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TSM1051

Constant voltage and constant current controller for battery chargers and adaptors

STMicroelectronics 

Application information

5

Application information

TSM1051

5.1

5.1.1

5.1.2

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Voltage and current control

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.

R1

=

R2

⋅

-(--V----O----U----T----–-----V----R----E---F---)-

VREF

Eq:1

where Vout is the desired output voltage.

To avoid the discharge of the load, the voltage divider R1, R2 should be highly resistive. For

this type of application, it is suggested a total value of 100 kΩ (or more) for resistors R1 and

R2

As an example, with R2 = 33 kΩ, VOUT = 5 V, VREF = 1.210 V, then R1 = 103.4 kΩ

Please note that if a low drop diode is inserted between the load and the voltage divider of

the voltage control loop in order to avoid current flowing from the load through the voltage

divider, the diode voltage drop should be taken into account in the computation of Equation

1 replacing Vout with Vout + Vdrop.

Current control

The current loop is controlled via the second trans-conductance operational amplifier, the

sense resistor Rsense, and the optocoupler.

The control equation verifies:

Rsense x Ilim = Vsense

Rsense = Vsense / Ilim

Eq:2

Eq:2a

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

control loop. As an example, with Ilim = 1 A, Vsense = -200 mV, then Rsense = 200 mΩ.

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

dissipation (Plim) through it during full load operation.

Plim = Vsense x Ilim.

Eq:3

As an example, with Ilim = 1 A, and Vsense = 200 mV, Plim = 200 mW.

Therefore, for most adaptor and battery charger applications, a quarter-watt, or half-watt

resistor to make the current sensing function is sufficient. Vsense threshold is achieved

internally by a voltage divider tied to the Vref voltage reference. Its middle point is tied to the

positive input of the current control operational amplifier, and its foot is to be connected to

lower potential point of the sense resistor as shown in Figure 3. The resistors of this voltage

divider are matched to provide the best precision possible. The current sinking outputs of

the two trans-conductance operational amplifiers are common (to the output of the IC). This

makes an ORing function which ensures that whenever the current or the voltage reaches

too high values, the optocoupler is activated. The relation between the controlled current

and the controlled output voltage can be described with a square characteristic as shown in

the following V/I output-power graph. (with power supply of the device indipendent from the

output voltage)

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