lx1665 Microsemi Corporation, lx1665 Datasheet - Page 12

• Manufacturer Part Number: lx1665
• Description: Dual Output Pwm Controllers With 5-bit Dac
• Manufacturer: Microsemi Corporation

LX1664/64A, LX1665/65A
12
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
that this inductor has the same current rating as the output
inductor.
C
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.
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:
transient and I
as the LX166x series, have adaptive output voltage positioning,
which adds 40mV to the DC set-point voltage — V
the difference between the low load voltage and the minimum
dynamic voltage allowed for the microprocessor.
and can be approximated as follows:
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.
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
2
1
(1 - 1.5µH), is used on the 5V rail to filter out the ripple. Ensure
FILTER CAPACITOR SELECTION
When a transient (step) load current change occurs, the output
Where V
Ripple current is a function of the output inductor value (L
Where f
Electrolytic capacitors can be used for the output filter capaci-
There is frequently strong pressure to use the least expensive
ESR * (I
I
RIPPLE
S
=
EX
is the switching frequency.
RIPPLE
RIPPLE
is the allowable output voltage excursion in the
V
f
IN
S
+ ∆ I) < V
is the inductor ripple current. Regulators such
* L
- V
OUT
OUT
EX
*
V
V
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
OUT
IN
D
UAL
U S I N G T H E L X 1 6 6 4 / 6 5 D E V I C E S
O
P
UTPUT
R O D U C T I O N
EX
PWM C
is therefore
OUT
ONTROLLERS WITH
),
D
C
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 LX1664 & 1665 have a threshold of 100mV, while the LX1664A
and LX1665A 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
for LX1664/65 and 60mV for LX1664A/65A) and I
current limit. Typical choices are shown below.
A smaller sense resistor will result in lower heat dissipation (I²R)
and also a smaller output voltage droop at higher currents.
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.
1
A T A
FILTER CAPACITOR SELECTION
The LX166x family offers two different comparator thresholds.
Where V
There are several alternative types of sense resistor. The
R
TABLE 2 - Current Sense Resistor Selection Guide
1
= V
S
TRIP
TRIP
H E E T
5-B
is the current sense comparator threshold (100mV
/ I
TRIP
IT
DAC
1
, is selected according to the formula:
(continued)
Ω
Ω
TRIP
is the desired