ADFS5758 datasheet(33/75 Pages) AD

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Data Sheet

ADFS5758

Rev. 0 | Page 33 of 75

Calculating Supply Voltage

Assuming DPC current mode, use the following equations to

calculate the voltage and current values:

VDPC+ Maximum = IOUT Maximum Voltage +

IOUT Headroom = 22.5 V

where:

IOUT Maximum = 20 mA (RLOAD = 1 kΩ).

IOUT Maximum Voltage is IOUT Maximum × RLOAD = 20 V.

IOUT Headroom = 2.5 V.

|VDPC+ to AVDD1| headroom can be calculated as 5% of 22.5 V =

1.125 V. Therefore, AVDD1 minimum = 22.5 V + 1.125 V =

23.625 V. Assuming a worst case AVDD1 supply rail tolerance

of ±10%, this example requires an AVDD1 supply rail of

approximately 26.25 V.

AVSS Considerations

AVSS is the negative supply rail and has a range of −33 V to

0 V. As in the case of AVDD1, AVSS must obey the maximum

operating range of |AVDD1 to AVSS| of 60 V. For bipolar current

output ranges, the maximum AVSS can be calculated as (IOUT_MAX ×

RLOAD) + IOUT footroom. For unipolar current output ranges,

AVSS can be tied to AGND (that is, 0 V). For unipolar voltage

output ranges, the maximum AVSS is −2 V to enable sufficient

footroom for the internal voltage output circuitry. To avoid

power supply sequencing issues, a Schottky diode must be

placed between AVSS and GND (the GND supply must always

be available).

AVDD2 Considerations

AVDD2 is the positive low voltage supply rail and has a range of

5 V to 33 V. If only one positive power rail is available, AVDD2

can be tied to AVDD1. However, to optimize for reduced power

dissipation, supply AVDD2 with a separate lower voltage supply.

VLOGIC Considerations

VLOGIC is the digital supply for the device and can range from

1.71 V to 5.5 V. The 3.3 V VLDO output voltage can be used to

drive VLOGIC.

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