8.1.1 Adjustable Output

As Figure 38 shows, the output voltage of the TPS7A9001 can be adjusted from 0.8 V to 5.2 V by using a resistor divider network.

Figure 38. Adjustable Operation

Use Equation 3 to calculate R₁ and R₂ for any output voltage range. This resistive network must provide a current greater than or equal to 5 µA for optimum noise performance.

Equation 3.

If greater voltage accuracy is required, take into account the output voltage offset contribution resulting from the feedback pin current (I_FB) and use 0.1%-tolerance resistors.

Table 2 lists the resistor combination required to achieve a few of the most common rails using commercially-available, 0.1%-tolerance resistors to maximize nominal voltage accuracy and also abiding to the formula given in Equation 3.

8.1.2 Start-Up

8.1.2.2 Noise-Reduction and Soft-Start Capacitor (C_NR/SS)

The C_NR/SS capacitor serves a dual purpose of both reducing output noise and setting the soft-start ramp during turn-on.

8.1.2.2.1 Noise Reduction

For low-noise applications, the C_NR/SS capacitor forms an RC filter for filtering output noise that is otherwise amplified by the control loop. For low-noise applications, a C_NR/SS of between 10 nF to 10 µF is recommended. Larger values for C_NR/SS can be used; however, above 1 µF there is little benefit in lowering the output voltage noise for frequencies above 10 Hz.

8.1.2.2.2 Soft-Start and In-Rush Current

Soft-start refers to the gradual ramp-up characteristic of the output voltage after the EN and UVLO thresholds are exceeded. Reducing how quickly the output voltage increases during startup also reduces the amount of current needed to charge the output capacitor, referred to as in-rush current. In-rush current is defined as the current going into the LDO during start-up. In-rush current consists of the load current, the current used to charge the output capacitor, and the ground pin current (that contributes very little to in-rush current). This current is difficult to measure because the input capacitor must be removed, which is not recommended. However, Equation 4 can be used to estimate in-rush current:

Equation 4.

where

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

The TPS7A90 features a monotonic, voltage-controlled soft-start that is set with an external capacitor (C_NR/SS). This soft-start helps reduce in-rush current, minimizing load transients to the input power bus that can cause potential start-up initialization problems when powering FPGAs, digital signal processors (DSPs), or other high current loads.

To achieve a monotonic start-up, the TPS7A90 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage exceeds approximately 97% of the internal reference. The final 3% of V_NR/SS is charged through the noise-reduction resistor (R_NR), creating an RC delay. R_NR is approximately 280 kΩ and applications that require the highest accuracy when using a large value C_NR/SS must take this RC delay into account.

The soft-start ramp time depends on the soft-start charging current (I_NR/SS), the soft-start capacitance (C_NR/SS), and the internal reference (V_REF). Use Equation 5 to calculate the approximate soft-start ramp time (t_SS):

Equation 5. t_SS = (V_REF × C_NR/SS) / I_NR/SS

The value for I_NR/SS is determined by the state of the SS_CTRL pin. When the SS_CTRL pin is connected to GND, the typical value for the I_NR/SS current is 6.2 µA. Connecting the SS_CTRL pin to IN increases the typical soft-start charging current to 100 µA. The larger charging current for I_NR/SS is useful if shorter start-up times are needed (such as when using a large noise-reduction capacitor).

8.1.3 Capacitor Recommendation

The TPS7A90 is designed to be stable using low equivalent series resistance (ESR) ceramic capacitors at the input, output, and noise-reduction pin. Multilayer ceramic capacitors are the industry standard for these types of applications and are recommended, but must be used with good understanding of their limitations. Ceramic capacitors that employ X7R-, X5R-, and COG-rated dielectric materials provide relatively good capacitive stability across temperature, whereas the use of Y5V-rated capacitors is discouraged precisely because the capacitance varies so widely. In all cases, ceramic capacitors vary a great deal with operating voltage and temperature and the design engineer must be aware of these characteristics. As a rule of thumb, ceramic capacitors are recommended to be derated by 50%. The input and output capacitors recommended herein account for a capacitance derating of 50%.

8.1.4 Power Dissipation (P_D)

Circuit reliability demands that proper consideration be given to device power dissipation, location of the circuit on the printed circuit board (PCB), and correct sizing of the thermal plane. The PCB area around the regulator must be as free as possible of other heat-generating devices that cause added thermal stresses.

To a first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. Equation 6 calculates P_D:

Power dissipation can be minimized, and thus greater efficiency achieved, by proper selection of the system voltage rails. For the lowest power dissipation use the minimum input voltage necessary for proper output regulation.

The primary heat conduction path for the DSK package is through the thermal pad to the PCB. Solder the thermal pad to a copper pad area under the device. This pad area should contain an array of plated vias that conduct heat to additional copper planes for increased heat dissipation.

The maximum power dissipation determines the maximum allowable ambient temperature (T_A) for the device. According to Equation 7, power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance (θ_JA) of the combined PCB and device package and the temperature of the ambient air (T_A).

Equation 7.

Unfortunately, the thermal resistance (θ_JA) is highly dependent on the heat-spreading capability built into the particular PCB design, and therefore varies according to the total copper area, copper weight, and location of the planes. The θ_JA recorded in the Thermal Information table is determined by the JEDEC standard, PCB, and copper-spreading area and is only used as a relative measure of package thermal performance.

8.1.5 Estimating Junction Temperature

The JEDEC standard recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures of the LDO when in-circuit on a typical PCB board application. These metrics are not strictly speaking thermal resistances, but rather offer practical and relative means of estimating junction temperatures. These psi metrics are determined to be significantly independent of the copper-spreading area. The key thermal metrics (Ψ_JT and Ψ_JB) are used in accordance with Equation 8 and are given in the Thermal Information table.

Equation 8.

where

• P_D is the power dissipated as explained in Equation 6
• T_T is the temperature at the center-top of the device package
• T_B is the PCB surface temperature measured 1 mm from the device package and centered on the package edge

For a more detailed discussion on thermal metrics and how to use them, see the application report Semiconductor and IC Package Thermal Metrics.

8.2.1 Design Requirements

For the design example shown in Figure 39, use the parameters listed in Table 3 as the input parameters.

8.2.2 Detailed Design Procedure

The output voltage can be set to 1.2 V by selecting the correct values for R₁ and R₂; see Equation 3.

Input and output capacitors are selected in accordance with the Capacitor Recommendation section. Ceramic capacitances of 10 µF for both input and output are selected to help balance the charge needed during startup when charging the output capacitor, thus reducing the input voltage drop.

To satisfy the required startup time (t_SS) and still maintain low-noise performance, a 0.01-µF C_NR/SS is selected for with SS_CTRL connected to V_IN. Equation 9 calculates this value. Using I_NR/SS(MAX) and the smallest C_NR/SS capacitance resulting from manufacturing variance (often ±20%) provides the fastest startup time, whereas using I_NR/SS(MIN) and the largest C_NR/SS capacitance resulting from manufacturing variance provides the slowest startup time.

With a 500-mA maximum load, the internal power dissipation is 800 mW, corresponding to a 23°C junction temperature rise. With an 85°C maximum ambient temperature, the junction temperature is at 108°C. To minimize noise, a feed-forward capacitance (C_FF) of 10 nF is selected.

See the Layout section for an example of how to layout the TPS7A90 to achieve best PSRR and noise.

8.2.3 Application Curves

Figure 40. PSRR vs Frequency

Figure 41. Output Noise vs Frequency