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ESR and Transient Response
R
SER
L
ESL
C
-
-
+
LDO
V
I
V
ESR
I
OUT
R
ESR
C
OUT
R
LOAD
V
OUT
+
TPS70845, TPS70848
TPS70851, TPS70858
TPS70802
SLVS301D – JUNE 2000 – REVISED DECEMBER 2007
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 37 .
Figure 37. 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 38 shows the output capacitor and its parasitic resistances in a typical LDO output stage.
Figure 38. LDO Output Stage with Parasitic Resistances ESR
In steady state (dc state condition), the load current is supplied by the LDO (solid arrow) and the voltage across
the capacitor is the same as the output voltage (V
(CO)
= V
OUT
). This condition means no current is flowing into the
C
O
branch. If I
OUT
suddenly increases (a transient condition), the following results occur:
• The LDO is not able to supply the sudden current need because of its response time. Therefore, capacitor C
O
provides the current for the new load condition (dashed arrow). C
O
now acts like a battery with an internal
resistance, ESR. Depending on the current demand at the output, a voltage drop occurs at R
ESR
. This voltage
is shown as V
ESR
in Figure 38 .
• When C
O
is conducting current to the load, initial voltage at the load will be V
O
= V
(CO)
– V
ESR
. As a result of
the discharge of C
O
, the output voltage V
O
drops continuously until the response time t
1
of the LDO is
reached and the LDO resumes supplying the load. From this point, the output voltage starts rising again until
it reaches the regulated voltage. This period is shown as t
2
in Figure 39 .
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