PM6686 datasheet(36/50 Pages) STMICROELECTRONICS

36 / 50 page

Monitoring and protections

PM6686

36/50

Doc ID 15281 Rev 4

9.5

Charge pump

The PM6686 can drive an external charge pump circuit whose typical application schematic

is shown in the next figure.

Figure 54.

Charge pump application circuit

The charge pump works in 4 phases:

LGATE is low. C1 is charged through the D1a diode at OUT1 voltage minus the diode

drop.

LGATE is driven high and C1 transfers the charge to C2. C2 voltage is OUT1 voltage

plus the LGATE voltage minus the voltage drops on D1 and D2.

LGATE is turned low and C2 shares its charge with C3 thought D3.

LGATE becomes high and C3 can charge C4 thought diode D4.

Every diode used to transfer charge introduces a voltage drop that decreases the charge

pump output voltage.

Repeating this cycle several times makes the charge pump output voltage equals to:

Equation 6

Where VCP is the charge pump output voltage, VOUT1 is the output voltage of the switching

section 1, VLGATE1 is the low-side MOSFET gate driving voltage and VDIODE is the forward

voltage droop of the diodes used in the application.

CP_FB pin must be connected to the output of the charge pump with a resistor divider;

when CP_FB pin droops below 2 V typ., OUT1 controller starts a NA SKIP cycle to boost the

voltage of the charge pump.

PHASE

LGATE

COU T

CP_FB

Vo ut

DIODE

LGATE

OUT

−

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