Datasheet

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Input Capacitor

Output Capacitor

ESR and Transient Response

ESR

ESL

TPS70745, TPS70748

TPS70751, TPS70758

TPS70702

SLVS291D – MAY 2000 – REVISED DECEMBER 2007

For a typical application, an input bypass capacitor (0.1 µ F to 1 µ F) is recommended. This capacitor filters any

high-frequency noise generated in the line. For fast transient conditions where droop at the input of the LDO may

occur because of high inrush current, it is recommended to place a larger capacitor at the input as well. The size

of this capacitor depends on the output current and response time of the main power supply, as well as the

distance to the V

pins of the LDO.

As with most LDO regulators, the TPS707xx requires an output capacitor connected between OUT and GND to

stabilize the internal control loop. The minimum recommended capacitance value is 10 µ F and the ESR

(equivalent series resistance) must be between 50m Ω and 2.5 Ω . Capacitor values 10 µ F or larger are acceptable,

provided the ESR is less than 2.5 Ω . Solid tantalum electrolytic, aluminum electrolytic, and multilayer ceramic

capacitors are all suitable, provided they meet the requirements described above. Larger capacitors provide a

wider range of stability and better load transient response. Table 3 provides a partial listing of surface-mount

capacitors suitable for use with the TPS707xx for fast transient response application.

This information, along with the ESR graphs, is included to assist in selection of suitable capacitance for the user

application. When necessary to achieve low height requirements along with high output current and/or high load

capacitance, several higher ESR capacitors can be used in parallel to meet the guidelines above.

Table 3. Partial Listing of TPS707xx-Compatible Surface-Mount Capacitors

VALUE MANUFACTURER MAXIMUM ESR MFR PART NO.

22F Kemet 345m Ω 7495C226K0010AS

33F Sanyo 100m Ω 10TPA33M

47F Sanyo 100m Ω 6TPA47M

68F Sanyo 45m Ω 10TPC68M

LDOs typically require an external output capacitor for stability. In fast transient response applications, capacitors

are used to support the load current while the LDO amplifier is responding. In most applications, one capacitor is

used to support both functions.

Besides its capacitance, every capacitor also contains parasitic impedances. These parasitic impedances are

resistive as well as inductive. The resistive impedance is called equivalent series resistance (ESR), and the

inductive impedance is called equivalent series inductance (ESL). The equivalent schematic diagram of any

capacitor can therefore be drawn as shown in Figure 43 .

Figure 43. ESR and ESL

In most cases one can neglect the effect of inductive impedance ESL. Therefore, the following application

focuses mainly on the parasitic resistance ESR.