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ESR and Transient Response
C
R
ESR
L
ESL
+
–
V
IN
R
ESR
LDO
I
OUT
V
ESR
C
OUT
R
LOAD
V
OUT
TPS751xxQ
TPS753xxQ
SLVS241C – MARCH 2000 – REVISED OCTOBER 2007
This information, along with the ESR graphs (see Figure 20 and Figure 21 ), 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
these guidelines.
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 22 .
Figure 22. 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 23 shows the output capacitor and its parasitic impedances in a typical LDO output stage.
Figure 23. LDO Output Stage With Parasitic Resistances ESR and ESL
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