A8650 datasheet(18/31 Pages) ALLEGRO

18 / 31 page

Low Input Voltage, Adjustable Frequency

2 A Synchronous Buck Regulator with EN/SYNC and Power OK

A8650

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Application Information

Design and Component Selection

Setting the Output Voltage (VOUT, RFB1, RFB2)

The output voltage of the A8650 is determined by connecting a

resistor divider from the output node (VOUT) to the FB pin as

shown in figure 9. There are trade-offs when choosing the value

of the feedback resistors. If the series combination (RFB1+RFB2)

is relatively low, then the light-load efficiency of the regula-

tor will be reduced. So, to maximize that efficiency, it is best to

choose high-value resistors. On the other hand, if the parallel

combination (RFB1//RFB2) is too high, then the regulator may be

susceptible to noise coupling into the FB pin.

In general, to produce the target output voltage, VOUT:

– 1

0.8 (V)

VOUT

RFB2

RFB1

(1)

Table 2 shows the most common output voltages and recom-

mended feedback resistors, assuming <0.2% efficiency loss

at light load of 100 mA, and a parallel combination of 4 kΩ

presented to the FB pin. For optimal system accuracy, it is recom-

mended that the feedback resistors have ≤1% tolerances.

Base PWM Switching Frequency (RFSET)

The base PWM switching frequency is set by connecting a resis-

tor from the FSET pin to ground. Figure 10 is a graph showing

the relationship between the typical switching frequency (y-axis)

and the FSET resistor (x-axis). For a particular base switching

Figure 9. Connecting a feedback resistor divider

RFB1

RFB2

VOUT

FB PIN

Table 2. Recommended Feedback Resistor Values

VOUT

(V)

RFB1

(VOUT to FB pin)

(kΩ)

RFB2

(FB pin to GND)

(kΩ)

1.2

6.04

12.1

1.5

7.50

8.45

1.8

9.09

7.15

2.5

12.4

5.76

3.3

16.5

5.23

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

5.0

15.0

25.0

35.0

45.0

55.0

65.0

75.0

85.0

95.0

RFSET (kΩ)

Figure 10. PWM switching frequency versus RFSET

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