AN938 Datasheet, PDF (7/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

AN938
2.1
Power supply, UVLO and voltage reference
Oscillator, sync and voltage feed-forward
One pin implements the oscillator function with inherent voltage feed-forward. A second pin
is dedicated to in/out synchronization.
A resistor R_osc and a capacitor C_osc, connected as shown in Figure 5, allow the setting of
the desired switching frequency given in Equation 1:
Equation 1
F_SW
=
----------------------------------------¹------------------------------------------
(R_osc ⋅ C_osc)In_⎝⎛⁶⁵--_⎠⎞ + 100 ⋅ C_osc
where F_sw is in kHz, R_osc in kΩ and C_osc in nF.
The oscillator capacitor C_osc is discharged by an internal mos transistor of 100 Ω of R_DS(on)
(Q1) and during this period the internal threshold is set at 1 V by a second mos, Q2. When
the oscillator voltage capacitor reaches the 1 V threshold, the output comparator turns off
mos Q1 and turns on mos Q2, charging the external capacitor C_osc. The oscillator block,
shown in Figure 5, generates a sawtooth wave signal that sets the switching frequency of
the system.
This signal, compared with the output of the error amplifier, generates the PWM signal that
modulates the conduction time of the power output stage.
The design of the oscillator implements the voltage feed-forward function without any
additional external components.
Figure 6.
Device switching frequency vs. R_osc and C_osc
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Doc ID 5410 Rev 10
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