Voltage and Current Control

TSM1011

5/9

5

VOLTAGE AND CURRENT CONTROL

5.1 Voltage Control

The

voltage

loop

is

controlled

via

a

first

transconductance

operational

amplifier,

the

is directly connected to the output.

chosen in accordance with

Equation 1

To avoid discharge of the load, the resistor bridge

of application, a total value of 100k

Ω (or more)

Note: 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 the above calculations by replacing

5.2 Current control

The current loop is controlled via the second

transconductance operational amplifier, the sense

Its midpoint 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

resistors of this bridge are matched to provide the

best precision possible.

The control equation verifies that:

Equation 2

Equation 2’

control loop.

taking into account the maximum dissipation

Equation 3

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

transconductance

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

R

1

R

2

V

ref

V

out

V

ref

–

---------------------------





⋅

=

R

sense

I

lim

⋅

V

sense

=

V

sense

R

5

V

ref

R

4

R

5

+

--------------------

⋅

=

I

lim

R

5

V

ref

⋅

R

4

R

5

+

() R

sense

⋅

------------------------------------------------

=

P

lim

V

⋅

=

Vout

Iout

Voltage regulation

TSM1011 Vcc : independent power supply

0

Secondary current regulation

TSM1011 Vcc : On power output

Primary current regulation