TPS79801-Q1, TPS79850-Q1

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SLVS822E – MARCH 2009 – REVISED SEPTEMBER 2015

Feature Description (continued)

7.3.2 Output Capacitance and Transient Response

The TPS798xx-Q1 is designed to be stable with a wide range of output capacitors. The ESR of the output

capacitor affects stability, most notably with small capacitors. To prevent oscillations, TI recommends a minimum

output capacitor of 1

μF with an ESR of 3 Ω or less. The TPS798xx-Q1 is a micropower device, and output

transient response is a function of output capacitance. Larger values of output capacitance decrease the peak

deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to

decouple individual components powered by the TPS798xx-Q1, increase the effective output capacitor value.

Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior over temperature and applied voltage. The most common

dielectrics used are Z5U, Y5 V, X5R, and X7R. The Z5U and Y5 V dielectrics are good for providing high

capacitances in a small package, but exhibit strong voltage and temperature coefficients. When used with a 5 V

regulator, a 10-

μF Y5 V capacitor can exhibit an effective value as low as 1 μF to 2 μF over the operating

temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for

use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less

expensive and is available in higher values.

Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates voltage across its terminals because of mechanical

stress, similar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor, the stress

can be induced by vibrations in the system or thermal transients.

7.3.3 Calculating Junction Temperature

Given an output voltage of 5 V, an input voltage range of 15 V to 24 V, an output current range of 0 mA to

50 mA, and a maximum ambient temperature of 50°C, the maximum junction temperature is calculated as

follows.

where

•

•

•

•

(2)

Therefore,

(3)

The thermal resistance is approximately 60°C/W, based on JEDEC 51-5 profile. Therefore, the junction

temperature rise above ambient is approximately equal to:

0.974 W × 60°C/W = 58.44°C

(4)

The maximum junction temperature is then equal to the maximum junction temperature rise above ambient plus

the maximum ambient temperature or:

(5)

7.3.4 Protection Features

The TPS798xx-Q1 incorporates several protection features that make it ideal for use in battery-powered circuits.

In addition to the normal protection features associated with monolithic regulators, such as current limiting and

thermal limiting, the device is protected against reverse-input voltages, and reverse currents from output to input.

Current limit protection and thermal-overload protection are intended to protect the device against current

overload conditions at the output of the device. The junction temperature should not exceed 125°C.

The input of the device withstands reverse voltages of –60 V. Current flow into the device is limited to less than

6 mA (typically, less than 100

μA), and no negative voltage appears at the output. The device protects both itself

and the load. This architecture also provides protection against batteries that may be plugged in backwards.

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