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PDF LX1912 Data sheet ( Hoja de datos )
| Número de pieza | LX1912 | |
| Descripción | VREF 800mV -1.0A -1.2MHz PWM | |
| Fabricantes | Microsemi | |
| Logotipo |  |
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LX1912
www.DataSheet4U.com
TM ® VREF @ 800mV, 1.0A, 1.2MHz PWM
PRODUCTION DATA SHEET
DESCRIPTION
The LX1912 operates as a Current
Mode PWM Buck regulator that
switches to PFM mode with light
loads. The entire regulator function is
implemented with few external
components.
The LX1912 responds quickly to
dynamic load changes using a high
bandwidth error amplifier and internal
compensation. Tight output voltage
regulation is maintained with the
compensated 800mV, ±2% reference
(line and temp regulation). With two
external resistors the output voltage is
easily programmed, from 800mV to
90% of VIN.
The regulator is capable of providing
an output load current of 1.0A, has no
minimum load current requirement for
stable operation. Current limit is
cycle-by-cycle to protect the switch.
Power conversion efficiency is
maximized with low regulator IQ and
PFM mode of operation
The LX1912 operational range
covers 4.0V to 6.0V, features include
power on delay; soft start to limit
inrush currents; and thermal shutdown
during fault conditions.
The 6-pin TSOT package provides a
small form factor with excellent power
dissipation capability.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
KEY FEATURES
Internal Reference 800mv ±2%
Accuracy (Line and Temp.)
4.0V to 6.0V Input Range
Internal Soft Start
Adj. Output From 0.8V to 90%
of VIN
Output Current up to 1.0A
Quiescent Current < 550μA,
Typical @ 23°C
1.2MHz PWM Frequency
Over Voltage Protection
APPLICATIONS
Portable Microprocessor Core
Voltage Supplies
5V to 3V
RoHS Compliant Product
4.7µF
4V to 6V
VPWR SW
VCC
LXP1a9rt12
GND
FB
N.C.
PRODUCT HIGHLIGHT
2.7µH
31.6K
3.3V @ 1.0A
30µF
10K
95%
90%
85%
80%
75%
70%
65%
60%
55%
50%
0.01
EFFICIENCY (VIN = 5V, VOUT @ 3.3V)
0.10
OUTPUT CURRENT (Amps)
Figure 1 – LX1912 Circuit Topology and Typical Efficiency Performance
1.00
TA (°C)
0 to +70
PACKAGE ORDER INFO
Output
Input Voltage Voltage
Range
Plastic TSOT
SG 6-Pin
RoHS Compliant / Pb-free
4.0V – 6.0V Adjustable
LX1912CSG
Part Marking
1912
Note: Available in Tape & Reel. Append the letters “TR” to the part number. (i.e. LX1912CSG-TR)
Copyright © 2005
Rev. 1.0, 2006-09-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
1 page  
LX1912
www.DataSheet4U.com
TM ® VREF @ 800mV, 1.0A, 1.2MHz PWM
PRODUCTION DATA SHEET
APPLICATION NOTE
FUNCTIONAL DESCRIPTION
The LX1912 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-
100µs 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 OVER VOLTAGE PROTECTION
The over voltage comparator compares the FB pin voltage to
a voltage that is 15% higher than the internal referenced VREF.
Once the FB pin voltage goes 15% above the internal reference,
the internal PMOS control switch is turned off, which allows the
output voltage to decrease toward regulation.
OUTPUT VOLTAGE PROGRAMMING
Resistors R1 and R2 program the output voltage. The total
impedance of both feedback resistors should not exceed 50K Ohms
to ensure optimal frequency stability. The value of R1 can be
determined using the following equation, note VREF is also referred
to as VFBT.
R1 =
R2⎢⎢⎣⎡⎜⎜⎝⎛
VOUT
VREF
⎟⎟⎠⎞
⎤
− 1⎥
⎥⎦
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 LX1912 stability performance is optimized by using an
inductor value of 2.7µH ±20%. 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 20 to 60µF have 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 > 4.7μF).
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 LX1912; (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 LX1912 VOUT pin and GND
pin to reduce high frequency noise and decoupling the VCC, VPWR
pins using a 0.1µF capacitor.
Copyright © 2005
Rev. 1.0, 2006-09-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet LX1912.PDF ] | |
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