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1. Voltage and Current Control

1.1. Voltage Control

The voltage loop is controlled via a first transcon-

ductance operational amplifier, the resistor bridge

R1, R2, and the optocoupler which is directly con-

nected to the output.

The relation between the values of R1 and R2

should be chosen as written in Equation 1.

R1 = R2 x Vref / (Vout - Vref)

Eq1

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 = 1.210V, then R1 = 41.9K

Ω.

Note that if the low drop diode should be inserted

between the load and the voltage regulation resis-

tor bridge to avoid current flowing from the load

through the resistor bridge, this drop should be

taken into account in the above calculations by re-

placing Vout by (Vout + Vdrop).

1.2. 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 re-

sistor bridge tied to the Vref voltage reference. Its

middle point is tied to the positive input of the cur-

rent 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 re-

sistors of this bridge are matched to provide the

best precision possible

The control equation verifies:

Rsense x Ilim = Vsense

eq2

Vsense = R5*Vref/(R4+R5)

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

eq2'

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.

eq3

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-con-

nuctance operational amplifiers are common (to

the output of the IC). This makes an ORing func-

tion which ensures that whenever the current or

the voltage reaches too high values, the optocou-

pler is activated.

The relation between the controlled current and

the controlled output voltage can be described

with a square characteristic as shown in the fol-

lowing V/I output-power graph.

Figure 3 : Output voltage versus output current

2. Compensation

The voltage-control trans-conductance operation-

al amplifier can be fully compensated. Both of its

output and negative input are directly accessible

for external compensation components.

An example of a suitable compensation network is

shown in Fig.2. It consists of a capacitor

Cvc1=2.2nF and a resistor Rcv1=22K

Ω in series.

Vout

Iout

Voltage regulation

TSM1012 Vcc : independent power supply

0

Secondary current regulation

TSM1012 Vcc : On power output

Primary current regulation

TSM1012

PRINCIPLE OF OPERATION AND APPLICATION HINTS