Datasheet

LT1963A Series
15
1963aff
For more information www.linear.com/LT1963A
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 20µs.
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
10µs. For improved transient response the value of the
bulk capacitor (tantalum or aluminum electrolytic) 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 gen
-
eral 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
LT1963A. 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.
Ceramic Capacitors
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior over
temperature and applied voltage. The most common
dielectrics used
are Z5U, Y5V, X5R and X7R. The Z5U and
Table 1. Capacitor Minimum ESR
V
OUT
10µF 22µF 47µF 100µF
1.2V 20mΩ 15mΩ 10mΩ 5mΩ
1.5V 20mΩ 15mΩ 10mΩ 5mΩ
1.8V 15mΩ 10mΩ 10mΩ 5mΩ
2.5V 5mΩ 5mΩ 5mΩ 5mΩ
3.3V 0mΩ 0mΩ 0mΩ 5mΩ
≥5V 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 LT1963A at three different output
voltages with various capacitors and various values of ESR.
The output load conditions are the same for all traces. In
all cases there is a DC load of 500mA. The load steps up
to 1A at the first transition and steps back to 500mA at
the second transition.
At the worst case point of 1.2V
OUT
with 10µF C
OUT
(Figure 3), a minimum
amount of ESR is required. While
20mΩ is enough to eliminate most of the ringing, a value
closer to 50mΩ provides a more optimum response. At
2.5V output with 10µF C
OUT
(Figure 4) the output rings
at the transitions with 0Ω ESR but still settles to within
10mV
in 20µs after the 0.5A load step. Once again a small
value of ESR will provide a more optimum response.
At 5
V
OUT
with 10µF C
OUT
(Figure 5) the response is well
damped with 0Ω ESR.
With a C
OUT
of 100µF at 0Ω ESR and an output of 1.2V
(Figure 6), the output rings although the amplitude is
only 20mV
p-p
. With C
OUT
of 100µF it takes only 5mΩ to
20mΩ of ESR to provide good damping at 1.2V output.
Performance at 2.5V and 5V output with 100µF C
OUT
shows
similar characteristics to the 10µF case (see Figures 7-8).
At 2.5V
OUT
5mΩ to 20mΩ can improve transient response.
At 5V
OUT
the response is well damped with 0Ω ESR.
Capacitor types with inherently higher ESR can be combined
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
applicaTions inForMaTion