PRODUCT DATABOOK 1996/1997
LX1662/62A, LX1663/63A
S
INGLE
-C
HIP
P
ROGRAMMABLE
PWM C
ONTROLLERS WITH
5-B
IT
DAC
P
R O D U C T I O N
D
A T A
S
H E E T
USING THE LX1662/63 DEVICES
INPUT INDUCTOR SELECTION
In order to cope with 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. This noise on the 5V rail can affect other loads, such as
graphics cards. It is recommended that a smaller input inductor,
L
2
(1 - 1.5µH), is used on the 5V rail to filter out the ripple. Ensure
that this inductor has the same current rating as the output
inductor.
C
1
FILTER CAPACITOR SELECTION
The capacitors on the output of the PWM section are used to filter
the output current ripple, as well as help during transient load
conditions, and the capacitor bank should be sized to meet ripple
and transient performance specifications.
When a transient (step) load current change occurs, the output
voltage will have a step which equals the product of the Effective
Series Resistance (ESR) of the capacitor and the current step (∆I).
when current increases from low (in sleep mode) to high, the
output voltage will drop below its steady state value. In the
advanced microprocessor power supply, the capacitor should
usually be selected on the basis of its ESR value, rather than the
capacitance or RMS current capability. Capacitors that satisfy the
ESR requirement usually have a larger capacitance and current
capability than needed for the application. The allowable ESR can
be found by:
ESR * (I
RIPPLE
+
∆
I) < V
EX
Where V
EX
is the allowable output voltage excursion in the
transient and I
RIPPLE
is the inductor ripple current. Regulators such
as the LX166x series, have adaptive output voltage positioning,
which adds 40mV to the DC set-point voltage — V
EX
is therefore
the difference between the low load voltage and the minimum
dynamic voltage allowed for the microprocessor.
Ripple current is a function of the output inductor value (L
OUT
),
and can be approximated as follows:
I
RIPPLE
=
C
1
FILTER CAPACITOR SELECTION
(continued)
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, meeting all Intel transient
specifications, at a reasonable cost. Beware of off-brand, very-low
cost filter capacitors, which have been shown to degrade in both
ESR and general electrolyte characteristics over time.
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 LX166x family offers two different comparator thresholds.
The LX1662 & 1663 have a threshold of 100mV, while the LX1662A
and LX1663A have a threshold of 60mV. The 60mV threshold is
better suited to higher current loads, such as a Pentium II or
Deschutes processor.
Sense Resistor
The current sense resistor, R
1
, is selected according to the formula:
R
1
= V
TRIP
/ I
TRIP
Where V
TRIP
is the current sense comparator threshold (100mV
for LX1662/63 and 60mV for LX1662A/63A) and I
TRIP
is the desired
current limit. Typical choices are shown below.
TABLE 2 - Current Sense Resistor Selection Guide
Load
Pentium-Class Processor (<10A)
Pentium II Class (>10A)
Sense Resistor
Value
5mΩ
2.5mΩ
Recommended
Controller
LX1662 or LX1663
LX1662A or LX1663A
V
IN
- V
OUT
V
OUT
*
f
S
* L
OUT
V
IN
Where f
S
is the switching frequency.
Electrolytic capacitors can be used for the output filter capaci-
tor bank, 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 are 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
demo boards use Sanyo MV-GX filter capacitors, which are
A smaller sense resistor will result in lower heat dissipation (I²R)
and also a smaller output voltage droop at higher currents.
There are several alternative types of sense resistor. The
surface-mount metal “staple” form of resistor has the advantage of
exposure to free air to dissipate heat and its value can be
controlled very tightly. Its main drawback, however, is cost. An
alternative is to construct the sense resistor using a copper PCB
trace. Although the resistance cannot be controlled as tightly, the
PCB trace is very low cost.
12
Copyright © 1999
Rev. 1.1 11/99
MICROSEMI [ MICROSEMI CORPORATION ]
