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TSM1011
(Rev.:2016)

Constant Voltage and Constant Current Controller for Battery Chargers and Adapters

STMicroelectronics 

TSM1011

6

Voltage and current control

Voltage and current control

6.1

Note:

6.2

Voltage control

The voltage loop is controlled via a first transconductance operational amplifier, the resistor

bridge R1, R2, and the optocoupler which is directly connected to the output.

The relative values of R1 and R2 should be chosen in accordance with Equation 1:

Equation 1

R1 = R2 x VRef / (Vout - VRef)

where Vout is the desired output voltage.

To avoid the discharge of the load, the resistor bridge R1, R2 should have high impedance.

For this type of application, a total value of 100 K (or more) would be appropriate for the

resistors R1 and R2.

For example, with R2 = 100 K, Vout = 4.10 V, VRef = 2.5 V, then R1 = 41.9 K.

If the low drop diode is to be inserted between the load and the voltage regulation resistor

bridge to avoid current flowing from the load through the resistor bridge, this drop should be

taken into account in Equation 1 by replacing Vout by (Vout + Vdrop).

Current control

The current loop is controlled via the second transconductance operational amplifier, the

sense resistor Rsense, and the optocoupler.

Vsense threshold is achieved externally by a resistor bridge tied to the VRef voltage

reference. Its midpoint is tied to the positive input of the current control operational amplifier,

and its foot is to be connected to the lower potential point of the sense resistor, as shown in

Figure 4. The resistors of this bridge are matched to provide the best precision possible.

The control equation verifies that:

Equation 2

Rsense  Ilim = Vsense

Vsense = --R-R----54-----+--V--R--r--e-5--f-

Equation 3

Ilim = ---R----4-----+--R--R--5---5----V-----r-R-e--f-s---e---n---s--e-

where Ilim is the desired limited current, and Vsense is the threshold voltage for the current

control loop.

Note that the Rsense resistor should be chosen taking into account the maximum dissipation

(Plim) through it during the full load operation.

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