Circuit description

AN2983

22/32

Doc ID 15679 Rev 1

2.8

Open-/short-circuit protection

As indicated, one of the requirements is that the module can sustain open and short circuits

indefinitely and restart the correct functionality as soon as the fault is removed.

From the "open circuit" point of view the module is intrinsically safe: if the load is

disconnected, no current runs through the sense resistor and the controller drives the power

MOSFET transistor in conduction and pin 10 of the J1 connector to ground. Whenever the

load is reconnected, the current restarts to flow and normal operation is resumed.

The short-circuit condition is more critical. At the end of the toff delay, the power MOSFET is

turned on, the current starts to flow and charges the inductor, and the voltage on the sense

resistor quickly reaches the threshold level. However, because of the controller’s internal

delay (175 nS nom.) the power MOSFET does not shut down immediately; as such, a

minimal amount of energy is still transferred to the inductor, but during the toff time – since

the load is a short-circuit – this energy is NOT dissipated.

The result is that the load current rises abnormally, leading to catastrophic failure unless the

cycle is blocked.

For this reason the circuit outlined in Figure 22 has been introduced.

Figure 22.

Short-circuit protection

In normal conditions the voltage developed across the sense resistor is not sufficient to turn

on Q3. The INV pin of the controller is then at:

Equation 50

On the other hand, in the case of a short-circuit, the load current increases until Q3 goes in

conduction, C5 is discharged and the INV pin goes to:

Equation 51

The controller then shuts down, the power MOSFET stops conducting, the inductor

discharges and the load current decays to zero. Since no current flows through the sense

V

44

.

1

)

K

22

K

33

K

220

(

K

22

*

)

V

18

(

9

R

7

R

6

R

9

R

*

)

Vcc

(

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

+

=

⎟

⎠

⎞

⎜

⎝

⎛

+

+

V

1

.

0

V

)

4

.

0

(

*

)

25

.

0

(

)

K

22

K

33

(

K

22

*

)

V

25

.

0

(

9

R

7

R

9

R

*

)

sat

_

Vce

_

3

Q

(

=

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

⎟

⎠

⎞

⎜

⎝

⎛

+