TSM1052 datasheet(9/15 Pages) STMICROELECTRONICS

9 / 15 page

TSM1052

Application information

9/15

4.2.2

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:

Equation 2

where Ilim is the desired limited current, and VSENSE is the threshold voltage for the current

control loop.

As an example, with Ilim = 1 A, VSENSE = 200 mV, then RSENSE = 200 mΩ.

Note:

The Rsense resistor should be chosen taking into account the maximum dissipation (Plim)

through it during full load operation.

Equation 3

As an example, with Ilim = 1 A, and Vsense = 200 mV, Plim = 200 mW.

Therefore, for most adapter and battery charger applications, a quarter-watt, or half-watt

resistor is sufficient. VSENSE threshold is made internally by a voltage divider tied to the Vref

voltage reference. Its middle point 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 Figure 15 on page 8. The resistors of this voltage divider are matched

to provide the best possible accuracy. 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 either the voltage control or the current control, driving the

optocoupler's photodiode to transmit the feedback to the primary side.

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 diagram.

(with the power supply of the device indipendent of the output voltage)

lim

sense

lim

sense

⋅

lim

sense

lim

⋅

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