Datasheet

ESR and Transient Response
3-2
Circuit Design
3.1 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
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 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 3–1.
Figure 3–1. ESR and ESL
R
ESR
L
ESL
C
In most cases one can neglect the effect of inductive impedance ESL. There-
fore, the following application focuses mainly on the parasitic resistance ESR.
Figure 3–2 shows the output capacitor and its parasitic impedances in a typical
LDO output stage.
Figure 3–2. LDO Output Stage With Parasitic Resistances ESR
–
LDO
V
I
V
ESR
I
O
R
ESR
C
O
R
LOAD
V
O
+
+
–
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(C
o
) = V
O
). This means no current is flowing into or out of the C
o
branch.
If I
O
suddenly increases (transient condition), the LDO is not able to supply the
sudden current need due to its response time (t
1
in Figure 3–3). Therefore, ca-
pacitor C
o
provides the current for the new load condition (dashed arrow). C
o
now acts like a battery with an internal resistance, R
ESR
. Depending on the
current demand at the output, a voltage drop will occur at R
ESR
. This voltage
is shown as V
ESR
in Figure 3–2.