AN901 Datasheet, PDF (8/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
2.7. Optional Regulator for VDDA Supply
VDDA valid operating range is between 3.0 V and 5.5 V. In applications where the only source available on the
primary side is above 5.5 V, Si884xx/Si886xx provides a voltage reference for an external regulator circuit.
The regulator circuit consists of transistor Q2, R14, and C14, as shown in Figure 4. The circuit behind the VREGA
pin can be modeled as a zener diode connected from VREGA to GNDA, and requires input current between
350 μA to 950 μA to establish a nominal 4.85 V reference at the VREGA pin. This reference is tied to the base of
Q2 and the emitter outputs approximately a 4.3 V supply suitable to power VDDA.
Figure 4. External Regulator Circuit
The governing equations for the circuit are:
I_R = I_b + I_REG = -^V----^I-^N-----–-_R----^V1----₄^R----^E---^G----^A--
Equation 20.
I_DDA = I_b + 1
Equation 21.
V_DDA = V_REGA – V_be
Equation 22.
It is recommended to set I_R to no more than 950 μA no matter I_DDA load. As I_DDA increases, more of I_R will flow into
the base of Q2. V_REGA reference voltage will be maintained as long as the I_REG > 350 μA. Choose Q2 with
adequate gain β to source the maximum expected I_DDA. The recommended value for C14 filter capacitor for the
VREGA reference is 100 nF.
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