TPS70745, TPS70748, TPS70751, TPS70758, TPS70702

DUAL-OUTPUT LOW-DROPOUT VOLTAGE REGULATORS

WITH POWER UP SEQUENCING FOR SPLIT VOLTAGE DSP SYSTEMS

SLVS291 – MAY 2000

28

POST OFFICE BOX 655303

APPLICATION INFORMATION

input capacitor

For a typical application, an input bypass capacitor (0.1

µF – 1 µF) is recommended. This capacitor will filter

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

output capacitor

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 values are 10

µF ceramic

capacitors with an ESR (equivalent series resistance) 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

described above. Larger capacitors provide a wider range of stability and better load transient response. Below

is a partial listing of surface-mount capacitors usable 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’s 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.

VALUE

MFR.

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

ESR and transient response

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

capacitors are used to support the load current while 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.

C

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.