lx1668 Microsemi Corporation, lx1668 Datasheet - Page 10

• Manufacturer Part Number: lx1668
• Description: Programmable Multiple Output Dc Dc Controller
• Manufacturer: Microsemi Corporation

LX1668
10
OUTPUT INDUCTOR
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.
value and ESR is the Effective Series Resistance of the output
capacitor.
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,
inductor-current rise and fall times are:
droop is usually negligible, due to the large capacitance.
sors, a 2.5µH inductor is recommended for the best combination
of fast response and manageable ripple voltage. For lower
I, resulting in more output-capacitor voltage droop.
The peak-to-peak output voltage ripple is:
where
I
I
and
When using electrolytic capacitors, the capacitor voltage
For higher current applications, such as Pentium II proces-
RIPPLE
RIPPLE
is the inductor ripple current, L is the output inductor
should typically be in the range of 20% to 40% of the
V
I
T
T
RIPPLE
RISE
FALL
RIPPLE
= L * I/(V
= L * I/V
=
= ESR * I
(V
IN
f
SW
- V
OUT
RIPPLE
* L
IN
OUT
– V
)
L
OUT
OUT
*
P
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
)
ROGRAMMABLE
= 5µH, L
V
V
OUT
IN
P
FIGURE 7 — Effect Of Different Inductor Values
A P P L I C A T I O N I N F O R M A T I O N
R O D U C T I O N
IN
= 0
M
U LTIPLE
Input Ripple Voltage
Output Voltage
(2.8V Set Point)
The
Input Current
O
UTPUT
D
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.
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,
supply, the output capacitor is usually selected for ESR instead
of capacitance or RMS current capability.
satisfies the ESR requirement usually has a larger capacitance and
current capability than strictly needed. The allowed ESR can be
found by:
load current step change, and V
A T A
Notice how, with a smaller inductor, transient response time
where I
DC:DC C
ESR * (I
S
RIPPLE
H E E T
is the inductor ripple current, I is the maximum
RIPPLE
ONTROLLER
I.
+ I ) < V
In an advanced microprocessor power
EX
EX
is the allowed output voltage
L
OUT
= 2.2µH, L
A capacitor that
IN
Copyright © 1999
= 0
Rev. 1.0
4/99