lx1668 Microsemi Corporation, lx1668 Datasheet - Page 14

• Manufacturer Part Number: lx1668
• Description: Programmable Multiple Output Dc Dc Controller
• Manufacturer: Microsemi Corporation

LX1668
14
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 22µF aluminum electrolytic is recommended for optimal
performance for power clock circuits at 2.5V.
External LDO
A 330µF aluminum electrolytic is recommended for powering V
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.
V
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
output with a minimum voltage of 1.5V. The dropout voltage
across the regulator is the product of R
limits the upper voltage.
Bus circuits in a Pentium II processor application at 1.5V. In this
case, the output can be connected directly to the L
the use of resistors to set the output voltage. If an alternative
output voltage is required, a resistor divider sets the output as
follows:
CC3
The power dissipated in the internal LDO is as follows:
where I
The linear regulator output, V
can be connected to 5V or 3.3V, but 5V is recommended for
P
V
D
OUT3
OUT2
= (V
= 1.5V * (1+R
is the output current from the linear regulator.
CC3
DRV
– V
and L
OUT2
FB
) * I
pins. The MOSFET can provide an
1
/R
OUT2
2
P
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
OUT3
)
ROGRAMMABLE
, can be used to power GTL+
DS(ON)
P
A P P L I C A T I O N I N F O R M A T I O N
R O D U C T I O N
and current – this
FB
M
pin without
U LTIPLE
TT
O
UTPUT
D
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
[V
supply when powering a 1.5V GTL+ Bus using the external LDO.
at 3A would be:
be necessary for a monolithic LDO such as the LX8384.
mined by the R
a 2SK1388 at 5A, the worst case dropout voltage will be 37m
* 5A = 185mV. Note that the R
dissipation, only dropout voltage. For reasons of economy,
choose a FET with higher R
output stages.
Disabling Linear Output
Linear regulator output can be disabled by pulling feedback pin
(L
be pulled higher than 5V.
A T A
FB
IN
Resistors Settings for Linear Regulator Output Voltage
For example, the heat dissipated in converting 3.3V to 1.5V
Note that the MOSFET requires the same heatsink as would
The dropout voltage of the linear regulator will be deter-
) up to 3.3V or 5V as shown in Figure 11. This pin should not
- V
DC:DC C
OUT3
TABLE 7 - Linear Enable (LIN EN) Function Table
Set Point (V)
P
Nominal
S
] * I
D
= (3.3 – 1.5) * 3 = 5.4W
3.3
3.2
3.1
3.0
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
H E E T
OUT3
DS(ON)
ONTROLLER
). For this reason, it is preferable to use a 3.3V
LIN EN
of the MOSFET (V
H
L
R1 (k )
11.3
11.3
10.3
9.76
8.87
8.87
8.87
8.87
8.87
8.87
7.15
7.15
7.15
12
11
10
10
10
0
TABLE 6 -
DS(ON)
DS(ON)
R2 (k )
than that used for the PWM
LIN OUTPUT
10.7
11.5
12.4
13.7
14.7
14.7
16.5
18.7
22.1
26.7
35.7
53.6
100
10
10
11
11
21
50
Disabled
Enabled
does not affect the power
DO
= R
DS(ON)
V
OUT
3.30
3.20
3.08
3.00
2.90
2.80
2.71
2.59
2.50
2.41
2.31
2.21
2.10
2.00
2.13
1.80
1.70
1.61
1.50
(V)
* I
I/O
Copyright © 1999
). Using
Rev. 1.0
D
=
4/99