www.irf.com

9

AHP270XXS Series

Attempts to adjust the output voltage to a value greater than

120% of nominal should be avoided because of the potential

of exceeding internal component stress ratings and

subsequent operation to failure. Under no circumstance

should the external setting resistor be made less than 500

Ω.

By remaining within this specified range of values, completely

safe operation fully within normal component derating is

assured.

The consequence is that if the +sense connection is

unintentionally broken, an AHP270XXS has a fail-safe output

voltage of Vout + 250mV, where the 250mV is independent

of the nominal output voltage. It can be further demonstrated

that in the event of both the + and - sense connections

being broken, the output will be limited to Vout + 500mV.

This 500mV is also essentially constant independent of the

nominal output voltage. While operation in this condition is

not damaging to the device, not all performance parameters

will be met.

Enable 2

Share

+Sense

- Sense

Return

+Vout

AHP270XXS

R

ADJ

To Load

Figure VI. Connection for V

Examination of the equation relating output voltage and

resistor value reveals a special benefit of the circuit topology

utilized for remote sensing of output voltage in the

AHP270XXS series of converters. It is apparent that as the

resistance increases, the output voltage approaches the

nominal set value of the device. In fact the calculated limiting

value of output voltage as the adjusting resistor becomes

very large is

≅ 250mV above nominal device voltage.