SI9140DQ VISHAY [Vishay Siliconix], SI9140DQ Datasheet - Page 15

• Manufacturer Part Number: SI9140DQ
• Description: MP Controller For High Performance Process Power Supplies
• Manufacturer: VISHAY [Vishay Siliconix]

DI = desired output current ripple. Typically DI = 25% of maximum
output current.
Finally, the time required to ramp up the current in the inductor
can be reduced with smaller inductance. A quick response
from the power supply relaxes the decoupling capacitance
required at the microprocessor, reducing the overall solution
cost and size.
Input Capacitor
The input capacitor’s function is to filter the raw power and
serve as the local power source to eliminate power-up and
transient surge failures. The type and characteristics of input
capacitors are determined by the input power and inductance
of the step-down converter.
requirement usually dominates the selection criteria. The
capacitance required to maintain regulation will automatically
be achieved once it meets the ripple current requirement. The
following equation calculates the ripple current of the input
capacitor:
An aluminum-electrolytic capacitor from Sanyo (OS-CON),
AVX (TPS Tantalum), or Nichicon (PL series) should be used
in high-power (30-W) applications to handle the ripple current.
The Sanyo capacitor is smaller and handles higher ripple
current than Nichicon, but at higher cost than the Nichicon
product. The AVX Tantalum capacitor has the best
capacitance and current handling capability per volume ratio,
but it takes extra surface area compared to OS-CON or PL
series.
increased drastically in the recent past due to high demand,
causing designers to shy away from the TPS Tantalum
capacitors. Nichicon capacitors can be used to provide an
economical solution if space is available or a large bulk
capacitance is already present on the input line. The number
Document Number: 70026
S-40699—Rev. H, 19-Apr-04
L +
I
RIPPLE
V
The TPS capacitors, lead time and cost have
IN
+ I
V
DI
O
RMSSW
2
f
OSC
2
– I
IN
2
The ripple current handling
of Sanyo (OS-CON) input capacitors required to handle
various output currents are specified in Table 2.
Output Capacitor
To regulate the microprocessor’s input voltage within 145 mV
during 10-A load transients, a large output capacitance with
low ESR is required. The output capacitor of the power supply
and decoupling capacitors at the microprocessor must hold up
the processor voltage until the power supply responds to the
change. Even with fastest known switching solution, it still
takes three 330-mF OS-CON capacitors to handle the load
transient. If it weren’t for the 10-A load transient, the output
capacitor would not need a low ESR value. The fundamental
output ripple current in a continuous step-down converter is
much lower than the input ripple current. Maintaining voltage
regulation during transients requires an ESR in the range of
30 mW.
requirements, the number of output and decoupling capacitors
can be reduced. The lower transient requirements also allows
greater consideration for Tantalum or Nichicon PL series
capacitors.
Conclusion
The Si9140 synchronous Buck controller’s ability to switch up
to 1 MHz combined with a 25-MHz error amplifier provides the
best solution in powering high- performance microprocessors.
The high switching frequency reduces inductor size without
compromising output ripple voltage.
bandwidth generated with the help of a 25-MHz error amplifier
reduces the amount of decoupling capacitors required to
handle the extreme transient requirement.
synchronous fixed-frequency operation eliminates the pulse
skipping mode that generates random unpredictable
EMI/EMC problems in desktop and notebook computers. The
synchronous rectification also allows the converter to operate
in continuous current mode, independent of output load
current.
bandwidth required to meet the transient demand of the
microprocessor as it transitions from stop clock and auto halt
to active mode. The synchronous rectification improves the
efficiency of the converter by substituting the much smaller I
MOSFET loss for the VI diode loss. The need for heatsinking
is eliminated by using low r
Si4435DY).
This preserves the wide closed-loop converter
For
microprocessors
DS(on)
TrenchFETs (Si4410DY and
Vishay Siliconix
with
The wide converter
lower
The Si9140’s
www.vishay.com
Si9140
transient
2
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