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PDF LX1668 Data sheet ( Hoja de datos )
| Número de pieza | LX1668 | |
| Descripción | PROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER | |
| Fabricantes | Microsemi Corporation | |
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T H E I N F I N I T E P O W E R O F I N N O VAT I O N
L I N D O C #: 1668
LX1668
P ROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER
PR O D U C T I O N D ATA S H E E T
DESCRIPTION
The LX1668 is a Monolithic Switching
Regulator Controller IC designed to pro-
vide a low cost, high performance adjustable
power supply for advanced microprocessors
and other applications requiring a very fast
transient response and a high degree of
accuracy. It provides a programmable
switching regulator output, together with one
internal linear low dropout regulator and one
adjustable linear regulator driver. The LX1668
offers a triple-output single-chip power sup-
ply for Pentium® II and other processors.
Programmable Synchronous Rectifier
Driver for CPU Core. The main output is
adjustable from 1.3 to 3.5V using a TTL-
compatible 5-bit digital code to meet Intel
specifications. The IC can read the signal
from a DIP-switch, hardwired to Pentium II
processor’s pins or from software. The 5-bit
code adjusts the output voltage between 1.30
and 2.05V in 50mV increments, and between
2.0 and 3.5V in 100mV increments. The
device can drive dual MOSFET’s resulting in
typical efficiencies of 85 – 90%, even with
loads in excess of 10A.
Internal Low Dropout [LDO] Regulator
provides a fixed 2.5V output for powering the
clock circuit at up to 250mA.
External Linear Regulator Driver out-
put can be connected to a MOSFET to provide
a high-current adjustable LDO function suit-
able for supplying the GTL+ bus circuitry on
a Pentium II processor motherboard at 1.5V.
Short-circuit Current Limiting without
Expensive Current Sense Resistors. The
current sensing mechanism can use a PCB
trace resistance or the parasitic resistance of
the main inductor. For applications requiring
a high degree of accuracy, a conventional
sense resistor can be used.
Ultra-Fast Transient Response Re-
duces System Cost. The fixed frequency
modulated off-time architecture results in the
fastest transient response for a given inductor.
Small Package Size. The LX1668 is available
in an economical 20-pin wide body SOIC or
a space-saving 20-pin TSSOP package.
NOTE: For current data & package dimensions, visit our web site: http://www.linfinity.com.
KEY FEATURES
s 5-Bit Programmable Output For CPU Core
Supply
s Internal Fixed 2.5V Low Dropout Regulator
s Adjustable Linear Regulator Driver
s Complete Single-Chip Power Solution For
Pentium II Processors
s No Sense Resistor Required For Short-Circuit
Current Limiting
s Soft-Start And Hiccup-Mode Current Limiting
Functions
s Modulated Constant Off-Time Control
Mechanism For Fast Transient Response And
Simple System Design
s Power Good Flag
s Over-Voltage Pin & SCR
s Digital-Compatible Inputs (Including VID Pins)
s Output Disable Function Shuts Off PWM While
Keeping 2.5V LDO Active - Compatible With
"Green PC" And "Instant On" Requirements
s Compatible to VRM8.2 - 8.4 Specifications
A P P L I C AT I O N S
s Pentium II & Pentium III Processor Supplies
s Voltage Regulator Modules
s General Purpose And Microprocessor DC:DC
Supplies
PRODUCT HIGHLIGHT
5V
12V
C8 C3
1µF 1µF
VOUT2
2.5V
C5
22µF
C9
1µF
3.3V
VOUT3
1.5V
C4
330µF
C6
330µF
Q3
IRLZ44
R1, 0
R2, 50k
L2
1µH
C7
1µF
1 TDRV
PGND 20
2 VCC12
BDRV 19
3 VCC5
AGND 18
LX16684 VOUT2
5 VCC3
6 LDRV
SS/EN
VFB
17
16 CSS
VCORE 15
7 LFB
PWRGD 14
8 VID0
OVP 13
9 VID1
VID4 12
10 VID2
VID3 11
Q1
IRL3102
0.1µF
C2
1500µFx3
L1 RSENSE
2.5µH 2.5mΩ
Q2
IRL3303
PWRGD
Q4*
SCR
2N6504
VID4
VID3
VID2
VID1
VID0
R3
10k
3.3V / 5V
CPU Core
VCORE
C1
1500µF x 6
* Q4 optional
OVP crowbar
PACKAGE ORDER INFORMATION
TA (°C)
DW
Plastic
20-pin
SOWB
PW
Plastic
20-pin
TSSOP
0 to 70
LX1668CDW
LX1668CPW
Note: All surface-mount packages are available in Tape & Reel, append the letter "T" to part number. (i.e. LX1668CPWT)
Copyright © 1999
Rev. 1.0 4/99
LINFINITY MICROELECTRONICS INC.
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841, 714-898-8121, FAX: 714-893-2570
1
1 page  
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
CHARACTERISTICS CURVES
LX1668
100 100
95 95
90 90
85 85
80
EFFICIENCY AT 3.1V
EFFICIENCY AT 2.8V
75 EFFICIENCY AT 1.8V
80
EFFICIE NCY A T 3.1V
EFFICIE NCY A T 2.8V
75
EFFICIE NCY A T 1.8V
70
1 2 3 4 5 6 7 8 9 10 11 12 13 14
IOUT (A)
FIGURE 1 — Efficiency Test Results:
Non-Synchronous Operation, V = 5V
IN
70
1 2 3 4 5 6 7 8 9 10 11 12 13 14
IOUT (A)
FIGURE 2 — Efficiency Test Results:
Synchronous Operation, V = 5V
IN
BLOCK DIAGRAM
+5V
2.5V
VOUT3
VCC3 5
2.5V
VCORE
15
VFB
16
60mV CS Comp
IRESET
PWM
RQ
VOUT2 4
LDRV 6
LFB 7
PWRGD 14
OVP 13
1.5V
Power Good
& OVP
VREF
8
9
VID[0:4] 10
11
12
DAC
VSET
40mV
Error Comp
RSS
20k
Hiccup
VRESET
SQ
Set
Hiccup
Off-Time
Control
UVLO
UVLO
17
SS/ENABLE
CSS
FIGURE 3 — Block Diagram
+12V VIN (5V)
2
VCC12
CIN
1 TDRV
19
BDRV
20 PGND
18 AGND
L RSENSE
VCORE
ESR
COUT
+5V
3
VCC5
Copyright © 1999
Rev. 1.0 4/99
5
5 Page 
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 VEX, allowing a higher ESR value and
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)
where IL is the inductor current and the d is the duty cycle. The
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 0.5IL.
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.
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:
ICout = COUT
dV
dt
=
COUT
CSS
* e-(t/RssCss )
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:
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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