A6986I datasheet(30/62 Pages) STMICROELECTRONICS

30 / 62 page

Equation (26) emphasizes how the selection of the transformer and, to some extent, the Schottky diode plays a

crucial role in the accurate regulation of the secondary output voltage. The figure below shows instead the effect

of the current drawn from the primary output on the isolated voltage, as described by equation (9).

Figure 32. Isolated voltage variation due to the primary output current

10%

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

OUT_pri

[mA]

Another factor affecting the secondary output voltage regulation comes from the leakage inductance of the

transformer. As described later on, the leakage inductance determines the peak current that can be reached in

the secondary winding. This peak current defines the maximum current from the secondary isolated output by

limiting the amount of charge delivered to the output. When the current demand from the secondary isolated

output exceeds the maximum deliverable charge limited by the peak current, the isolated output voltage drops.

Figure 33. Secondary output maximum current

To some extent also the duty cycle can affect the secondary output voltage. Since the energy transfer from

primary to secondary occurs only during the TOFF, a too high duty cycle could reduce the achievable peak

current, hence limiting the deliverable current to the secondary output. Under this condition the secondary output

could drop.

7.2

Switching frequency

A resistor connected to the FSW pin features the selection of the switching frequency. The pinstrapping is

performed at power-up, before the soft-start takes place. The FSW pin is pinstrapped and then driven floating in

order to minimize the quiescent current from VIN. Please refer to Table 6 to identify the pull-up / pull-down resistor

value.

Setting fSW = 250 kHz or fSW = 500 kHz does not require any external resistor.

The selection of the switching frequency must take into account the minimum on-time of the device (TON MIN, as

indicated in the electrical characteristics table), as described by the following equation.

fSW_max≤ VOUT

TON MIN∙VIN∙η

(27)

The switching frequency affects the selection of the primary inductance as well as the transformer construction.

A6986I

Switching frequency

DS13481 - Rev 2

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