AN938 Datasheet, PDF (9/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

AN938
Power supply, UVLO and voltage reference
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
Tch = R_osc ⋅ C_osc ⋅ In_⎝⎛⁶⁵--_⎠⎞
Tdis = 100 ⋅ C_osc
The maximum duty cycle is a function of Tch, Tdis and an internal delay and is represented
by the equation below:
Equation 4
R_osc ⋅ C_osc ⋅ In_⎝⎛⁶⁵--_⎠⎞ – 80 ⋅ 10^–9
Dmax = ---------------------------------------------------------------------------------
R_osc ⋅ C_osc ⋅ In_⎝⎛⁶⁵--_⎠⎞ + 100 ⋅ C_osc
which is represented in Figure 9.
Figure 9.
Vo-Io output characteristics -
hiccup protection limits output
power
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Figure 10. Vo-Io output characteristics -
effective pulse-by-pulse protection
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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 T_B = 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.
Doc ID 5410 Rev 10
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