TPS73601DBVT datasheet(20/51 Pages) TI

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7.2.1 Design Requirements

For this design example, use the parameters listed below as the input parameters.

Table 7-1. Design Parameters (Fixed-Voltage Version)

PARAMETER

DESIGN REQUIREMENT

Input voltage

5 V, ±3%

Output voltage

3.3 V, ±1%

Output current

400 mA (maximum), 20 mA (minimum)

RMS noise, 10 Hz to 100 kHz

< 30 μVRMS

Ambient temperature

55°C (maximum)

Table 7-2. Design Parameters (Adjustable-Voltage Version)

PARAMETER

DESIGN REQUIREMENT

Input voltage

5 V, ±3%, provided by the dc/dc converter switching at 1 MHz

Output voltage

2.5 V, ±1%

Output current

0.4 A (maximum), 10 mA (minimum)

RMS noise, 10 Hz to 100 kHz

< 35 μVRMS

Ambient temperature

55°C (maximum)

7.2.2 Detailed Design Procedure

The first step when designing with a linear regulator is to examine the maximum load current along with the

input and output voltage requirements to determine if the device thermal and dropout voltage requirements can

be met. At 0.4 A, the dropout voltage of the TPS73633 is a maximum of 200 mV over temperature; thus, the

dropout headroom is sufficient for operation over both input and output voltage accuracy.

The maximum power dissipated in the linear regulator is the maximum voltage dropped across the pass

transistor from the input to the output times the maximum load current. In this example, the maximum voltage

drop across in the pass transistor is 5 V + 3% (5.15 V) minus 3.3 V – 1% (3.267 V) or 1.883 V. The power

dissipated in the pass transistor is calculated by taking this voltage drop multiplied by the maximum load current.

For this example, the maximum power dissipated in the linear regulator is 942 mW. Once the power dissipated

in the linear regulator is known, the corresponding junction temperature rise can be calculated. To calculate the

junction temperature rise above ambient, the power dissipated must be multiplied by the junction-to-ambient

thermal resistance. For thermal resistance information see the Thermal Protection section. For this example,

using the DRB package, the maximum junction temperature rise is calculated to be 45°C. The maximum junction

temperature rise is calculated by adding junction temperature rise to the maximum ambient temperature. In this

example, the maximum junction temperature is 100°C. Keep in mind the maximum junction temperate must be

below 125°C for reliable operation. Addition ground planes, added thermal vias, and air flow all help to lower the

maximum junction temperature. Using the DCQ or DBV packages are not recommended for this application due

to the excessive junction temperature rise that would be incurred.

To get the noise level below 30 µVRMS, a noise reduction capacitance (CNR) of 10 nF is selected along with

an output capacitance of 10 μF. Referencing the Output Noise section, the RMS noise can be calculated to be

28 µVRMS.

Use of an input capacitor is optional. However, in systems where the input supply is located several inches away

from the LDO, a small 0.1-µF input capacitor is recommended to negate the adverse effects that input supply

inductance has on stability and ac performance.

In the same way as with designing with a fixed output voltage, the first step is to examine the maximum load

current along with the input and output voltage requirements to determine if the device thermal and dropout

voltage requirements are met. At 0.4 A, the maximum dropout voltage is 200 mV. Since the input voltage is 5

V and the output voltage is 2.5 V, there is more than sufficient voltage headroom to avoid dropout and maintain

good PSRR.

TPS736

SBVS038V – SEPTEMBER 2003 – REVISED SEPTEMBER 2024

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