lx1665 Microsemi Corporation, lx1665 Datasheet - Page 15

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

LINEAR REGULATOR
Referring to the front page Product Highlight, a schematic is
presented which uses a MOSFET as a series pass element for a
linear regulator. The MOSFET is driven by the LX1664 controller,
and down-converts a +5V or +3.3V supply to the desired V
level, between 1.5 & 3.5V, as determined by the feedback
resistors.
the supply capability, as well as the MOSFET ratings, and will
typically lie in the 3-5 ampere range. This output is well suited
for I/O buffers, memory, chipset and other components. Using
3.3V supply to convert to 1.5V for GTL+ Bus will significantly
reduce heat dissipation in the MOSFET.
MOSFET Comments
Heatsinking the MOSFET becomes important, since the linear
stage output current could approach 5 amperes in some applica-
tions. Since there are no switching losses, power dissipation in
the MOSFET is simply defined by P
current. This means that a +5V
the MOSFET dissipate (5-3.3) * 5 = 8.5 watts. This amount of
power in a MOSFET calls for a heatsink, which will be the same
physical size as that required for a monolithic LDO, such as the
LX8384 device.
of R
voltage will be (worst case) 37 milliohms x 5A = 185mV.
affect heat dissipation, only dropout voltage. For reasons of
economy, a FET with a higher resistance can be chosen for the
linear regulator, e.g. 2SK1388 or IRLZ44.
Avoiding Crosstalk
To avoid a load transient on the switching output affecting the
linear regulator, follow these guidelines:
The current available from the Linear regulator is dictated by
The dropout voltage for the linear regulator stage is the product
Note that the R
1) Separate 5V supply traces to switching & linear FETs as
2) Place capacitor C
Typical transient response is shown in Figure 13.
DS
TABLE 5 - Linear Regulator MOSFET Selection Guide
much as possible.
ON * I
OUT
. Using a 2SK1388 device at 5A, the dropout
DS
ON of the (linear regulator) MOSFET does not
9
Ω Ω Ω Ω Ω
as close to drain of Q
IN
to +3.3V
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
D
UAL
D
U S I N G T H E L X 1 6 6 4 / 6 5 D E V I C E S
= (V
OUT
O
P
UTPUT
at 5A will require that
IN
R O D U C T I O N
- V
4
as possible.
OUT
PWM C
) * I output
ONTROLLERS WITH
OUT
LX1664/1664A, LX1665/65A
D
LINEAR REGULATOR
Output Voltage Setting
As shown in Application Information Figures 6-9, two resistors (R
& R
V
of 10-12k ohm for optimal noise performance. Please refer to
Table 6.
A T A
OUT
V
As an example, to set resistor magnitudes, assume a desired
1.5 * (12.1k + 10k) / 10k = 3.3 volts (approximately)
In general, the divider resistor values should be in the vicinity
6
) set the linear regulator stage output voltage:
OUT
of 3.3 volts:
Channel 2 = Linear Regulator Output.
Channel 4 = Switching Regulator Output.
Channel 3 = Switching Regulator Load Current
= 1.5 * (R
S
FIGURE 13 — Typical Transient Response
H E E T
5-B
5
+ R
IT
Set point = 3.3V @ 2A (20mV/div.)
V
Transient 0 - 13A
(continued)
6
CC_CORE
) / R
DAC
6
set point = 2.8V
15
5