AN1818 datasheet(2/6 Pages) STMICROELECTRONICS

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AN1818

Power dissipation under no-load conditions

2/6

The CC-CV (Constant Current - Constant Voltage) standard is a monolithic IC that includes one

independent op-amp and another op-amp for which the non-inverting input is wired to a 2.5V fixed voltage

reference. A good example of such a secondary-side device is ST’s TSM103W.

Normally the CC-CV voltage reference is “shunted”, meaning that the internal current generator requires

an external power supply in order to polarize and fix the voltage reference at 2.5V (Vref = 2.5V).

If we assume that Vout is connected to a discharge battery, the resulting characteristic curve, Vout-Iout, is

shown in Figure 2.

Figure2: Characteristic Vout - Iout for adapter application

In Figure 2, we can see that the load charges gradually, by increasing the current and the voltage in order

to reach a minimal voltage drop. This gradual increase guarantees a limited and stable current. Following

this, the voltage value increases (while the current stays constant) up to the constant voltage value.

A typical adapter application will have Vout_max = 20V (at no-load conditions) and Vout_min = 5V (which is

the minimal voltage necessary to have a constant current).

In order to have Vout_min = 5V, Vcc_min = 5V. If the minimum current value to bias Vref is 1mA, this means

that:

Therefore, in order to have Vout_min = 5V, we must fix Rref = 2.5kΩ.

Now that we have fixed the value of Rref, let's consider a no-load condition where Vout_max = 20V. It

follows that:

234

Iout (A)

Vout_min in CC mode

234

Iout (A)

Vout_min in CC mode

re f

1mA

re f

outmin

re f

–

re f

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

2.5V

–

1mA

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

2.5k

Ω

re f

out

re f

–

re f

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

2.5

–

2.5

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

7mA

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