LX1668 Ver la hoja de datos (PDF) - Microsemi Corporation
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LX1668 Datasheet PDF : 15 Pages
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PRODUCT DATABOOK 1996/1997
P ROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER
PR O D U C T I O N D ATA S H E E T
LX1668
APPLICATION INFORMATION
OUTPUT CAPACITOR (continued)
excursion in the transient. Adaptive voltage positioning in-
creases the value of V , allowing a higher ESR value and
EX
reducing the cost of the output capacitor. Typically, the
positioning voltage is 40mV (peak), using the LX1668, and the
transient tolerance is 100mV, resulting in a VEX of 140mV (see
Figure 4).
Electrolytic capacitors can be used for the output capacitor,
but are less stable with age than tantalum capacitors. As they age,
their ESR degrades, reducing the system performance and
increasing the risk of failure. It is recommended that multiple
parallel capacitors be used, so that, as ESR increases with age,
overall performance will still meet the processor’s requirements.
There is frequently strong pressure to use the least expensive
components possible, however, this could lead to degraded
long-term reliability, especially in the case of filter capacitors.
Linfinity’s demonstration boards use Sanyo MV-GX filter capaci-
tors, which are aluminum electrolytic, and have demonstrated
reliability. The Oscon series from Sanyo generally provides the
very best performance in terms of long term ESR stability and
general reliability, but at a substantial cost penalty. The MV-GX
series provides excellent ESR performance at a reasonable cost.
Beware of off-brand, very low-cost filter capacitors, which have
been shown to degrade in both ESR and general electrolytic
characteristics over time.
INPUT CAPACITOR
The input capacitor and the input inductor are to filter the
pulsating current generated by the buck converter to reduce
interference to other circuits connected to the same 5V rail. In
addition, the input capacitor provides local de-coupling the buck
converter. The capacitor should be rated to handle the RMS
current requirement. The RMS current is:
IRMS = IL √ d(1-d)
SOFT-START CAPACITOR (continued)
The SS pin voltage can be expressed as:
VSS = VSET (1-e-t/RssCss)
where VSET is the output of the DAC. RSS and CSS are soft start
resistor and capacitor, as shown in Figure 4. The required
inductor current for the output capacitor to follow the SS-pin
voltage equals the required capacitor current plus the load
current. The soft-start capacitor should be selected so that the
overall inductor current does not exceed it maximum.
The capacitor current to follow the SS-pin voltage is:
I =C
dV
C
=
* e OUT
-(t/RssCss )
Cout
OUT dt
C
SS
where COUT is the output capacitance. The typical value of CSS
should be in the range of 0.1 to 0.2µF.
During the soft-start interval, before the PWRGD signal
becomes valid, the load current from a microprocessor is
negligible; therefore, the capacitor current is approximately the
required inductor current.
CURRENT LIMIT
Current limiting occurs when a sensed voltage, proportional to
load current, exceeds the current-sense comparator threshold
value. The current can be sensed either by using a fixed sense
resistor in series with the inductor to cause a voltage drop
proportional to current, or by using a resistor and capacitor in
parallel with the inductor to sense the voltage drop across the
parasitic resistance of the inductor. The LX1668 has a threshold
of 60mV.
Sense Resistor
The current sense resistor, RSENSE, is selected according to the
formula:
where I is the inductor current and the d is the duty cycle. The
L
maximum value, when d = 50%, IRMS = 0.5IL. For 5V input and
output in the range of 2 to 3V, the required RMS current is very
close to 0.5I .
L
A high-frequency (ceramic) capacitor should be placed
across the drain of the top MOSFET and the source of the bottom
one to avoid ringing due to the parasitic inductor being switched
ON and OFF. See capacitor C7 in the Product Highlight.
SOFT-START CAPACITOR
The value of the soft-start capacitor determines how fast the
output voltage rises and how large the inductor current is
required to charge the output capacitor. The output voltage will
follow the voltage at SS pin if the required inductor current does
not exceed the maximum current in the inductor.
RSENSE = VTRIP / ITRIP
Where VTRIP is the current sense comparator threshold (60mV)
and ITRIP is the desired current limit. Typical choices are shown
below.
TABLE 2 - Current Sense Resistor Selection Guide
Load
Sense Resistor
Value
Pentium-Class Processor (<10A)
Pentium II Class (>10A)
5mΩ
2.5mΩ
A smaller sense resistor will result in lower heat dissipation
(I²R) and also a smaller output voltage droop at higher currents.
Copyright © 1999
Rev. 1.0 4/99
11
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