L7987L datasheet(34/41 Pages) STMICROELECTRONICS

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Application ideas

L7987L

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DocID026362 Rev 3

Application ideas

7.1

Positive buck-boost

The L7987L device can implement the step-up/down conversion with a positive output

voltage.

Figure 23 shows the complete schematic: one power MOSFET and one Schottky diode are

added to the standard buck topology to provide 24 V output voltage, with input voltage from

12 V to 60 V. In this design example, the programmed switching frequency is 570 kHz and

the maximum expected load is 0.5 A.

Figure 23. L7987L - positive buck-boost schematic example

In this topology the relationship between input and output voltage is:

Equation 33

So the duty cycle is given by:

Equation 34

The output voltage isn't limited by the maximum operating voltage of the device, because

the output voltage is sensed only through the resistor divider. The external power MOSFET

maximum drain to source voltage must be higher than the output voltage and also the

additional diode, D2, must be rated for the same maximum voltage.

In Figure 23 a clamping network has been added to limit the Q1 gate to source voltage

(C10, R15 and D5) and to speed up Q1 turn-off time (D4).

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