PM6670AS datasheet(23/54 Pages) STMICROELECTRONICS

23 / 54 page

PM6670AS

Device description

23/54

7.1.2

Output ripple compensation and loop stability

The loop is closed connecting the center tap of the output divider (internally, when the fixed

output voltage is chosen, or externally, using the MODE pin in the adjustable output voltage

mode). The feedback node is the negative input of the error comparator, while the positive

input is internally connected to the reference voltage (Vr = 0.9 V). When the feedback

voltage becomes lower than the reference voltage, the PWM comparator goes to high and

sets the control logic, turning on the high-side MOSFET. After the On-Time (calculated as

previously described) the system releases the high-side MOSFET and turns on the

synchronous rectifier.

The voltage drop along ground and supply PCB paths, used to connect the output capacitor

to the load, is a source of DC error. Furthermore the system regulates the output voltage

valley, not the average, as shown in Figure 26. Thus, the voltage ripple on the output

capacitor is an additional source of DC error. To compensate this error, an integrative

network is introduced in the control loop, by connecting the output voltage to the COMP pin

through a capacitor (CINT) as shown in Figure 29.

Figure 28. Switching section simplified block diagram

driver

Level

shifter

driver

Ton

Ton--min

min

Anti cross-

conduction

circuitry

Toff

Toff--min

min

PWM Comparator

Positive Current Limit comparator

0.9V

Min

Min fsw

fsw

counter

Zero

Zero--crossing

crossing

Comparator

bandgap

1.236V

PHASE

PGND

LGATE

BOOT

CSNS

COMP

Integrator

VSNS

VBG

PULSE

PULSE -- SKIP

SKIP

MODE

MODE<4V

1-Shot generator

Ton

VSNS

VOSC

DDRSEL

0.9V

100uA

500mV

2.5V

HGATE

VREF

SGND

VOSC

VCC

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