Switching Power Supplies A - Z

Appendix 1

0 2 4 6 8 101214161820

0.5

1

1.5

25 degC

100 degC

DRAIN

CURRENT (A)

DRAIN

VOLTAGE

(

V

)

Extraplolated to

max current

Figure A-3: The Missing Half of the Topswitch Datasheet Curve

This estimate is way too optimistic. As mentioned previously, a more correct estimate is the

RDS averaged over the RDS at the extreme values. At peak value

RDS

MAX

=

17

1.5 − 0.625

= 19.4 Ω

We also know that the R

DSON−MIN

is 10 Ω from the datasheet since the R

DSON

is rather

typically stated at only 1/10

th

the maximum current, that is, 10 Ω at 150 mA in this case.

So the correct effective R

DSON

is the average of the two

RDS =

RDS

MIN

+ RDS

MAX

2

=

10 + 19.4

2

= 14.7 Ω

Now that we know the RDS is ...