lt1963aes8-trpbf Linear Technology Corporation, lt1963aes8-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt1963aes8-trpbf
• Description: 1.5a, Low Noise, Fast Transient Response Ldo Regulators
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1963A Series
Table 1. Capacitor Minimum ESR
V
1.2V
1.5V
1.8V
2.5V
3.3V
≥ 5V
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 val-
ues 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
(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
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
damped with 0Ω ESR.
With a C
(Figure 6), the output rings although the amplitude is only
20mV
of ESR to provide good damping at 1.2V output. Perfor-
mance at 2.5V and 5V output with 100µF C
ilar characteristics to the 10µF case (see Figures 7-8). At
2.5V
At 5V
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
12
OUT
OUT
OUT
OUT
p-p
OUT
. With C
5mΩ to 20mΩ can improve transient response.
the response is well damped with 0Ω ESR.
with 10µF C
20mΩ
20mΩ
15mΩ
10
5mΩ
0mΩ
0mΩ
of 100µF at 0Ω ESR and an output of 1.2V
µ
F
OUT
U
of 100µF it takes only 5mΩ to 20mΩ
OUT
15mΩ
15mΩ
10mΩ
22
5mΩ
0mΩ
0mΩ
OUT
U
(Figure 5) the response is well
µ
F
(Figure 4) the output rings
W
OUT
10mΩ
10mΩ
10mΩ
47
5mΩ
0mΩ
0mΩ
µ
with 10µF C
F
OUT
shows sim-
U
100
5mΩ
5mΩ
5mΩ
5mΩ
5mΩ
0mΩ
µ
F
OUT
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 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
LT1963A. These capacitors can also be used in conjunc-
tion with ceramic capacitors. These tend to be the cheap-
est 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
Y5V dielectrics are good for providing high capacitances
in a small package, but exhibit strong voltage and tem-
perature coefficients as shown in Figures 10 and 11. When
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