AN938 datasheet(9/33 Pages) STMICROELECTRONICS

9 / 33 page

AN938

Power supply, UVLO and voltage reference

Doc ID 5410 Rev 10

9/33

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

the order of some tenths of a volt, for both offline and DC-DC converters using mains

transformers.

The charge and discharge time is approximately equal to:

Equation 3

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

by the equation below:

Equation 4

which is represented in Figure 9.

2.2

Current protection

The L4973 has two current limiting functions: pulse-by-pulse and hiccup modes.

Increasing the output current to the pulse-by-pulse current limiting threshold (Ith1 typ. value

of 4.5 A), the controller reduces the on-time to the value of TB = 300 ns (the blanking time at

which the current limiting 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.

Tch

osc

⋅

---

⎝⎠

⎛⎞

⋅

Tdis

100 C

osc

⋅

Dmax

osc

---

⎝⎠

⎛⎞

80 10

–

⋅

–

⋅⋅

osc

---

⎝⎠

⎛⎞

100 C

osc

⋅

⋅⋅

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

Figure 9.

Vo-Io output characteristics -

hiccup protection limits output

power

Figure 10.

Vo-Io output characteristics -

effective pulse-by-pulse protection

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