(

)

f

IN

OUT

OUT

O LL

SW

IN

(V

V

) V

1

I

2 L

V

-

´

=

´

´

´

TPS51218

SLUS935B

– MAY 2009 – REVISED FEBRUARY 2012

www.ti.com

LIGHT LOAD CONDITION IN AUTO-SKIP OPERATION

conditions to maintain high efficiency. As the output current decreases from heavy load condition, the inductor

current is also reduced and eventually comes to the point that its rippled valley touches zero level, which is the

boundary between continuous conduction and discontinuous conduction modes. The rectifying MOSFET is

turned off when this zero inductor current is detected. As the load current further decreases, the converter runs in

to discontinuous conduction mode. The on-time is kept almost the same as it was in the continuous conduction

mode so that it takes longer time to discharge the output capacitor with smaller load current to the level of the

discontinuous conduction mode) can be calculated in Equation 3.

where

•

(3)

ADAPTIVE ZERO CROSSING

The TPS51218 has an adaptive zero crossing circuit which performs optimization of the zero inductor current

detection at skip mode operation. This function pursues ideal low-side MOSFET turning off timing and

compensates inherent offset voltage of the ZC comparator and delay time of the ZC detection circuit. It prevents

SW-node swing-up caused by too late detection and minimizes diode conduction period caused by too early

detection. As a result, better light load efficiency is delivered.

FORCED CONTINUOUS CONDUCTION MODE

When the RF pin is tied high, the controller keeps continuous conduction mode (CCM) in light load condition. In

this mode, switching frequency is kept almost constant over the entire load range which is suitable for

applications need tight control of the switching frequency at a cost of lower efficiency. To set the switching

tied low prior to soft-start operation to set frequency and tied high after powergood indicates high.

OUTPUT DISCHARGE CONTROL

When EN is low, the TPS51218 discharges the output capacitor using internal MOSFET connected between SW

and GND while high-side and low-side MOSFETs are kept off. The current capability of this MOSFET is limited to

discharge slowly.

LOW-SIDE DRIVER

represented by its internal resistance, which are 1.0

Ω for V5IN to DRVL and 0.5Ω for DRVL to GND. A dead time

to prevent shoot through is internally generated between high-side MOSFET off to low-side MOSFET on, and

low-side MOSFET off to high-side MOSFET on. 5-V bias voltage is delivered from V5IN supply. The

instantaneous drive current is supplied by an input capacitor connected between V5IN and GND. The average

drive current is equal to the gate charge at Vgs=5V times switching frequency. This gate drive current as well as

the high-side gate drive current times 5V makes the driving power which need to be dissipated from TPS51218

package.

HIGH-SIDE DRIVER

floating driver, 5 V of bias voltage is delivered from V5IN supply. The average drive current is also equal to the

capacitor between VBST and SW pins. The drive capability is represented by its internal resistance, which are

1.5

Ω for VBST to DRVH and 0.7 Ω for DRVH to SW.

14

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