7.3.1 Adjustable Operation

The TPS73801 has an adjustable output voltage range of 1.21 V to 20 V. The output voltage is set by the ratio of two external resistors as shown in Figure 24. The device maintains the voltage at the FB pin at 1.21 V referenced to ground. The current in R1 is then equal to (1.21 V/R1), and the current in R2 is the current in R1 plus the FB pin bias current. The FB pin bias current, 3 μA at 25 °C, flows through R2 into the FB pin. The output voltage can be calculated using the formula shown in Equation 1. The value of R1 should be less than 4.17 kΩ to minimize errors in the output voltage caused by the FB pin bias current. Note that in shutdown the output is turned off, and the divider current is zero.

Figure 24. Adjustable Operation

The output voltage can be set using the following equations:

Equation 1.

7.3.2 Fixed Operation

The TPS73801 can be used in a fixed voltage configuration. By connecting the FB pin to OUT the TPS73801 will regulate the output to 1.21 V. During fixed voltage operation, the FB pin can be used for a Kelvin connection if routed separately to the load. This allows the regulator to compensate for voltage drop across parasitic resistances (RP) between the output and the load. This becomes more crucial with higher load currents.

Figure 25. Kelvin Sense Connection

7.3.3 Overload Recovery

Like many IC power regulators, the TPS73801 has safe operating area protection. The safe area protection decreases the current limit as input-to-output voltage increases and keeps the power transistor inside a safe operating region for all values of input-to-output voltage. The protection is designed to provide some output current at all values of input-to-output voltage up to the device breakdown.

When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads. During start up, as the input voltage is rising, the input-to-output voltage differential is small, allowing the regulator to supply large output currents. With a high input voltage, a problem can occur wherein removal of an output short does not allow the output voltage to recover. Other regulators also exhibit this phenomenon, so it is not unique to the TPS73801.

The problem occurs with a heavy output load when the input voltage is high and the output voltage is low. Common situations occur immediately after the removal of a short circuit or when the shutdown pin is pulled high after the input voltage has already been turned on. The load line for such a load may intersect the output current curve at two points. If this happens, there are two stable output operating points for the regulator. With this double intersection, the input power supply may need to be cycled down to zero and brought up again to make the output recover.

7.3.4 Output Voltage Noise

The TPS73801 regulators have been designed to provide low output voltage noise over the 10-Hz to 100-kHz bandwidth while operating at full load. Output voltage noise is typically 40 nV/√Hz over this frequency bandwidth for the TPS73801. For higher output voltages (generated by using a resistor divider), the output voltage noise is gained up accordingly. This results in RMS noise over the 10-Hz to 100-kHz bandwidth of 14 μVRMS for the TPS73801.

Higher values of output voltage noise may be measured when care is not exercised with regards to circuit layout and testing. Crosstalk from nearby traces can induce unwanted noise onto the output of the TPS73801. Powersupply ripple rejection must also be considered; the TPS73801 regulators do not have unlimited power-supply rejection and pass a small portion of the input noise through to the output.

7.3.5 Protection Features

The TPS73801 regulators incorporate several protection features that make them ideal for use in battery-powered circuits. In addition to the normal protection features associated with monolithic regulators, such as current limiting and thermal limiting, the devices are protected against reverse input voltages, reverse output voltages and reverse voltages from output to input.

Current limit protection and thermal overload protection are intended to protect the device against current overload conditions at the output of the device. For normal operation, the junction temperature should not exceed 125°C.

The input of the device withstands reverse voltages of 20 V. Current flow into the device is limited to less than 1 mA (typically less than 100 μA), and no negative voltage appears at the output. The device protects both itself and the load. This provides protection against batteries that can be plugged in backward.

The output of the TPS73801 can be pulled below ground without damaging the device. If the input is left open circuit or grounded, the output can be pulled below ground by 20 V. The output acts like an open circuit; no current flows out of the pin. If the input is powered by a voltage source, the output sources the short-circuit current of the device and protects itself by thermal limiting. In this case, grounding the EN pin turns off the device and stops the output from sourcing the short-circuit current.

The FB pin can be pulled above or below ground by as much as 7 V without damaging the device. If the input is left open circuit or grounded, the FB pin acts like an open circuit when pulled below ground and like a large resistor (typically 5 kΩ) in series with a diode when pulled above ground.

In situations where the FB pin is connected to a resistor divider that would pull the FB pin above its 7-V clamp voltage if the output is pulled high, the FB pin input current must be limited to less than 5 mA. For example, a resistor divider is used to provide a regulated 1.5-V output from the 1.21-V reference when the output is forced to 20 V. The top resistor of the resistor divider must be chosen to limit the current into the FB pin to less than 5 mA when the FB pin is at 7 V. The 13-V difference between OUT and FB pins divided by the 5-mA maximum current into the FB pin yields a minimum top resistor value of 2.6 kΩ.

In circuits where a backup battery is required, several different input/output conditions can occur. The output voltage may be held up while the input is either pulled to ground, pulled to some intermediate voltage, or is left open circuit. When the IN pin of the TPS73801 is forced below the OUT pin or the OUT pin is pulled above the IN pin, input current typically drops to less than 2 μA. This can happen if the input of the device is connected to a discharged (low voltage) battery and the output is held up by either a backup battery or a second regulator circuit. The state of the EN pin has no effect on the reverse output current when the output is pulled above the input.

See the device modes in Table 1.