Dual-Output Power-Supply

Controller for Notebook Computers

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11

second half-cycle, the PWM turns on the high-side

MOSFET by connecting the capacitor to the MOSFET

gate by closing an internal switch between BST_ and

DH_. This provides the necessary enhancement voltage

to turn on the high-side switch, an action that “boosts”

the 5V gate-drive signal above the battery voltage.

Ringing seen at the high-side MOSFET gates (DH3 and

DH5) in discontinuous-conduction mode (light loads) is

a natural operating condition caused by the residual

energy in the tank circuit formed by the inductor and

stray capacitance at the LX_ nodes. The gate driver

negative rail is referred to LX_, so any ringing there is

directly coupled to the gate-drive supply.

1X

CS_

FB_

BST_

DH_

LX_

LEVEL

SHIFT

VL

SHOOT-

THROUGH

CONTROL

DL_

PGND

LEVEL

SHIFT

Q

R

S

SYNCHRONOUS

RECTIFIER CONTROL

N

3.3V

1R

30R

VL

4

µA

0mV TO 100mV

CURRENT

LIMIT

MINIMUM

CURRENT

(IDLE MODE)

25mV

SLOPE COMP

Σ

REF, 3.3V

(OR INTERNAL

5V REFERENCE)

MAIN PWM

COMPARATOR

60kHz

LPF

R

S

Q

OSC

SS_

ON_

Figure 3. PWM Controller Block Diagram