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ZHCSG07 February 2017 TPS7A4701-EP

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7 Detailed Description

7.1 Overview

The TPS7A4701-EP is a positive voltage (36 V), ultra-low-noise (4 µV_RMS) LDOs capable of sourcing a 1-A load. The TPS7A4701-EP is designed with bipolar technology primarily for high-accuracy, high-precision instrumentation applications where clean voltage rails are critical to maximize system performance. This feature makes the device ideal for powering operational amplifiers, analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and other high-performance analog circuitry.

7.2 Functional Block Diagram

7.3 Feature Description

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 while load impedance decreases. Note also that 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 Under-Voltage Lockout (UVLO)

The TPS7A4701-EP 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 turnon 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 turnon and shutdown of the LDO when the input voltage is present. EN can be connected directly to V_I if independent turnon 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 turnon after EN and UVLO have achieved threshold voltage. The noise reduction capacitor serves a dual purpose of both governing output noise reduction and programming the soft-start ramp during turnon.

Inrush current is defined as the current through the LDO from IN to OUT during the time of the turnon ramp up. Inrush current then consists primarily of the sum of load and charge current to the output capacitor. Inrush current can be estimated by Equation 1:

Equation 1. TPS7A4701-EP q_iout-t_bvs204.gif

where

7.4 Device Functional Modes

The TPS7A4701-EP has the following functional modes:

Enabled: When EN goes above V_+EN(HI), the device is enabled.
Disabled: When EN goes below V_+EN(LO), the device is disabled. During this time, OUT is high impedance, and the current into IN does not exceed I_(SHDN).

7.5 Programming

7.5.1 ANY-OUT Programmable Output Voltage

TPS7A4701-EP can be used in ANY-OUT mode. For ANY-OUT operation, the device does not use external resistors to set the output voltage, but uses 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. TPS7A4701-EP q_vout_anyout_bvs204.gif

Table 1. ANY-OUT Programmable Output Voltage

ANY-OUT PROGRAM PINS (Active Low)	ADDITIVE OUTPUT VOLTAGE LEVEL
Pin 4 (6P4V2)	6.4 V
Pin 5 (6P4V1)	6.4 V
Pin 6 (3P2)	3.2 V
Pin 8 (1P6)	1.6 V
Pin 9 (0P8)	800 mV
Pin 10 (0P4)	400 mV
Pin 11 (0P2)	200 mV
Pin 12 (0P1)	100 mV

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.

Table 2. Common Output Voltages and Corresponding Pin Settings

V_O (V)	PIN NAMES AND VOLTAGE PER PIN
V_O (V)	0P1V 100 mV	0P2V 200 mV	0P4V 400 mV	0P8V 800 mV	1P6V 1.6 V	3P2V 3.2 V	6P4V1 6.4 V	6P4V2 6.4 V
1.4	Open	Open	Open	Open	Open	Open	Open	Open
1.5	GND	Open	Open	Open	Open	Open	Open	Open
1.8	Open	Open	GND	Open	Open	Open	Open	Open
2.5	GND	GND	Open	GND	Open	Open	Open	Open
3	Open	Open	Open	Open	GND	Open	Open	Open
3.3	GND	GND	Open	Open	GND	Open	Open	Open
4.5	GND	GND	GND	GND	GND	Open	Open	Open
5	Open	Open	GND	Open	Open	GND	Open	Open
10	Open	GND	GND	Open	GND	Open	GND	Open
12	Open	GND	Open	GND	Open	GND	GND	Open
15	Open	Open	Open	GND	Open	Open	GND	GND
18	Open	GND	GND	Open	Open	GND	GND	GND
20.5	GND	GND	GND	GND	GND	GND	GND	GND

7.5.2 Adjustable Operation

The TPS7A4701-EP has an output voltage range of 1.4 V to 34 V. For adjustable operation, set the nominal output voltage of the device using two external resistors, as shown in Figure 23.

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. R₁ can be calculated using Equation 3.

Equation 3. TPS7A4701-EP q_r1_bvs204.gif

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%.

Table 3. Suggested Resistors for Common Voltage Rails

V_OUT	R₁, Calculated	R₁, Closest 1% Value	R₂
1.4 V	0 Ω	0 Ω	∞
1.8 V	2.782 kΩ	2.8 kΩ	9.76 kΩ
3.3 V	13.213 kΩ	13.3 kΩ	9.76 kΩ
5 V	25.650 kΩ	25.5 kΩ	10 kΩ
12 V	77.032 kΩ	76.8 kΩ	10.2 kΩ
15 V	101.733 kΩ	102 kΩ	10.5 kΩ
18 V	118.276 kΩ	118 kΩ	10 kΩ
24 V	164.238 kΩ	165 kΩ	10.2 kΩ

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.