7.3.1 Adjustable Operation

The TPS798xx-Q1 has an output voltage range of 1.275 V to 28 V. The output voltage is set by the ratio of two external resistors as shown in Figure 18. The feedback loop monitors the output to maintain the voltage at the adjust pin at 1.275 V referenced to ground. The current in R₁ is then equal to 1.275 V/R₁, and the current in R₂ is the current in R₁ plus the FB pin bias current. The FB pin bias current, 0.2 μA at 25°C, flows through R₂ into the FB pin. The output voltage can be calculated using the formula in Figure 18. The value of R₁ should be less than 250 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 18. Adjustable Operation

A 100-pF capacitor (C₁) placed in parallel with the top resistor (R₂) of the output divider is necessary for stability and transient performance of the adjustable TPS798xx-Q1. The impedance of C₁ at 10 kHz should be less than the value of R₂.

The adjustable device is tested and specified with the FB pin tied to the OUT pin and a 1 mA-DC load (unless otherwise specified) for an output voltage of 1.275 V. Specifications for output voltages greater than 1.275 V are proportional to the ratio of the desired output voltage to 1.275 V (V_OUT/1.275 V). For example, load regulation for an output current change of 1 mA to 50 mA is –10 mV (typical) at V_OUT = 1.275 V.

At V_OUT = 12 V, load regulation is:

7.3.2 Output Capacitance and Transient Response

The TPS798xx-Q1 is designed to be stable with a wide range of output capacitors. The ESR of the output capacitor affects stability, most notably with small capacitors. To prevent oscillations, TI recommends a minimum output capacitor of 1 μF with an ESR of 3 Ω or less. The TPS798xx-Q1 is a micropower device, and output transient response is a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the TPS798xx-Q1, increase the effective output capacitor value.

Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior over temperature and applied voltage. The most common dielectrics used are Z5U, Y5 V, X5R, and X7R. The Z5U and Y5 V dielectrics are good for providing high capacitances in a small package, but exhibit strong voltage and temperature coefficients. When used with a 5 V regulator, a 10-μF Y5 V capacitor can exhibit an effective value as low as 1 μF to 2 μF over the operating temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expensive and is available in higher values.

Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals because of mechanical stress, similar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor, the stress can be induced by vibrations in the system or thermal transients.

7.3.3 Calculating Junction Temperature

Given an output voltage of 5 V, an input voltage range of 15 V to 24 V, an output current range of 0 mA to 50 mA, and a maximum ambient temperature of 50°C, the maximum junction temperature is calculated as follows.

The power dissipated (P_DISS) by the DGN package is equal to:

Therefore,

The thermal resistance is approximately 60°C/W, based on JEDEC 51-5 profile. Therefore, the junction temperature rise above ambient is approximately equal to:

The maximum junction temperature is then equal to the maximum junction temperature rise above ambient plus the maximum ambient temperature or:

7.3.4 Protection Features

The TPS798xx-Q1 incorporates several protection features that make it 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 device is protected against reverse-input voltages, and reverse currents 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. The junction temperature should not exceed 125°C.

The input of the device withstands reverse voltages of –60 V. Current flow into the device is limited to less than 6 mA (typically, less than 100 μA), and no negative voltage appears at the output. The device protects both itself and the load. This architecture also provides protection against batteries that may be plugged in backwards.

The FB pin of the adjustable device can be pulled above or below ground by as much as 7 V without damaging the device. If the input is left open or grounded, the FB pin behaves as an open circuit when pulled below ground, or as a large resistor (typically, 100 kΩ) in series with a diode when pulled above ground. If the input is powered by a voltage source, pulling the FB pin below the reference voltage increases the output voltage. This configuration causes the output to go to a unregulated high voltage. Pulling the FB pin above the reference voltage turns off all output current.

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 provides a regulated 1.5-V output from the 1.275-V reference when the output is forced to 28 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 21-V difference between the OUT and FB pins divided by the 5-mA maximum current into the FB pin yields a minimum top resistor value of 5.8 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. The rise in reverse output current above 7 V occurs from the breakdown of the 7-V clamp on the FB pin. With a resistor divider on the regulator output, this current is reduced, depending on the size of the resistor divider.

When the IN pin of the TPS798xx-Q1 is forced below the OUT pin, or the OUT pin is pulled above the IN pin, input current typically drops to less than 0.6 mA. This scenario can occur if the input of the TPS798xx-Q1 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.

7.4.1 Low-Voltage Tracking

At low input voltages, the regulator drops out of regulation and the output voltage tracks input minus a voltage based on the load current and switch resistance. This allows for a smaller input capacitor and can possibly eliminate the need of using a boost convertor during cold-crank conditions.