MPZ2012S221A datasheet(25/56 Pages) STMICROELECTRONICS

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where IOUT_pri and IOUT_sec are respectively the primary and secondary output currents, N is the transformer turn

ratio, D the duty cycle and ΔIL the current ripple in the primary winding.

Figure 31 depicts how these peak currents vary depending on the secondary output current, under the specified

conditions.

Figure 31. Peak currents in LS and HS MOSFET depending on the isolated output current (VIN = 12 V,

VOUT_prim = 5 V, N = 6)

Depending on the application conditions (for example, duty cycle and therefore the input voltage), the peak or the

reverse current limit can be first exceeded and hence represents the limitation of the secondary isolated output

current (see Figure 32).

Figure 32. Left: VIN = 12 V, IPRIM exceeds IVY_SINK. Right: VIN = 18 V, IPRIM exceeds IPK.

Once the reverse current limit is crossed, the device turns the LS MOSFET off and the current flows in the body

diode of the HS MOSFET until it reaches zero. Both HS and LS MOSFET remain off until the next clock cycle.

7.3

Thermal shutdown

The shutdown block disables the switching activity if the junction temperature is higher than a fixed internal

threshold (TSHDWN refer to Table 6. Electrical characteristics). The thermal sensing element is close to the power

elements, ensuring fast and accurate temperature detection. A hysteresis of approximately 30 °C prevents the

device from turning ON and OFF too fast. After a thermal protection event has expired, the L6983I restarts with a

new soft-start.

7.4

Power good

The PGOOD pin indicates whether the output voltage is within its regulation level. The pin output is an open drain

MOSFET. The PG is pulled low:

When the FB pin voltage is lower than 90 % (typ.) of the nominal internal reference for more than 10 µs.

When the FB pin voltage is higher than 120% (typ.) of the nominal internal reference for more than 10 µs.

During the soft-start procedure also with pre-charged VOUT.

If a thermal shutdown event occurs.

If a UVLO event occurs.

L6983I

Thermal shutdown

DS14040 - Rev 1

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MPZ2012S221A 数据表(PDF) 25 Page - STMicroelectronics