Power supply, UVLO and voltage reference

AN937

6/29

Doc ID 5655 Rev 15

Figure 7.

Maximum duty cycle vs. Rosc and Cosc as parameter

In fact, the slope of the ramp is modulated by the input ripple voltage, generally present in

the order of some tents of Volt, for both off-line and dc-dc converters using mains

transformers.

The charge and discharge time is approximately to:

The maximum duty cycle is a function of Tch, Tdis and an internal delay and is represented

by the equation:

and is represented in Figure 7:

2.2

Current protection

The L4971 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

time in which the current limit protection does not trigger. This minimum time is necessary to

avoid undesirable intervention of the protection due to the spike current generated during

the recovery time of the freewheeling diode.

0

4

8

Ω)

0.60

0.70

0.80

0.90

D97IN685

0.8nF

1.2nF

2.2nF

4.7nF

5.3nF

Tch

R

osc

C

osc

⋅

In

6

5

---

⎝⎠

⎛⎞

⋅

=

Tdis

100 C

osc

⋅

=

Dmax

R

osc

C

osc

In

6

5

---

⎝⎠

⎛⎞

80 10

9

–

⋅

–

⋅⋅

R

osc

C

osc

In

6

5

---

⎝⎠

⎛⎞

100 C

osc

⋅

+

⋅⋅

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

=