APPLICATION INFORMATION
Capacitor Recommendations for the PTV12020 Series of Power Modules
Input Capacitors
Output Capacitor (Optional)
Ceramic Capacitors
Tantalum Capacitors
Capacitor Table
PTV12020W/L
SLTS231A – NOVEMBER 2004 – REVISED FEBRUARY 2005
The required input capacitors are a 22-µF ceramic and 560-µF electrolytic type. For V
O
> 2.1 V and I
O
≥ 11 A ,
the 560-µF capacitance must be rated for 1,200 mArms ripple current capability. For other conditions, V
O
> 2.1 V
at I
O
< 11 A and V
O
≤ 2.1 V for all loads, the ripple current rating must be at least 750 mArms. Where applicable,
Table 1 gives the maximum output voltage and current limits for a capacitor's rms ripple current rating.
The above ripple current requirements are conditional that the 22-µF ceramic capacitor is present. The 22-µF
X5R/X7R ceramic capacitor is necessary to reduce both the magnitude of ripple current through the electroytic
capacitor and the amount of ripple current reflected back to the input source. Ceramic capacitors should be
located within 0.5 in. (1,3 cm) of the module’s input pins. Additional ceramic capacitors can be added to reduce
the RMS ripple current requirement for the electrolytic capacitor.
Ripple current (Arms) rating, less than 100-m Ω equivalent series resistance (ESR), and temperature are the
major considerations when selecting input capacitors. Unlike polymer-tantalum capacitors, regular tantalum
capacitors have a recommended minimum voltage rating of 2 × (max. DC voltage + AC ripple). This is standard
practice to ensure reliability. Only a few tantalum capacitors were found to have sufficient voltage rating to meet
this requirement. At temperatures below 0 ° C, the ESR of aluminum electrolytic capacitors increases. For these
applications Os-Con, polymer-tantalum, and polymer-aluminum types should be considered.
For applications with load transients (sudden changes in load current), regulator response benefits from external
output capacitance. The recommended output capacitance of 330 µF allows the module to meet its transient
response specification. For most applications, a high-quality computer-grade aluminum electrolytic capacitor is
adequate. These capacitors provide decoupling over the frequency range, 2 kHz to 150 kHz, and are suitable
when ambient temperatures are above 0 ° C. For operation below 0 ° C, tantalum, ceramic, or Os-Con type
capacitors are recommended. When using one or more nonceramic capacitors, the calculated equivalent ESR
should be no lower than 4 m Ω (7 m Ω using the manufacturer's maximum ESR for a single capacitor). A list of
preferred low-ESR type capacitors are identified in Table 1 .
Above 150 kHz, the performance of aluminum electrolytic capacitors is less effective. Multilayer ceramic
capacitors have low ESR and a resonant frequency higher than the bandwidth of the regulator. They can be
used to reduce the reflected ripple current at the input as well as improve the transient response of the output.
When used on the output their combined ESR is not critical as long as the total value of ceramic capacitance
does not exceed approximately 300 µF. Also, to prevent the formation of local resonances, do not place more
than five identical ceramic capacitors in parallel with values of 10 µF or greater.
Tantalum-type capacitors can only be used on the output bus, and are recommended for applications where the
ambient operating temperature can be less than 0 ° C. The AVX TPS, Sprague 593D/594/595 and Kemet
T495/T510 capacitor series are suggested over many other tantalum types due to their higher rated surge, power
dissipation, and ripple current capability. As a caution, many general-purpose tantalum capacitors have
considerably higher ESR, reduced power dissipation and lower ripple current capability. These capacitors are
also less reliable as they have reduced power dissipation and surge current ratings. Tantalum capacitors that
have no stated ESR or surge current rating are not recommended for power applications.
When specifying Os-con and polymer tantalum capacitors for the output, the minimum ESR limit is encountered
before the maximum capacitance value is reached.
Table 1 identifies the characteristics of capacitors from a number of vendors with acceptable ESR and ripple
current (rms) ratings. The recommended number of capacitors required at both the input and output buses is
identified for each capacitor type.
8
