7.3.1 Internal Current Limit (I_CL)

The internal current limit circuit is used to protect the LDO against high-load current faults or shorting events. The LDO is not designed to operate at a steady-state current limit. During a current-limit event, the LDO sources constant current. Therefore, the output voltage falls when load impedance decreases. Also, when a current limit occurs while the resulting output voltage is low, excessive power is dissipated across the LDO, which results in a thermal shutdown of the output.

7.3.2 Enable (EN) And Undervoltage Lockout (UVLO)

The TPS7A47-Q1 only turns on when both EN and UVLO are above the respective voltage thresholds. The UVLO circuit monitors input voltage (V_I) to prevent device turn-on before V_I rises above the lockout voltage. The UVLO circuit also causes a shutdown when V_I falls below lockout. The EN signal allows independent logic-level turn-on and shutdown of the LDO when the input voltage is present. EN can be connected directly to V_I if independent turn-on is not needed.

7.3.3 Soft-Start And Inrush Current

Soft-start refers to the ramp-up characteristic of the output voltage during LDO turn-on after EN and UVLO have achieved the threshold voltage. The noise-reduction capacitor serves a dual purpose of both governing output noise reduction and programming the soft-start ramp during turn-on.

Inrush current is defined as the current through the LDO from IN to OUT during the time of the turn-on ramp up. Inrush current then consists primarily of the sum of load and charge current to the output capacitor. Use Equation 1 to estimate in-rush current:

Equation 1.

where

• V_OUT(t) is the instantaneous output voltage of the turn-on ramp
• dV_OUT(t) / dt is the slope of the V_O ramp
• R_LOAD is the resistive load impedance

7.5.1 ANY-OUT Programmable Output Voltage

For ANY-OUT operation, do not use external resistors to set the output voltage, but use device pins 4, 5, 6, 8, 9, 10, 11, and 12 to program the regulated output voltage. Each pin is either connected to ground (active) or is left open (floating). The ANY-OUT programming is set by Equation 2 as the sum of the internal reference voltage (V_(REF) = 1.4 V) plus the accumulated sum of the respective voltages assigned to each active pin; that is, 100 mV (pin 12), 200 mV (pin 11), 400 mV (pin 10), 800 mV (pin 9), 1.6 V (pin 8), 3.2 V (pin 6), 6.4 V (pin 5), or 6.4 V (pin 4). Table 1 summarizes these voltage values associated with each active pin setting for reference. By leaving all program pins open, or floating, the output is thereby programmed to the minimum possible output voltage equal to V_(REF).

Equation 2.

Table 2 shows a list of the most common output voltages and the corresponding pin settings. The voltage setting pins have a binary weight; therefore, the output voltage can be programmed to any value from 1.4 V to 20.5 V in 100-mV steps.

7.5.2 Adjustable Operation

The TPS7A47-Q1 has an output voltage range of 1.4 V to 34 V. For adjustable operation, set the nominal output voltage of the device (as shown in Figure 23) using two external resistors.

Figure 23. Adjustable Operation for Maximum AC Performance

R₁ and R₂ can be calculated for any output voltage within the operational range. The current through feedback resistor R₂ must be at least 5 µA to ensure stability. Additionally, the current into the FB pin (I_(FB), typically 350 nA) creates an additional output voltage offset that depends on the resistance of R₁. For high-accuracy applications, select R₂ such that the current through R₂ is at least 35 µA to minimize any effects of I_(FB) variation on the output voltage; 10 kΩ is recommended. Equation 3 calculates R₁.

Equation 3.

where

• V_REF = 1.4 V
• I_FB = 350 nA

Use 0.1% tolerance resistors to minimize the effects of resistor inaccuracy on the output voltage.

Table 3 shows the resistor combinations to achieve some standard rail voltages with commercially available 1% tolerance resistors. The resulting output voltages yield a nominal error of < 0.5%.

To achieve higher nominal accuracy, two resistors can be used in the place of R₁. Select the two resistor values such that the sum results in a value as close as possible to the calculated R₁ value.

There are several alternative ways to set the output voltage. The program pins can be pulled low using external general-purpose input/output pins (GPIOs), or can be hardwired by the given layout of the printed circuit board (PCB) to set the ANY-OUT voltage. The TPS7A4701 evaluation module (EVM), available for purchase from the TI eStore, allows the output voltage to be programmed using jumpers.