8

Application ideas

8.1

Positive buck-boost

The A7987 device can implement the step-up/down conversion with a positive output voltage.

The figure below 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 an input voltage from 18 V to 60 V. In this design

example, the programmed switching frequency is 510 kHz and the maximum expected load is 0.8 A.

Figure 24. Positive buck-boost schematic example

+

+

R8

47k

EN

C10

68nF

C2

33uF

SYNCH

C3

1uF

VIN

GND

C1

4.7uF

U1

A7987

PGOOD

VIN_2

SS

SYNCH

FSW

ILIM

COMP

8

EN

FB

GND

16

VCC

LX_1

LX_2

VIN_1

VBIAS

BOOT

TP

17

3

6

7

10

11

5

4

2

R11

1.6k

12

9

13

14

1

15

C7

100nF

C11

680pF

R6

1.1k

C9

220nF

C10

470pF

R7

R5

560

D1

D4

R15

100R

D5

L1

VOUT

47k

C8

4.7nF

GND

Q1

C18

1uF

D2

STPS3L60

C6

47uF

47uH

PGOOD

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

VOUT=VIN⋅ D1−D

(33)

so the duty cycle is given by:

D= VOUT

VOUT+VIN

(34)

The output voltage is not 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 24. Positive buck-boost schematic example a clamping network has been added to limit Q1 gate to

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

The current flowing through the internal power MOSFET is transferred to the load only during the OFF-time, so

according to the maximum allowed A7987 DC switch current (3.0 A), the maximum output current for the buck-

boost topology can be calculated by the equation below.

ISW=IOUT1−D<3A

(35)

where ISW is the average current in the embedded power MOSFET during the ON-time.

In addition to these constraints, the thermal considerations summarized in Section 6.5 Thermal considerations

must also be evaluated.

A7987

Application ideas

DS12928 - Rev 3

page 26/36