INT

⎟

⎠

⎞

⎜

⎝

⎛

−

OUT

Zout

INT

filt

INT

filt

INT

CUT

INT

RIPPLEout

filt

INT

RIPPLEout

RIPPLEINT

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