Data Sheet

ADP1706/ADP1707/ADP1708

THEORY OF OPERATION

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Rev. B | 13 of 19

where:

R1 is the resistor from OUT to ADJ.

R2 is the resistor from ADJ to GND.

The maximum bias current into ADJ is 100 nA, so for less than

0.5% error due to the bias current, use values less than 60 kΩ for

R2.

TRACK MODE (ADP1707)

The ADP1707 includes a tracking mode feature. As shown in Fig-

ure 30, if the voltage applied at the TRK pin is less than the nominal

output voltage, OUT is equal to the voltage at TRK. Otherwise,

OUT regulates to its nominal output value.

Figure 30. ADP1707 Output Voltage vs. Tracking Voltage

For example, consider an ADP1707 with a nominal output voltage

of 3.3 V. If the voltage applied to its TRK pin is greater than 3.3

V, OUT maintains a nominal output voltage of 3.3 V. If the voltage

applied to TRK is reduced below 3.3 V, OUT tracks this voltage.

OUT can track the TRK pin voltage from the nominal value all the

way down to 0 V. A voltage divider is present from TRK to the error

amplifier input with a divider ratio equal to the divider from OUT

to the error amplifier, which sets the output voltage equal to the

tracking voltage. Both divider ratios are set by postpackage trim,

depending on the desired output voltage.

ENABLE FEATURE

The ADP1706/ADP1707/ADP1708 use the EN pin to enable and

disable the OUT pin under normal operating conditions. As shown

in Figure 31, when a rising voltage on EN crosses the active

threshold, OUT turns on. When a falling voltage on EN crosses the

inactive threshold, OUT turns off.

Figure 31. ADP1706 Typical EN Pin Operation

As shown in Figure 31, the EN pin has hysteresis built in. This

prevents on/off oscillations that can occur due to noise on the EN

pin as it passes through the threshold points.

The EN pin active/inactive thresholds are derived from the IN

voltage. Therefore, these thresholds vary when changing the input

voltage. Figure 32 shows typical EN active/inactive thresholds when

the input voltage varies from 2.5 V to 5.5 V.

Figure 32. Typical EN Pin Thresholds vs. Input Voltage