SEA05L Datasheet, PDF (7/12 Pages) STMicroelectronics – Advanced constant voltage and constant current controller with very efficient LED pilot-lamp driver

SEA05L
Application information
Current control
The current loop is controlled via a transconductance operational amplifier, the sense
resistor R_sense, and the optocoupler.
The control equation verifies:
Equation 3
R_sense ∗ I_{O max} = V_csth
Equation 4
R_sense
=
V_csth
I _{O max}
where I_Omax is the desired limited current, and V_csth is the threshold voltage for the current
control loop. As an example, with I_omax = 1 A, V_csth = 50 mV, then R_sense = 50 mΩ.
Note that the R_sense resistor should be chosen, taking into account the maximum dissipation
(P_lim) through it during full load operation.
Equation 5
P_lim = V_csth ∗ I_{O max}
As an example, with I_Omax = 1 A and V_csth = 50 mV, P_lim = 50 mW.
Therefore, for most adaptor and battery charger applications, a low-power resistor is
suitable for the current sensing function.
V_csth threshold is achieved internally by a voltage divider tied to an internal precise voltage
reference. Its midpoint is tied to the positive input of the current control operational amplifier,
and its endpoint is connected to GND. The resistors of this voltage divider are matched to
provide the best precision possible. The current sinking outputs of the two transconductance
operational amplifiers are common (to the output of the IC). This makes an OR function
which ensures that whenever the current or the voltage reaches excessively high values, the
optocoupler is activated.
The relationship 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
(with power supply of the device independent from the output voltage).
Doc ID 18462 Rev 1
7/12