PM6685 datasheet(24/52 Pages) STMICROELECTRONICS

24 / 52 page

Device description

PM6685

24/52

Figure 30. Circuitry for output ripple compensation

The integrator amplifier generates a current, proportional to the DC errors, which decreases

the output voltage in order to compensate the total static error, including the voltage drop on

PCB traces. In addition, CINT provides an AC path for the output ripple. In steady state, the

voltage on COMP5/COMP3 pin is the sum of the reference voltage Vr and the output ripple

(see Figure 30). In fact when the voltage on the COMP pin reaches Vr, a fixed Ton begins

and the output increases.

For example, we consider VOUT=5V with an output ripple of ∆V=50mV. Considering

CINT>>CFILT, the CINT DC voltage drop VCINT is about 5V-Vr+25mV=4.125V. CINT ensures

an AC path for the output voltage ripple. Then the COMP pin ripple is a replica of the output

ripple, with a DC value of Vr+25mV=925mV.

For more details about the output ripple compensation network, see the paragraph “Closing

the integrator loop

” in the Design guidelines.

7.4

Pulse skip mode

If the SKIP pin is tied to ground, the device works in skip mode.

At light loads a zero-crossing comparator truncates the low-side switch on-time when the

inductor current becomes negative. In this condition the section works in discontinuous

conduction mode. The threshold between continuous and discontinuous conduction mode

is:

Equation 4

COMP

OUT

RFb1

RFb2

PWM

Comparator

CINT

RINT

CFILT

COUT

COMP PIN

VOLTAGE

OUTPUT VOLTAGE

I=gm(V1-Vr)

∆V

VCINT

ROUT

COMP

OUT

RFb1

RFb2

PWM

Comparator

CINT

RINT

CFILT

COUT

COMP PIN

VOLTAGE

OUTPUT VOLTAGE

I=gm(V1-Vr)

∆V

VCINT

ROUT

OUT

)

SKIP

(

ILOAD

−

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