LT1764A Series
APPLICATIO S I FOR ATIO
ing the capacitor value improves the stability and reduces
the ringing. Table 1 gives some recommended values of
ESR to minimize ringing caused by fast, hard current
transitions.
Table 1. Capacitor Minimum ESR
VOUT
10µF
22µF
1.2V
10mΩ
5mΩ
1.5V
7mΩ
5mΩ
1.8V
5mΩ
5mΩ
2.5V
0mΩ
0mΩ
3.3V
0mΩ
0mΩ
≥ 5V
0mΩ
0mΩ
47µF
3mΩ
3mΩ
3mΩ
0mΩ
0mΩ
0mΩ
100µF
0mΩ
0mΩ
0mΩ
0mΩ
0mΩ
0mΩ
Figures 3 through 8 show the effect of ESR on the transient
response of the regulator. These scope photos show the
transient response for the LT1764A at three different
output voltages with various capacitors and various val-
ues of ESR. The output load conditions are the same for all
traces. In all cases there is a DC load of 1A. The load steps
up to 2A at the first transition and steps back to 1A at the
second transition.
At the worst case point of 1.2VOUT with 10µF COUT
(Figure 3), a minimum amount of ESR is required. While
5mΩ is enough to eliminate most of the ringing, a value
closer to 20mΩ provides a more optimum response. At
2.5V output with 10µF COUT (Figure 4) the output rings
at the transitions with 0Ω ESR but still settles to within
10mV in 20µs after the 1A load step. Once again a small
value of ESR will provide a more optimum response.
At 5VOUT with 10µF COUT (Figure 5) the response is well
damped with 0Ω ESR.
With a COUT of 100µF at 0Ω ESR and an output of 1.2V
(Figure 6), the output rings although the amplitude is only
10mVp-p. With COUT of 100µF it takes only 5mΩ to 20mΩ
of ESR to provide good damping at 1.2V output. Perfor-
mance at 2.5V and 5V output with 100µF COUT shows sim-
ilar characteristics to the 10µF case (see Figures 7-8). At
2.5VOUT 5mΩ to 20mΩ can improve transient response.
At 5VOUT the response is well damped with 0Ω ESR.
Capacitor types with inherently higher ESR can be com-
bined with 0mΩ ESR ceramic capacitors to achieve both
good high frequency bypassing and fast settling time.
Figure 9 illustrates the improvement in transient response
that can be seen when a parallel combination of ceramic
and POSCAP capacitors are used. The output voltage is at
the worst case value of 1.2V. Trace A, is with a 10µF
ceramic output capacitor and shows significant ringing
with a peak amplitude of 25mV. For Trace B, a 22µF/45mΩ
POSCAP is added in parallel with the 10µF ceramic. The
output is well damped and settles to within 10mV in less
than 5µs.
For Trace C, a 100µF/35mΩ POSCAP is connected in
parallel with the 10µF ceramic capacitor. In this case the
peak output deviation is less than 20mV and the output
settles in about 5µs. For improved transient response the
value of the bulk capacitor (tantalum or aluminum electro-
lytic) should be greater than twice the value of the ceramic
capacitor.
Tantalum and Polytantalum Capacitors
There is a variety of tantalum capacitor types available,
with a wide range of ESR specifications. Older types have
ESR specifications in the hundreds of mΩ to several
Ohms. Some newer types of polytantalum with multi-
electrodes have maximum ESR specifications as low as
5mΩ. In general the lower the ESR specification, the larger
the size and the higher the price. Polytantalum capacitors
have better surge capability than older types and generally
lower ESR. Some types such as the Sanyo TPE and TPB
series have ESR specifications in the 20mΩ to 50mΩ
range, which provide near optimum transient response.
Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors can also be used with the
LT1764. These capacitors can also be used in conjunction
with ceramic capacitors. These tend to be the cheapest
and lowest performance type of capacitors. Care must be
used in selecting these capacitors as some types can have
ESR which can easily exceed the 3Ω maximum value.
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