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參數(shù)資料
| 型號： | LX1665CDWT |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.5 A SWITCHING CONTROLLER, 200 kHz SWITCHING FREQ-MAX, PDSO18 |
| 封裝： | PLASTIC, SOIC-18 |
| 文件頁數(shù)： | 7/17頁 |
| 文件大?。?/td> | 354K |
| 代理商： | LX1665CDWT |
DUAL OUTPUT PWM CONTROLLERS WITH 5-BIT DAC
LX1664/1664A, LX1665/65A
PRODUCT DA T ABOOK 1996/1997
15
Copyright 1999
Rev. 1.2 11/99
P RODUCTION D ATA S HEET
USING THE LX1664/65 DEVICES
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
OUT
level, between 1.5 & 3.5V, as determined by the feedback
resistors.
The current available from the Linear regulator is dictated by
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
D = (VIN - VOUT) * I output
current. This means that a +5V
IN to +3.3VOUT at 5A will require that
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.
The dropout voltage for the linear regulator stage is the product
of R
DS ON * IOUT.
Using a 2SK1388 device at 5A, the dropout
voltage will be (worst case) 37 milliohms x 5A = 185mV.
Note that the R
DS ON of the (linear regulator) MOSFET does not
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.
LINEAR REGULATOR (continued)
Device
R
DS(ON) @
I
D @
Max. Break-
10V (m
)T
C = 100°C
down Voltage
IRFZ24N
70
12
55
IRL2703
40
17
30
IRLZ44N
22
29
55
TABLE 5 - Linear Regulator MOSFET Selection Guide
FIGURE 13 — Typical Transient Response
Avoiding Crosstalk
To avoid a load transient on the switching output affecting the
linear regulator, follow these guidelines:
1) Separate 5V supply traces to switching & linear FETs as
much as possible.
2) Place capacitor C
9 as close to drain of Q4 as possible.
Typical transient response is shown in Figure 13.
Channel 2 = Linear Regulator Output.
Set point = 3.3V @ 2A (20mV/div.)
Channel 4 = Switching Regulator Output.
V
CC_CORE set point = 2.8V
Channel 3 = Switching Regulator Load Current
Transient 0 - 13A
Output Voltage Setting
As shown in Application Information Figures 6-9, two resistors (R
5
& R
6) set the linear regulator stage output voltage:
V
OUT = 1.5 * (R5 + R6) / R6
As an example, to set resistor magnitudes, assume a desired
V
OUT of 3.3 volts:
1.5 * (12.1k + 10k) / 10k = 3.3 volts (approximately)
In general, the divider resistor values should be in the vicinity
of 10-12k ohm for optimal noise performance. Please refer to
Table 6.
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