14
LT1533
APPLICATIONS INFORMATION
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Ceramic: Generally used for high frequency and high
voltage bypass. If all ceramic capacitors are used, they can
have such a low ESR as to cause loop stability problems.
Often they can resonate with their ESL before ESR be-
comes effective.
Input Capacitor
The requirements for the input capacitor are less stringent
for this part. Input current ripple is lower because of the
push-pull action and low noise features of the part. How-
ever, the input capacitor should have low ESR at high
frequencies since this will be an important factor concern-
ing how much conducted noise is created. Values of input
capacitor will typically be in the 1µF to 22µF range with
ESR under 0.3Ω.
The input capacitor can see a high surge current when a
battery of high capacitance source is connected “live.”
Some solid tantalum capacitors can fail under this condi-
tion. Several manufacturers have developed a line of solid
tantalum capacitors specially tested for surge capability
(e.g., AVX TPS series). However, even these units may fail
if the input voltage approaches the maximum voltage
rating of the capacitor. AVX recommends derating capaci-
tor voltage by 2:1 for high surge applications.
Output Filter Capacitor
Output capacitors are usually chosen on the basis of ESR
since this will determine output ripple. Typical required
ESR will be in the 0.05Ω to 0.3Ω range.
The specific value for capacitance will depend on topol-
ogy. A typical output capacitor is an AVX type TPS, 22µF
and 25V with a guaranteed ESR less than 0.2Ω. To further
reduce ESR, multiple output capacitors can be used in
parallel. The value in microfarads is not particularly impor-
tant. A small 22µF tantalum capacitor will have high ESR
and higher output voltage ripple. Table 1 shows some
typical surface mount capacitors.
Table 1
SIZE CAPACITOR ESR (MAX Ω)
E CASE AVX TPS, Sprague 593D 0.1 to 0.3
AVX TAJ 0.7 to 0.9
D CASE AVX TPS, Sprague 593D 0.1 to 0.3
AVX TAJ 0.9 to 2.0
Panasonic CD 0.05 to 0.18
C CASE AVX TPS 0.2 (Typ)
AVX TAJ 1.8 to 3.0
B CASE AVX TAJ 2.5 to 10
Switching Diodes
In general, switching diodes should be Schottky diodes
such as 1N5818 or MBR130 (1A/30V). Low output current
applications may use 1N4148 switching diodes.
Unregulated Applications
The LT1533 can be used to create a low noise “DC
transformer” unregulated power supply. DC transformers
are open-loop switching regulators where the output
voltage is controlled by the turns ratio of the transformer.
A DC transformer provides a low cost isolated supply.
For such applications, the DUTY pin of the LT1533 should
be grounded. This will force the outputs into a 50% on,
50% off mode. Note that because of slew control there will
be some variance from 50%. Figure 6 shows a 5V to ±12V
DC transformer.
One concern with this type of application is having both
switch outputs transition at the same time. This can cause
both primary side windings to have positive EMF added to
the winding, causing the current to run away. Since this
part controls slew rate this won’t happen. It is possible to
see slightly increased total current draw when both drivers
are on, but this will be controlled and observable. Since the
outputs share a common sense resistor, the outputs will
turn off when the total current in both exceeds the limit set
by the V
C
pin.
The FB pin should be DC biased between 0.7V and 1.2V to
prevent frequency shifting from occurring. This also en-
sures that the V
C
pin is set to its upper clamp, providing
peak output current.
