Datasheet

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Input Capacitor
Output Capacitor
ESR and Transient Response
R
ESR
L
ESL
C
TPS70145, TPS70148
TPS70151, TPS70158
TPS70102
SLVS222D – DECEMBER 1999 – REVISED NOVEMBER 2004
For a typical application, an input bypass capacitor (0.1µF – 1µF) 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 dependent on the output current and response time of the main power supply, as well as the
distance to the V
I
pins of the LDO.
As with most LDO regulators, the TPS701xx requires an output capacitor connected between OUT and GND to
stabilize the internal control loop. The minimum recommended capacitance value is 10µF and the ESR
(equivalent series resistance) must be between 50m Ω and 2.5 Ω . Capacitor values 10µF 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 usable with the TPS701xx 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.
Table 3. Partial Listing of TPS701xx-Compatible Surface-Mount Capacitors
VALUE MANUFACTURER MAXIMUM ESR MFR PART NO.
22µF Kemet 345m Ω 7495C226K0010AS
33µF Sanyo 100m Ω 10TPA33M
47µF Sanyo 100m Ω 6TPA47M
68µF 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 47 .
Figure 47. 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.
Figure 48 shows the output capacitor and its parasitic impedances in a typical LDO output stage.
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