5/21

AN1061 APPLICATION NOTE

Figure 7. Maximum Duty Cycle vs Rosc and Cosc as Parameter

3.1 Current Protection

The L4978 has two current limit levels, pulse by pulse and hiccup modes.

Increasing the output current till the pulse by pulse limiting current threshold (Ith1 typ. value of

3A) the controller reduces the on-time till the value of TB = 300ns that is a blanking time in

which the current limit protection does not trigger. This minimun time is necessary to avoid un-

desirable intervention of the protection due to the spike current generated during the recovery

time of the freewheeling diode.

In this condition, because of this fixed balnking time, the output current is given by:

Where Ro is the load resistance, Vf is the diode forward voltage. RD and RL are the series

resistance of,

respectively, the freewheeling diode and the choke.

Typical output characteristics are represented in figure 8 and 9.

In fig 8, the pulse by pulse protection is sufficient to limit the current.

In fig 9 the pulse by pulse protection is no more effective to limit the current due to the minimun

Ton fixed by the blanking time TB, and the hiccup protection intervenes because the output

peak current reachs the relative threshold.

0

4

8

Ω)

0.60

0.70

0.80

0.90

D97IN685

0.8nF

1.2nF

2.2nF

4.7nF

5.3nF

I

max

V

CC

T

B

F

SW

V

f

1T

B

F

SW

⋅

–

()

⋅

–

⋅⋅

[]

R

O

R

D

R

L

+

() 1T

B

F

SW

⋅

–

()

R

dson

R

L

+

()T

B

F

SW

⋅

++

[]

----------------------------------------------------------------------------------------------------------------------------------------------------

=

Figure 8. Output Characteristic

Figure 9. Output Characteristic

3A

3.6A

D98IN909A

A

B

C

3A

3.6A

D99IN1077

A