L7987L datasheet(37/41 Pages) STMICROELECTRONICS

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L7987L

Application ideas

Figure 26. Negative (or inverting) buck-boost PCB layout (top and bottom)

Equation 33 and Equation 34 for positive buck-boost can be used to program the output

voltage and estimate the working duty cycle, assuming for VOUT a positive voltage. The

other considerations summarized in Section 7.1 are also applied to the inverting buck-boost.

In this topology the device GND is shorted to VOUT, so the resulting voltage stress on the

integrated power MOS is the sum of VIN and VOUT. Consequently, the maximum input

voltage must be lower than:

Equation 39

VIN,MAX = 61 V is the maximum operating input voltage for the L7987L device, as shown in

Table 5: Electrical characteristics on page 8.

Therefore, if the output voltage is -12 V, the maximum operating input voltage is close

to 49 V, if also the freewheeling diode has the same reverse voltage rating.

For control loop stability analysis and compensation, the transfer function model and

considerations summarized in Section 7.1 are also applied to this topology.

O U T

M A X

I N

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L7987L Datasheet(PDF) 37 Page - STMicroelectronics