LT1963A Series
applicaTions inForMaTion
Table 1. Capacitor Minimum ESR
POSCAP capacitors are used. The output voltage is at the
worst case value of 1.2V. Trace A, is with a 10µF ceramic
outputcapacitorandshowssignificantringingwithapeak
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.
V
10µF
20mΩ
20mΩ
15mΩ
5mΩ
22µF
15mΩ
15mΩ
10mΩ
5mΩ
47µF
10mΩ
10mΩ
10mΩ
5mΩ
100µF
5mΩ
5mΩ
5mΩ
5mΩ
5mΩ
0mΩ
OUT
1.2V
1.5V
1.8V
2.5V
3.3V
≥5V
0mΩ
0mΩ
0mΩ
ForTraceC,a100µF/35mΩPOSCAPisconnectedinparallel
withthe10µFceramiccapacitor.Inthiscasethepeakoutput
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.
0mΩ
0mΩ
0mΩ
Figures3through8showtheeffectofESRonthetransient
response of the regulator. These scope photos show the
transientresponsefortheLT1963Aatthreedifferentoutput
voltageswithvariouscapacitorsandvariousvaluesofESR.
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.
Tantalum and Polytantalum Capacitors
There is a variety of tantalum capacitor types available,
with a wide range of ESR specifications. Older types have
ESRspecificationsinthehundredsofmΩtoseveralOhms.
Some newer types of polytantalum with multi-electrodes
havemaximumESRspecificationsaslowas5mΩ. Ingen-
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.
At the worst case point of 1.2V
with 10µF C
OUT
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
(Figure 4) the output rings
OUT
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 5V
with 10µF C
(Figure 5) the response is well
OUT
OUT
Aluminum Electrolytic Capacitors
damped with 0Ω ESR.
Aluminumelectrolyticcapacitorscanalsobeusedwiththe
LT1963A.Thesecapacitorscanalsobeusedinconjunction
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.
With a C of 100µF at 0Ω ESR and an output of 1.2V
OUT
(Figure 6), the output rings although the amplitude is
only 20mV . With C of 100µF it takes only 5mΩ to
p-p
OUT
20mΩ of ESR to provide good damping at 1.2V output.
Performanceat2.5Vand5Voutputwith100µFC shows
OUT
similar characteristics to the 10µF case (see Figures ±-8).
At2.5V
5mΩto20mΩcanimprovetransientresponse.
OUT
Ceramic Capacitors
At 5V
the response is well damped with 0Ω ESR.
OUT
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 X±R. The Z5U and
CapacitortypeswithinherentlyhigherESRcanbecombined
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
1963aff
15
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