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| 型號： | LX1668CPW-TR |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1 A SWITCHING CONTROLLER, 250 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封裝： | ROHS COMPLIANT, PLASTIC, TSSOP-20 |
| 文件頁數(shù)： | 6/15頁 |
| 文件大?。?/td> | 392K |
| 代理商： | LX1668CPW-TR |
PROGRAMMABLE MULTIPLE OUTPUT DC:DC CONTROLLER
LX1668
PRODUCT DA T ABOOK 1996/1997
Copyright 1999
Rev. 1.0a
14
P RODUCTION D ATA S HEET
LIN EN
LIN OUTPUT
H
Disabled
L
Enabled
TABLE 7 - Linear Enable (LIN EN) Function Table
APPLICA TION INFORMA TION
LINEAR REGULATOR
Capcitor Selection
The internal LDO regulator and the external linear regulator
driver are both designed to be insensitive to selection of
capacitors. Most Aluminum or Tantalum electrolytic capacitor
will make for suitable operation. Capacitors should be chosen
so that the voltage drop during a load transient does not exceed
the relevant specifications.
Internal LDO
A 22F aluminum electrolytic is recommended for optimal
performance for power clock circuits at 2.5V.
External LDO
A 330F aluminum electrolytic is recommended for powering V
TT
at 1.5V. This will provide sufficient voltage hold-up for any
expected transients on the GTL+ Bus.
Internal Fixed-Voltage LDO Regulator
The internal LDO regulator has a fixed 2.5V output voltage and
is intended for powering the clock circuit. Its current is limited
to 250mA. The limitation for using this LDO regulator is heat
dissipation. Power dissipation should not exceed the limit of
400mW on average for SOIC package and 300mW for TSSOP
package.
The power dissipated in the internal LDO is as follows:
P
D = (VCC3 – VOUT2 ) * IOUT2
where I
OUT2 is the output current from the linear regulator.
V
CC3 can be connected to 5V or 3.3V, but 5V is recommended for
optimal performance.
Adjustable External LDO Regulator
The LX1668 has a linear regulator driver function. A low dropout
linear regulator can be constructed by connecting an external
MOSFET to the L
DRV and LFB pins.
The MOSFET can provide an
output with a minimum voltage of 1.5V. The dropout voltage
across the regulator is the product of R
DS(ON) and current – this
limits the upper voltage.
The linear regulator output, V
OUT3, can be used to power GTL+
Bus circuits in a Pentium II processor application at 1.5V. In this
case, the output can be connected directly to the L
FB pin without
the use of resistors to set the output voltage. If an alternative
output voltage is required, a resistor divider sets the output as
follows:
V
OUT3 = 1.5V * (1+R1 /R2 )
The maximum output current from the linear regulator section is
around 5 - 7A, and is limited only by the MOSFET’s thermal
dissipation performance (Power dissipation is equal to the
voltage drop across the MOSFET multiplied by the current; P
D =
[V
IN - VOUT3] * IOUT3 ). For this reason, it is preferable to use a 3.3V
supply when powering a 1.5V GTL+ Bus using the external LDO.
For example, the heat dissipated in converting 3.3V to 1.5V
at 3A would be:
P
D = (3.3 – 1.5) * 3 = 5.4W
Note that the MOSFET requires the same heatsink as would
be necessary for a monolithic LDO such as the LX8384.
The dropout voltage of the linear regulator will be deter-
mined by the R
DS(ON) of the MOSFET (VDO = RDS(ON) * II/O).
Using
a 2SK1388 at 5A, the worst case dropout voltage will be 37m
* 5A = 185mV. Note that the R
DS(ON) does not affect the power
dissipation, only dropout voltage.
For reasons of economy,
choose a FET with higher R
DS(ON) than that used for the PWM
output stages.
Disabling Linear Output
Linear regulator output can be disabled by pulling feedback pin
(L
FB) up to 3.3V or 5V as shown in Figure 11. This pin should not
be pulled higher than 5V.
3.3
12
10
3.30
3.2
11.3
10
3.20
3.1
11.3
10.7
3.08
3.0
11
3.00
2.9
10.3
11
2.90
2.8
10
11.5
2.80
2.7
10
12.4
2.71
2.6
10
13.7
2.59
2.5
9.76
14.7
2.50
2.4
8.87
14.7
2.41
2.3
8.87
16.5
2.31
2.2
8.87
18.7
2.21
2.1
8.87
22.1
2.10
2.0
8.87
26.7
2.00
1.9
8.87
21
2.13
1.8
7.15
35.7
1.80
1.7
7.15
53.6
1.70
1.6
7.15
100
1.61
1.5
0
50
1.50
Nominal
Set Point (V)
R1 (k
)
R2 (k
)V
OUT (V)
TABLE 6 -
Resistors Settings for Linear Regulator Output Voltage
O
B
SO
LETE
PRO
D
UCT
NOT
RECOMMENDED
FOR
NEW
DESIGNS
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