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參數(shù)資料
| 型號： | LX1911CLD-TR |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.8 A SWITCHING REGULATOR, 1400 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封裝： | 3 X 3 MM, 0.90 MM HEIGHT, ROHS COMPLIANT, PLASTIC, MO-229, 6 PIN |
| 文件頁數(shù)： | 4/7頁 |
| 文件大?。?/td> | 152K |
| 代理商： | LX1911CLD-TR |
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 4
Copyright
2004
Rev. 1.1, 2006-05-15
WWW
.Microse
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.CO
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LX1911
1.5A, 1.1MHZ PWM Step-Down Regulator
PRODUCTION DATA SHEET
TM
APPLICATION NOTE
FUNCTIONAL DESCRIPTION
The LX1911 is a Current Mode PWM regulator with
internal compensation.
The internal PMOS high side switch is protected with
current limit on a pulse by pulse basis and with thermal
shutdown. Thermal shutdown is activated with a junction
temperature of 160°C (typical) and has 20°C of hysteresis.
The regulator has an internal Power On Reset delay of
50-100s to ensure all circuitry is operating before enabling
the Switch output.
Soft Start is activated upon initial power-on, or
following recovery from either thermal shutdown or short
circuit.
The Soft start control block generates a voltage
ramp
that clamps the error amplifier non-inverting
reference voltage.
As this clamp voltage rises, the duty
cycle is gradually increased, thus limiting the peak inrush
currents.
PWM / PFM mode of operation is determined by the
load current condition. The PFM mode increases system
efficiency by reducing the switching frequency thus
switching losses.
During light loading, IOUT < 200mA
typically, PFM mode becomes active, the switching
frequency begins to decrease, the frequency change occurs
over a continuous range, decreasing further as IOUT
decreases.
OUTPUT VOLTAGE PROGRAMMING
Resistors R1 and R2 program the output voltage. The
value of R2 (lower resistor of divider) should be less than
10K
Ω. The value of R1 can be determined using the
following equation, note VREF is also referred to as VFBT.
1
V
R2
R1
REF
OUT
=
DIODE SELECTION
A Schottky diode is required for switching speed and low
forward voltage.
Efficiency is determined mostly by the
diode’s forward voltage.
The diode conducts 1-D%, for
VOUT = 1.2V this becomes 76% in a 5V system.
INDUCTOR SELECTION
Selecting the appropriate inductor type and value ensures
optimal performance of the converter circuit for the
intended application. A primary consideration requires the
selection of an inductor that will not saturate at the peak
current level. EMI, output voltage ripple, and overall circuit
efficiency affect inductor choice. The inductor that works
best depends upon the application’s requirements and some
experimentation with actual devices in-circuit is typically
necessary to make the most effective choice.
INDUCTOR SELECTION, CONT.
The internal compensation of the LX1911 design was
optimized for a 3.3H inductor but will operate with an
inductor value in the range of 2.2H and 22H for a majority
of applications. The benefit of a larger inductor value can
increase efficiency at the lower output currents and reduces
output voltage ripple, thus output capacitance related to
ripple filtering. Smaller inductors typically provide smaller
package size (critical in many portable applications) at the
expense of increasing output ripple current. Regardless of
inductor value, selecting a device manufactured with a
ferrite-core produces lower losses at higher switching
frequencies and thus better overall performance.
Larger
inductors may lead to diminished Step-Load response.
CAPACITOR SELECTION
To minimize ripple voltage, output capacitors with a low
series resistance (ESR) are recommended.
Multi-layer
ceramic capacitors with X5R or X7R dielectric make an
effective choice because they feature small size, very low
ESR, a temperature stable dielectric, and can be connected in
parallel to increase capacitance. Typical output capacitance
values of 10 to 30F has proven effective.
Other low ESR
capacitors such as solid tantalum, specialty polymer, or
organic semiconductor, make effective choices provided that
the capacitor is properly rated for the output voltage and
ripple current.
Finally, choose an input capacitor of
sufficient size to effectively decouple the input voltage
source impedance (e.g., CIN > 4.7F).
LAYOUT CONSIDERATIONS
The high peak currents and switching frequencies present in
DC/DC converter applications require careful attention to
device layout for optimal performance. Basic design rules
include: (1) maintaining wide traces for power components
(e.g., width > 50mils); (2) place CIN, COUT, the Schottky
diode, and the inductor close to the LX1911; (3) minimizing
trace capacitance by reducing the etch area connecting the
SW pin to the inductor; and (4) minimizing the etch length to
the FB pin to reduce noise coupling into this high impedance
sense input. Other considerations include placing a 0.1uF
capacitor between the LX1911 VOUT pin and GND pin to
reduce high frequency noise and decoupling the VIN pin
using a 0.1F capacitor.
The LX1911 Switch has fast
switching speeds which may generate noise spikes when a
high capacitance Schottky diode is selected for the catch
diode. A simple snubber circuit, as shown in Figure 1, R=10
ohms and C=680pF has proven effective to reduce the spike
voltage generated at the Switch Pin / Diode connection.
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