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VDDQ SMPS, Light Load Condition

I

1

2

L

f

(V

V

OUT

V

IN

(1)

Low-Side Driver

High-Side Driver

Current Sensing Scheme

TPS51116

SLUS609A – MAY 2004 – REVISED JUNE 2004

DETAILED DESCRIPTION (continued)

TPS51116 automatically reduces switching frequency at light load condition to maintain high efficiency. This

operation is described as follows. As the output current decreases from heavy load condition, the inductor current

is also reduced and eventually comes to the point that its valley touches zero current, 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 decreased, the converter runs in discontinuous

conduction mode and it takes longer and longer to discharge the output capacitor to the level that requires next

ON cycle. The ON-time is kept the same as that in the heavy load condition. In reverse, when the output current

increase from light load to heavy load, switching frequency increases to the constant 400 kHz as the inductor

threshold between continuous and discontinuous conduction mode) can be calculated in Equation 1:

where

•

f is the PWM switching frequency (400 kHz)

represented by its internal resistance, which are 3

Ω for V5IN to DRVL and 0.9 Ω for DRVL to PGND. A

dead-time to prevent shoot through is internally generated between top MOSFET off to bottom MOSFET on, and

bottom MOSFET off to top 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

drive current times 5 V makes the driving power which needs to be dissipated from TPS51116 package.

floating driver, 5-V bias voltage is delivered from V5IN supply. The average drive current is also calculated by the

capacitor between VBST and LL pins. The drive capability is represented by its internal resistance, which are 3

Ω

for VBST to DRVH and 0.9

Ω for DRVH to LL.

In order to provide both good accuracy and cost effective solution, TPS51116 supports both of external resistor

should be connected between the source terminal of the bottom MOSFET and PGND. CS pin is connected to the

MOSFET source terminal node. The inductor current is monitored by the voltage between PGND pin and CS pin.

inductor current is monitored by the voltage between PGND pin and LL pin so that LL pin should be connected to

that PGND should be connected to the proper current sensing device, i.e. the sense resistor or the source

terminal of the bottom MOSFET.

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