PM6680ATR datasheet(36/48 Pages) STMICROELECTRONICS

Part #	PM6680ATR

Description	Dual synchronous step-down controller with adjustable output voltages plus LDO

PDF	48 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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36 / 48 page

Design guidelines

PM6680A

36/48

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

Equation 25

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 13). The cutoff frequency fCUT must be much greater (10 or more

times) than the switching frequency of the section:

Equation 26

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

Equation 27

Where VRIPPLEout is the output ripple and q is the attenuation factor of the output ripple.

If the ripple is very small (lower than approximately 30 mV), a further compensation network,

named virtual ESR network, is needed. This additional part generates a triangular ripple

that is added to the ESR output voltage ripple at the input of the integrator network. The

complete control schematic is represented in Figure 37.

OUT

Zout

INT

filt

INT

filt

INT

CUT

INT

RIPPLEout

filt

INT

RIPPLEout

RIPPLEINT

PM6680ATR Datasheet(PDF) 36 Page - STMicroelectronics