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PDF LX1669CD Data sheet ( Hoja de datos )
| Número de pieza | LX1669CD | |
| Descripción | PROGRAMMABLE 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 #: 1669
LX1669
PROGRAMMABLE DC:DC CONTROLLER
PR O D U C T I O N D ATA S H E E T
DESCRIPTION
The LX1669 is a Monolithic Switching
Regulator Controller IC designed to
provide 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 out-
put suitable for powering Pentium® II and
other processors.
Programmable Synchronous Recti-
fier 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 soft-
ware. 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 effi-
ciencies of 85 – 90%, even with loads in
excess of 10A.
Short-circuit Current Limiting with-
out Expensive Current Sense Resis-
tors. 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 LX1669 is
available in an economical 16-pin narrow
body SOIC 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 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 Can Drive SCR Crowbar Or
Turn Off Signal Silver-Box Power Supply
s Digital-Compatible Inputs (Including VID Pins)
A P P L I C AT I O N S
s Socket 7 Processor Supplies
s Pentium II Processor Supplies
s Deschutes CPU & L2-Cache Memory
Supplies
s Voltage Regulator Modules
s General Purpose And Microprocessor
DC:DC Supplies
5V 12V
C8 C3
1µF 1µF
R3
10kΩ
PWRGD
PRODUCT HIGHLIGHT
1 TDRV
2 VCC12
3 VCC5
4 PWRGD
5 OVP
6 VID0
7 VID1
8 VID2
L2, 1µH
Q1
IRL3102
PGND 16
BDRV 15
AGND
SS/EN
14
13
CSS
VFB 12 0.1µF
VCORE 11
VID4 10
VID3 9
VID4
VID3
VID2
VID1
VID0
C2
1500µFx3
L1 RSENSE
2.5µH 2.5mΩ
Q2
IRL3303
Q4*
SCR
2N6504
CPU Core
VCORE
C1
1500µF x 6
PACKAGE ORDER INFO
* Q4 optional OVP crowbar
TA (°C)
D
Plastic SOIC
16-pin
0 to 70
LX1669CD
Note: All surface-mount packages are available in Tape & Reel, append the letter "T" to part number. (i.e. LX1669CDT)
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
PROGRAMMABLE DC:DC CONTROLLER
PR O D U C T I O N D ATA S H E E T
CHARACTERISTICS CURVES
LX1669
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
VCORE
11
VFB
12
60mV CS Comp
IRESET
40mV
Error Comp
VRESET
PWM
RQ
SQ
Set
PWRGD 4
OVP 5
Power Good
& OVP
VREF
6
7
VID[0:4] 8
9
10
DAC
VSET
RSS
20k
Hiccup
Hiccup
Off-Time
Control
UVLO
UVLO
13
SS/ENABLE
CSS
FIGURE 3 — Block Diagram
+12V VIN (5V)
2
VCC12
CIN
1 TDRV
15
BDRV
16 PGND
14 AGND
L RSENSE
VCORE
ESR
COUT
+5V
3
VCC5
Copyright © 1999
Rev. 1.0 4/99
5
5 Page 
PRODUCT DATABOOK 1996/1997
PROGRAMMABLE DC:DC CONTROLLER
PR O D U C T I O N D ATA S H E E T
LX1669
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. The positioning
voltage is 40mV (peak), using the LX1669, 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 3. 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:
dV
ICout = COUT 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 LX1669 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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