TPS51116 datasheet(9/29 Pages) TI

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VDDQ SMPS, Light Load Condition

OUT(LL) +

IN * VOUT)

OUT

(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

reduction of frequency is achieved smoothly and without increase of VOUTripple or load regulation. Detail

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

current reaches to the continuous conduction. The transition load point to the light load operation IOUT(LL) (i.e. the

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

where

•

f is the PWM switching frequency (400 kHz)

Switching frequency versus output current in the light load condition is a function of L, f, VIN and VOUT, but it

decreases almost proportional to the output current from the IOUT(LL) given above. For example, it is 40 kHz at

IOUT(LL)/10 and 4 kHz at IOUT(LL)/100.

The low-side driver is designed to drive high-current, low-RDS(on), N-channel MOSFET(s). The drive capability is

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

to the gate charge at VGS = 5 V times switching frequency. This gate drive current as well as the high-side gate

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

The high-side driver is designed to drive high-current, low-RDS(on) N-channel MOSFET(s). When configured as a

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

gate charge at VGS = 5V times switching frequency. The instantaneous drive current is supplied by the flying

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

sensing and MOSFET RDS(on) sensing. For resistor sensing scheme, an appropriate current sensing 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.

For RDS(on) sensing scheme, CS pin should be connected to V5IN through the trip voltage setting resistor, RTRIP.

In this scheme, CS terminal sinks 10-µA ITRIP current and the trip level is set to the voltage across the RTRIP. The

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

the drain terminal of the bottom MOSFET. ITRIP has 4500ppm/°C temperature slope to compensate the

temperature dependency of the RDS(on). In either scheme, PGND is used as the positive current sensing node so

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

terminal of the bottom MOSFET.

TPS51116 Datasheet(PDF) 9 Page - Texas Instruments

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