SEA05LTR datasheet(7/12 Pages) STMICROELECTRONICS

7 / 12 page

SEA05L

Application information

Doc ID 18462 Rev 1

7/12

Current control

The current loop is controlled via a transconductance operational amplifier, the sense

resistor Rsense, and the optocoupler.

The control equation verifies:

Equation 3

Equation 4

where IOmax is the desired limited current, and Vcsth is the threshold voltage for the current

control loop. As an example, with Iomax = 1 A, Vcsth = 50 mV, then Rsense = 50 mΩ.

Note that the Rsense resistor should be chosen, taking into account the maximum dissipation

(Plim) through it during full load operation.

Equation 5

As an example, with IOmax = 1 A and Vcsth = 50 mV, Plim = 50 mW.

Therefore, for most adaptor and battery charger applications, a low-power resistor is

suitable for the current sensing function.

Vcsth 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).

csth

sense

∗

max

csth

sense

max

lim

csth

∗

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