DocID9310 Rev 2 7/14
TSM1011 Voltage and current control
14
6 Voltage and current control
6.1 Voltage control
The voltage loop is controlled via a first transconductance operational amplifier, the resistor
bridge R
1
, R
2
, and the optocoupler which is directly connected to the output.
The relative values of R
1
and R
2
should be chosen in accordance with Equation 1:
Equation 1
R
1
= R
2
x V
Ref
/ (V
out
- V
Ref
)
where V
out
is the desired output voltage.
To avoid the discharge of the load, the resistor bridge R
1
, R
2
should have high impedance.
For this type of application, a total value of 100 K (or more) would be appropriate for the
resistors R
1
and R
2
.
For example, with R
2
= 100 K, V
out
= 4.10 V, V
Ref
= 2.5 V, then R
1
= 41.9 K.
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 Equation 1 by replacing V
out
by (V
out
+ V
drop
).
6.2 Current control
The current loop is controlled via the second transconductance operational amplifier, the
sense resistor R
sense
, and the optocoupler.
V
sense
threshold is achieved externally by a resistor bridge tied to the V
Ref
voltage
reference. Its midpoint is tied to the positive input of the current control operational amplifier,
and its foot is to be connected to the lower potential point of the sense resistor, as shown in
Figure 4. The resistors of this bridge are matched to provide the best precision possible.
The control equation verifies that:
Equation 2
Equation 3
where I
lim
is the desired limited current, and V
sense
is the threshold voltage for the current
control loop.
Note that the R
sense
resistor should be chosen taking into account the maximum dissipation
(P
lim
) through it during the full load operation.
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
------------------------------------------------=
