IRF7805 datasheet(4/8 Pages) IRF

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IRF7805/IRF7805A

Figure 2: Q

oss Characteristic

For the synchronous MOSFET Q2, R

ds(on) is an im-

portant characteristic; however, once again the impor-

tance of gate charge must not be overlooked since it

impacts three critical areas. Under light load the

MOSFET must still be turned on and off by the con-

trol IC so the gate drive losses become much more

significant. Secondly, the output charge Q

oss and re-

verse recovery charge Q

rr both generate losses that

are transfered to Q1 and increase the dissipation in

that device. Thirdly, gate charge will impact the

MOSFETs’ susceptibility to Cdv/dt turn on.

The drain of Q2 is connected to the switching node

of the converter and therefore sees transitions be-

tween ground and V

in. As Q1 turns on and off there is

a rate of change of drain voltage dV/dt which is ca-

pacitively coupled to the gate of Q2 and can induce

a voltage spike on the gate that is sufficient to turn

the MOSFET on, resulting in shoot-through current .

The ratio of Q

gd/Qgs1 must be minimized to reduce the

potential for Cdv/dt turn on.

Spice model for IRF7805 can be downloaded in ma-

chine readable format at www.irf.com.

IRF7805 Datasheet(PDF) 4 Page - International Rectifier