TPS746 datasheet(20/45 Pages) TI

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TPS746-Q1

OUT

VIN

VOUT

CIN

COUT

RPG*

*Pull-up resistor not required

for push-pull option

GND

Cff

TPS746-Q1

SBVS358A – JUNE 2018 – REVISED OCTOBER 2019

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Product Folder Links: TPS746-Q1

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8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

8.1.1 Adjustable Device Feedback Resistors

Figure 50 shows that the output voltage of the TPS746P-Q1 can be adjusted from 0.55 V to 5.5 V by using a

resistor divider network.

Figure 50. Adjustable Operation

The adjustable-version device requires external feedback divider resistors to set the output voltage. VOUT is set

using the feedback divider resistors, R1 and R2, according to the following equation:

VOUT = VFB × (1 + R1 / R2)

(2)

To ignore the FB pin current error term in the VOUT equation, set the feedback divider current to 100x the FB pin

current listed in the Electrical Characteristics table. This setting provides the maximum feedback divider series

resistance, as shown in the following equation:

R1 + R2 ≤ VOUT / (IFB × 100)

(3)

8.1.2 Input and Output Capacitor Selection

The TPS746-Q1 requires an output capacitance of 0.47 µF or larger for stability. Use X5R- and X7R-type

ceramic capacitors because these capacitors have minimal variation in value and equivalent series resistance

(ESR) over temperature. When choosing a capacitor for a specific application, pay attention to the dc bias

characteristics for the capacitor. Higher output voltages cause a significant derating of the capacitor. For best

performance, the maximum recommended output capacitance is 220 µF.

Although an input capacitor is not required for stability, good analog design practice is to connect a capacitor

from IN to GND. Some input supplies have a high impedance, thus placing the input capacitor on the input

supply helps reduce the input impedance. This capacitor counteracts reactive input sources and improves

transient response, input ripple, and PSRR. If the input supply has a high impedance over a large range of

frequencies, several input capacitors can be used in parallel to lower the impedance over frequency. Use a

higher-value capacitor if large, fast, rise-time load transients are anticipated, or if the device is located several

inches from the input power source.

8.1.3 Dropout Voltage

The TPS746-Q1 uses a PMOS pass transistor to achieve low dropout. When (VIN – VOUT) is less than the

dropout voltage (VDO), the PMOS pass device is in the linear region of operation and the input-to-output

resistance is the RDS(ON) of the PMOS pass element. VDO scales approximately with output current because the

PMOS device behaves like a resistor in dropout mode. As with any linear regulator, PSRR and transient

response degrade as (VIN – VOUT) approaches dropout operation.

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