The output voltage of the MAX757 is set by two resistors,
R1 and R2 (Figure 1), which form a voltage divider
between the output and the FB pin. The output voltage
is set by the equation:
VOUT = (VREF) [(R2 + R1) / R2]
where VREF = 1.25V.
To simplify resistor selection:
R1 = (R2) [(VOUT / VREF) - 1]
Since the input bias current at FB has a maximum
value of 100nA, large values (10k½ to 200k½) can be
used for R1 and R2 with no significant loss of accuracy.
For 1% error, the current through R1 should be at least
100 times FB’s bias current