SEA05 Datasheet, PDF (6/10 Pages) STMicroelectronics – Advanced constant voltage and constant current controller

Voltage and current control
6
Voltage and current control
SEA05
6.1
Voltage control
The voltage loop is controlled via a first transconductance operational amplifier, the voltage
divider R1, R2, and the optocoupler which is directly connected to the output. Its possible to
choose the values of R1 and R2 resistors using Equation 1-2:
Equation 1
V_o
=
Vctrl
∗
(R₁ + R₂ )
R₂
Equation 2
R₁
=
R₂
∗
(V_O − Vctrl)
Vctrl
where Vo is the desired output voltage.
As an example, with R1 = 100 kΩ and R2 = 15 kΩ Vo = 19.17 V
6.2
Current control
The current loop is controlled via the second trans-conductance operational amplifier, the
sense resistor R_sense, and the optocoupler. The control equation verifies:
Equation 3
R_sense∗I_omax = Vcsth
Equation 4
R_sense
=
Vcsth
I_{o max}
where I_omax is the desired limited current, and V_csth is the threshold voltage for the current
control loop.
As an example, with I_omax = 1 A, V_csth = 50 mV, then R_sense = 50 m Ω.
Note that the R_sense resistor should be chosen taking into account the maximum dissipation
(Plim) through it during full load operation.
Equation 5
P_Lim = V_csth ⋅ l_omax
As an example, with I_omax = 1 A, and V_csth = 50 mV, Plim = 50 mW.
Therefore, for most adaptor and battery charger applications, it is suitable a low power
resistor to make the current sensing function.
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Doc ID 17014 Rev 1