Datasheet

ESR and Transient Response
3-3
Circuit Design
When C
o
is conducting current to the load, initial voltage at the load will be
V
O
= V(C
o
) – V
ESR
. Due to the discharge of C
o
, the output voltage V
O
will drop
continuously until the response time t
1
of the LDO is reached and the LDO will
resume 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 3–3.
The figure also shows the impact of different ESRs on the output voltage. The
left brackets show different levels of ESRs where number 1 displays the lowest
and number 3 displays the highest ESR.
From above, the following conclusions can be drawn:
The higher the ESR, the larger the droop at the beginning of load transient.
The smaller the output capacitor, the faster the discharge time and the
bigger the voltage droop during the LDO response period.
3.1.1 Conclusion
To minimize the transient output droop, capacitors must have a low ESR and
be large enough to support the minimum output voltage requirement for a
given LDO response time.
Figure 3–3. Correlation of Different ESRs and Their Influence to the Regulation of V
O
at a Load Step From Low-to-High Output Current
ESR 1
ESR 2
ESR 3
3
1
2
t
1
t
2
I
O
V
O