LX1668 Ver la hoja de datos (PDF) - Microsemi Corporation
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LX1668 Datasheet PDF : 15 Pages
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LX1668
PRODUCT DATABOOK 1996/1997
P ROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER
PRODUCTION DATA SHEET
APPLICATION INFORMATION
Output Voltage
(2.8V Set Point)
Input Ripple Voltage
LOUT = 5µH, LIN = 0
Input Current
LOUT = 2.2µH, LIN = 0
OUTPUT INDUCTOR
FIGURE 7 — Effect Of Different Inductor Values
The output inductor should be selected to meet the requirements
of the output voltage ripple in steady-state operation and the
inductor current slew-rate during transient.
The peak-to-peak output voltage ripple is:
V = ESR * I
RIPPLE
RIPPLE
where
I=
RIPPLE
(VIN - VOUT )
fSW * L
*
VOUT
VIN
I is the inductor ripple current, L is the output inductor
RIPPLE
value and ESR is the Effective Series Resistance of the output
capacitor.
I should typically be in the range of 20% to 40% of the
RIPPLE
maximum output current. Higher inductance results in lower
output voltage ripple, allowing slightly higher ESR to satisfy the
transient specification. Higher inductance also slows the induc-
tor current slew rate in response to the load-current step change,
∆I, resulting in more output-capacitor voltage droop. The
inductor-current rise and fall times are:
TRISE = L * ∆I/(VIN – VOUT )
and
T
FALL
=
L
*
∆I/V
OUT
When using electrolytic capacitors, the capacitor voltage
droop is usually negligible, due to the large capacitance.
For higher current applications, such as Pentium II proces-
sors, a 2.5µH inductor is recommended for the best combination
of fast response and manageable ripple voltage. For lower
current applications, such as Pentium and other Socket 7
processors, a 5µH inductor is sufficient. The effect of different
inductor values is shown in Figure 7 above.
Notice how, with a smaller inductor, transient response time
is improved, but at the expense of much greater ripple.
INPUT INDUCTOR
In order to supply faster transient load changes, a smaller output
inductor is needed. However, reducing the size of the output
inductor will result in a higher ripple voltage on the input supply,
as shown in Figure 8 above. This noise on the 5V rail can affect
other system components, such as graphics cards. It is recom-
mended that a 1 – 1.5µH inductor, L2, is used on input to the
regulator, to filter the ripple on the 5V supply. Ensure that this
inductor has the same current rating as the output inductor.
OUTPUT CAPACITOR
The output capacitor is sized to meet ripple and transient
performance specifications. Effective Series Resistance (ESR) is
a critical parameter. When a step load current occurs, the output
voltage will have a step that equals the product of the ESR and
the current step, ∆I. In an advanced microprocessor power
supply, the output capacitor is usually selected for ESR instead
of capacitance or RMS current capability. A capacitor that
satisfies the ESR requirement usually has a larger capacitance and
current capability than strictly needed. The allowed ESR can be
found by:
ESR
*
(I
RIPPLE
+
∆I
)
<
V
EX
where I is the inductor ripple current, ∆I is the maximum
RIPPLE
load current step change, and V is the allowed output voltage
EX
10
Copyright © 1999
Rev. 1.0 4/99
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