Mosfet Selection | Texas Instruments TPS40090EVM-002 instruction

SLUU195 − June 2004

3.Transient consideration. An additional consideration in the selection of the output inductor and capacitance value can be derived from examining the transient voltage overshoot which can be initiated with a load step from full load to no load. By equating the inductive energy with the capacitive energy the equation (10) can be derived.

LEQ

* I

0.6mH

(100 A)

(10)

COUT +

+ 1846 mF

VOUT2

(1.75 V)

* VOUT1

* (1.5 V)

where

•IOH is full load

•IOL is no load

•VOUT2 is the the allowed transient voltage rise

•VOUT1 is the initial voltage

In this 100-A design the capacitance required for limiting the transient is significantly larger than the capacitance required to keep the ripple acceptably low. Eight 220-∝F POSCAP capacitors are in parallel with four 22-∝F ceramic capacitors. The ESR of each POSCAP is 15mΩ.

4.5MOSFET Selection

There are different requirements for switching FET(s) and rectifier FET(s) in the high-ratio step down application. The duty cycle is around 12%. So the rectifier FET(s) is on for most of the

cycle. The conduction loss is dominant. Low-RDS(on)FET(s) are preferred. Also due to the dV/dt turn on of the rectifier FET(s) and cross conduction, choose a rectifier FET with Qgs > Qgd. When the switch node is falling, the Qgd can pull the gate of the lower FET below GND, which

upsets the driver. Two Si7880DP from Siliconix are in parallel for the rectifier FET. The RDS(on) of this FET is 3 mΩ and Qgs=18nC, and Qgd=10.5nC.

The switching FET switches at high voltage and high current, the switching loss is dominant. One single Si7860DP is selected for its low total gate charge.

Both types of FET(s) are offered in the Powerpak SO−8 package.

The PCB is layed out for two FETs in parallel, for both switching FET(s) and rectifier FET(s), to give the feasibility to modify the board for different applications.

4.6Current Sensing

TPS40090 supports both resistor current sensing and DCR current sensing approach. DCRs of the output inductors are used in this design as the current sensing components. The DCR

current sensing circuit is shown in Figure 5. The idea is to parallel a R-C network to the inductor. If the two time constants are same (L/DCR=R ⋅ C), then VC=VDCR. Extra circuit, shown in (b), is used to compensate the positive temperature coefficient of copper specific resistance, which is 0.385%/°C. See detail explanation in the datasheet.

TPS40090 Multi-Phase Buck Converter and TPS2834 Drivers Steps-Down from 12-V to 1.5-V at 100 A