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3

VOLTAGE AND CURRENT CONTROL

3.1 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 relation between the values of R1 and R2

should be chosen as writen in Equation 1.

R1 = R2 x Vref / (Vout - Vref)

Equation 1

Where Vout is the desired output voltage.

To avoid the discharge of the load, the resistor

bridge R1, R2 should be highly resistive. For this

type of application, a total value of 100K

Ω (or

more) would be appropriate for the resistors R1

and R2.

As an example, with R2 = 100K

Ω, Vout = 4.10V,

Vref = 2.5V, then R1 = 41.9K

Ω.

Note that if the low drop diode should 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 the above calculations by

replacing Vout by (Vout + Vdrop).

3.2 Current Control

The current loop is controlled via the second

trans-conductance

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 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 on the following figure.

The resistors of this bridge are matched to provide

the best precision possible

The control equation verifies:

Rsense x Ilim = Vsense

Equation 2

Vsense = R5*Vref/(R4+R5)

Ilim = R5*Vref/(R4+R5)*Rsense

Equation 3

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 full load operation.

Plim = Vsense x Ilim.

Equation 4

Therefore, for most adapter and battery charger

applications, a quarter-watt, or half-watt resistor to

make the current sensing function is sufficient.

The current sinking outputs of the two trans-

connuctance 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.

Fig. 3: Output voltage versus output current

4

COMPENSATION

Vout

Iout

Voltage regulation

TSM1013 Vcc : independent power supply

0

Secondary current regulation

TSM1013 Vcc : On power output

Primary current regulation

TSM1013

Principle of Operation and Application Hints