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Input Capacitor

Output Capacitor

ESR and Transient Response

R

ESR

L

ESL

C

TPS70751-EP

DUAL-OUTPUT LOW-DROPOUT VOLTAGE REGULATOR

WITH POWER-UP SEQUENCING FOR SPLIT-VOLTAGE DSP SYSTEMS

SLVS718 – DECEMBER 2006

APPLICATION INFORMATION (continued)

For a typical application, an input bypass capacitor (0.1

µF through 1 µF) is recommended. This capacitor filters

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

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

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

As with most LDO regulators, the TPS70751 requires an output capacitor connected between OUT and GND to

stabilize the internal control loop. The minimum recommended capacitance values are 10-

µF ceramic capacitors

with an equivalent series resistance (ESR) between 50 m

Ω and 2.5 Ω, or 6.8-µF tantalum capacitors with ESR

between 250 m

Ω and 4 Ω. Solid tantalum electrolytic, aluminum electrolytic, and multilayer ceramic capacitors

with capacitance values greater than 10

µF are all suitable, provided they meet the requirements previously

described. Larger capacitors provide a wider range of stability and better load transient response. The following

is a partial listing of surface-mount capacitors usable with the TPS70751 for fast transient response application.

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

user's application. When it is 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 previous guidelines.

VALUE

MFR.

MAX ESR

PART NO.

22

µF

Kemet

345 m

Ω

7495C226K0010AS

33

µF

Sanyo

100 m

Ω

10TPA33M

47

µF

Sanyo

100 m

Ω

6TPA47M

68

µF

Sanyo

45 m

Ω

10TPC68M

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

capacitors are used to support the load current the 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 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 40.

Figure 40. 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.

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