Application Notes
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PT4840 Input/ Output Filter Capacitance Selection
for Excalibur
Triple-Ouput DC/DC Converters
General Requirements
The capacitors on the input bus are optional but may be
required to insure dynamic response to load transients.
The suggested capacitors on the input bus include
ceramic noise attenuation components. The PT4840
series has an internal 1μH input inductor. This inductor
provides an effective input differential noise filter when
1μF ceramic capacitors are connected across the input
terminals. This low impedance filter has an attenuation
factor of typically 15dB.
The output capacitors are all optional and may be used
to optimize dynamic and transient load performance. The
maximum capacitance allowed for each bus is given in the
electrical specification table on p.3.
Input Capacitors
The input capacitors are all optional. The 33μF/100V
electrolytic capacitor should have a minimum ripple
current 400mA. Ripple current and <350m
equivalent
series resistance (ESR) are the major considerations along
with temperature when selecting electrolytic capacitors.
The ceramic capacitors, each 1μF (3
×
0.33μF), X7R type,
have a wide temperature range and are suggested for
noise reduction. The module’s internal inductor and the
external ceramic capacitors form an excellent differential
noise filter. Additional filter components, L
1
and L
2
,
will reject common-mode input noise (
See Figure 1
). A
common-mode choke can also be used to reduce common-
mode noise levels. These optional filter components may
be required to meet FCC Class A limits for conducted
emissions.
PT4840 Series
Figure 1; Optional Capacitor and FIlter Components
IC Chip Set
+V
IN
–
V
IN
+15VDC
+3.3VDC
+1.5VDC
PT4841
1
2
11
15,16
22,23
18
–
20
12,13
3
Vo
1
COM
COM
Vo
2
Vo
3
+V
IN
–
V
IN
EN 1
Vo
2
adj
Vo
3
adj
17
21
C
OUT 1
C
OUT 3
C
OUT 2
4
EN 2
L1
1μH
Optional
L1
1μH
Optional
C
0.33μFx3
Ceramic
C
IN
C
0.33μFx3
Ceramic
Optional Output Capacitors; Vo
1
(Table 1)
The ESR of the 330μF output capacitor for Vo
1
(+15V)
must be
≥
40m
. Electrolytic capacitors have minimal
effect on ripple at frequencies greater than 200kHz but
excellent low-frequency transient response. At the ripple
frequency, ceramic decoupling capacitors improve the re-
sponse to fast transients and reduce any high-frequency
noise components during higher current excursions.
The preferred capacitor part numbers are identified in the
Table 1. The table identifies vendors with acceptable
ESR and ripple current (rms) ratings. The suggested
minimum quantities for Vo
1
are identified. Tantalum
capacitors, rated 30V or greater with an equivalent ESR
of
≥
40m
,
are also recommended for Vo
1
.
Optional Output Capacitors; Vo
2
& Vo
3
(Table 2)
The combined ESR of the output capacitors selected for
Vo
2
and Vo
3
(lower bus voltages) must be
≥
10m
. Low
ESR electrolytic capacitors have minimal effect on ripple
at frequencies greater than 200kHz but excellent low-
frequency transient response. Above the ripple frequency,
ceramic decoupling capacitors improve the response to
fast transients and reduce any high-frequency noise
components during higher current excursions. The
preferred lower ESR type capacitor part numbers for
Vo
2
and Vo
3
are identified in Table 2. The table identifies
vendors with acceptable ESR and ripple current (rms)
ratings. The tantalum and Oscon type capacitors have
both low ESR and stable characteristics. These are
recommended for Vo
2
and Vo
3
in applications where the
temperature range extends below 0°C.
Capacitors from other vendors may also be suitable. The speci-
fications in both tables provide guidance for the selection of
alternative parts. The ripple current (rms) rating and ESR
(Equivalent Series Resistance at 100kHz) are the critical
parameters necessary to insure both optimum regulator and
long term capacitor life cycles.