7

LTC1155

S

APPLICATI

I FOR ATIO

For this example, we assume a worst-case scenario; i.e.,

that the power supply to the power MOSFET is “hard” and

provides a constant 5V regardless of the current. In this

and the drain sense resistance. Therefore:

= 62.5A

The drop across the drain sense resistor under these

conditions is much larger than 100mV and is equal to the

drain current times the sense resistance:

= 1.88V

By consulting the power MOSFET data sheet SOA graph,

we note that the IRLZ34 is capable of delivering 62.5A at

a drain-to-source voltage of 3.12V for approximately

10ms.

An RC time constant can now be calculated which satisfies

this requirement:

Figure 2 is a graph of normalized overcurrent shutdown

time versus normalized MOSFET current. This graph can

be used instead of the above equation to calculate the RC

time constant. The Y axis of the graph is normalized to one

RC time constant. The X axis is normalized to the set

current. (The set current is defined as the current required

to develop 100mV across the drain sense resistor).

This time constant should be viewed as a maximum safe

delay time and should be reduced if the competing

requirement of starting a high inrush current load is less

stringent; i.e., if the inrush time period is calculated at

20ms, the RC time constant should be set at roughly two

or three times this time period and not at the maximum of

182ms. A 60ms time constant would be produced with a

270k resistor and a 0.22

µF capacitor (as shown in

Figure 1).

RC

t

In

V

RI

RC

In

ms

SEN

SEN

MAX

=

−

•

















=

−

•

















=

=

–

–.

.

..

–.

/– .

1

001

1

010

0 030 62 5

0 01 0 054

182

Note that the shutdown time is shorter for increasing

levels of MOSFET current. This ensures that the total

energy dissipated by the MOSFET is always within the

bounds established by the MOSFET manufacturer for safe

operation.

In the example presented above, we established that the

power MOSFET should not be allowed to pass 62.5A for

more than 10ms. 62.5A is roughly 18 times the set current

of 3.3A. By drawing a line up from 18 and reflecting it off

the curve, we establish that the RC time constant should

be set at 10ms divided by 0.054, or 180ms. Both methods

result in the same conclusion.

Using a Speed Up Diode

A way to further reduce the amount of time that the power

MOSFET is in a short-circuit condition is to “bypass”the

delay resistor with a small signal diode as shown in Figure

3. The diode will engage when the drop across the drain

sense resistor exceeds 0.7V, providing a direct path to the

MOSFET CURRENT (1 = SET CURRENT)

1

0.01

0.1

1

10

5

10

20

100

1155 F02

250

Figure 2. Shutdown Time vs MOSFET Current