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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 writen 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.

The control equation verifies:

Rsense x Ilim = Vsense

eq2

Rsense = Vsense / Ilim

eq2’

where Ilim is the desired limited current, and

Vsense is the threshold voltage for the current

control loop.

As an example, with Ilim = 1A, Vsense = -200mV,

then Rsense = 200m

Ω.

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

As an example, with Ilim = 1A, and Vsense =

200mV, Plim = 200mW.

Therefore, for most adapter 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 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 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 its out-

put and the negative input are directly accessible

for external compensation components.

Vout

Iout

Voltage regulation

TSM105 Vcc : independent power supply

0

Secondary current regulation

TSM105 Vcc : On power output

Primary current regulation

TSM105

PRINCIPLE OF OPERATION AND APPLICATION HINTS