PM6680 datasheet(26/49 Pages) STMICROELECTRONICS

Part #	PM6680

Description	No Rsense dual step-down controller with adjustable voltages for notebook system power

PDF	49 Pages

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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Device description

PM6680

26/49

Figure 32.

No audible skip mode

The low side switch is turned on until the output voltage crosses about Vreg+1%. Then the

high side MOSFET is turned on for a fixed on time period. Afterwards the low side switch is

enabled until the inductor current reaches the zero-crossing threshold. This keeps the

switching frequency higher than 33KHz. As a consequence of the control, the regulated

voltage can be slightly higher than Vreg (up to 1%).

If, due to the load, the frequency is higher than 33KHz, the device works like in skip mode.

No-audible skip mode reduces audio frequency noise that may occur in pulse skip mode at

very light loads, keeping the efficiency higher than in PWM mode.

7.6

Current limit

The current-limit circuit employs a "valley" current-sensing algorithm. During the conduction

time of the low side MOSFET the current flowing through it is sensed. The current-sensing

element is the low side MOSFET on-resistance (Figure 33).

Figure 33.

RDSON sensing technique

Inductor current

Time

No audible skip mode

Low side

∼30us

LGATE

HGATE

CSENSE

PHASE

Rcsense

RDSon

PM6680 Datasheet(PDF) 26 Page - STMicroelectronics