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PDF LX1993 Data sheet ( Hoja de datos )
| Número de pieza | LX1993 | |
| Descripción | High Efficiency LED Driver | |
| Fabricantes | Microsemi Corporation | |
| Logotipo |  |
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INTEGRATED PRODUCTS
LX1993
High Efficiency LED Driver
PRODUCTION
DESCRIPTION
The LX1993 is a high efficiency
step-up boost converter that features a
psuedo-hysteretic pulse frequency
modulation topology for driving
white or color LEDs in backlight or
frontlight systems. Designed for
maximum efficiency, reduced board
size, and minimal cost, the LX1993 is
ideal for PDA and digital camera
applications. The LX1993 features an
internal N-Channel MOSFET and
control circuitry that is optimized for
portable system design applications.
The LX1993 promotes improved
performance in battery-operated
systems by operating with a quiescent
supply current 70µA (typical) and a
shutdown current of less than 1µA.
The input voltage range is from 1.6V
to 6.0V thus allowing for a broad
selection of battery voltage
applications and start-up is
guaranteed at 1.6V input.
The LX1993 is capable of switching
currents in excess of 300mA and the
output current is readily programmed
using one external current sense
resistor in series with the LEDs. This
configuration provides a feedback
signal to the FB pin thus maintaining
constant output current regardless of
varying LED forward voltage (VF).
The LX1993 provides an additional
feature for simple dynamic adjustment
of the output current (i.e., up to 100%
of the maximum programmed current).
Designers can make this adjustment by
generating an analog reference signal
or a PWM signal applied directly to
the ADJ pin and any PWM amplitude
is readily accommodated via a single
external resistor. The LX1993 is
available in the 8-Pin MSOP and thus
requires a very small PCB area.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
PRODUCT HIGHLIGHT
KEY FEATURES
> 80% Maximum Efficiency
70µA Typical Quiescent Supply
Current
Externally Programmable Peak
Inductor Current Limit For
Maximum Efficiency
Logic Controlled Shutdown
< 1µA Shutdown Current
Dynamic Output Current
Adjustment Via Analog
Reference Or Direct PWM Input
8-Pin MSOP Package
APPLICATIONS
Pagers
Wireless Phones
PDAs
Handheld Computers
LED Driver
Digital Camera Displays
Li-Ion
ON OFF
SW
IN
SHDN
OUT
LX1993
FB
ADJ CS
GND
Copyright © 2000
Rev. 1.0b, 2005-03-03
PACKAGE ORDER INFO
TA (°C)
0 to 70
Plastic MSOP
DU 8-Pin
RoHS Compliant / Pb-free
Transition DC: 0432
LX1993CDU
Note: Available in Tape & Reel. Append the letters “TR” to the part
number. (i.e. LX1993CDU-TR)
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
1 page  
INTEGRATED PRODUCTS
LX1993
High Efficiency LED Driver
PRODUCTION
APPLICATION INFORMATION
OPERATING THEORY
The LX1993 is a PFM boost converter that is
optimized for driving a string of series connected LEDs. It
operates in a pseudo-hysteretic mode with a fixed switch
“off time” of 300ns. Converter switching is enabled as
LED current decreases causing the voltage across RSET to
decrease to a value less than the voltage at the VADJ pin.
When the voltage across RSET (i.e., VFB) is less than VADJ,
comparator A1 activates the control logic. The control
logic activates the DRV output circuit that connects to the
gate of the internal FET. The output (i.e., SW) is switched
“on” (and remains “on”) until the inductor current ramps up
to the peak current level. This current level is set via the
external RCS resistor and monitored through the CS input by
comparator A2.
The LED load is powered from energy stored in the
output capacitor during the inductor charging cycle. Once
the peak inductor current value is achieved, the output is
turned off (off-time is typically 300ns) allowing a portion
of the energy stored in the inductor to be delivered to the
load (e.g., see Figure 6, channel 2). This causes the output
voltage to continue to rise across RSET at the input to the
feedback circuit. The LX1993 continues to switch until the
voltage at the FB pin exceeds the control voltage at the ADJ
pin. The value of RSET is established by dividing the
maximum adjust voltage by the maximum series LED
current. A minimum value of 15Ω is recommended for
RSET. The voltage at the FB pin is the product of IOUT (i.e.,
the current through the LED chain) and RSET.
R
SET
=
⎡VADJmax
⎢⎣
⎤
ILEDmax⎥⎦
The application of an external voltage source at the
ADJ pin provides for output current adjustment over the
entire dimming range and the designer can select one of
two possible methods. The first option is to connect a
PWM logic signal to the ADJ pin (e.g., see Figure 1). The
LX1993 includes an internal 50pF capacitor to ground that
works with an external resistor to create a low-pass filter
(i.e., filter out the AC component of a pulse width
modulated input of fPWM ≥ 100KHz). The second option is
to adjust the reference voltage directly at the ADJ pin by
applying a DC voltage from 0.0 to 0.3V (e.g., see Figure
2). The adjustment voltage level is selectable (with limited
accuracy) by implementing the voltage divider created
between the external series resistor and the internal 2.5MΩ
resistor. Disabling the LX1993 is achieved by driving the
SHDN pin with a low-level logic signal thus reducing the
device power consumption to approximately 0.5µA (typ).
INDUCTOR SELECTION AND OUTPUT CURRENT LIMIT
PROGRAMMING
Setting the level of peak inductor current to approximately
2X the expected maximum DC input current will minimize
the inductor size, the input ripple current, and the output
ripple voltage. The designer is encouraged to use inductors
that will not saturate at the peak inductor current level. An
inductor value of 47µH is recommended. Choosing a lower
value emphasizes peak current overshoot while choosing a
higher value emphasizes output ripple voltage. The peak
switch current is defined using a resistor placed between the
CS terminal and ground and the IPEAK equation is:
( )I PEAK
= I MIN
+ ⎜⎛ V IN
⎝
L
⎟⎞
⎠
t
D
+
I SCALE
R CS
The maximum IPEAK value is limited by the ISW value
(max. = 500mA rms). The minimum IPEAK value is defined
when RCS is zero. The minimum IPEAK value is defined when
RCS is zero. A typical value for the minimum peak current
(IMIN) at 25oC is 197mA. The parameter tD is related to
internal operation of comparator A. A typical value at 25oC
is 850ns. A typical value of ISCALE at 25oC is 44mA per KΩ.
All of these parameters have an effect on the final IPEAK
value.
DESIGN EXAMPLE:
Determine IPEAK where VIN equals 3.0V and RCS equals
4.02KΩ using nominal values for all other parameters.
( )IPEAK =197mA+ ⎜⎝⎛3.0V47µ7 ⎟⎠⎞×850ns+ 44mAkΩ × 4.02KΩ
The result of this example yields a nominal IPEAK of
approximately 428mA.
OUTPUT RIPPLE AND CAPACITOR SELECTION
Output voltage ripple is a function of the inductor value
(L), the output capacitor value (COUT), the peak switch
current setting (IPEAK), the load current (IOUT), the input
voltage (VIN) and the output voltage (VOUT) for a this boost
converter regulation scheme. When the switch is first turned
on, the peak-to-peak voltage ripple is a function of the output
droop (as the inductor current charges to IPEAK), the feedback
transition error (i.e., typically 10mV), and the output
overshoot (when the stored energy in the inductor is
delivered to the load at the end of the charging cycle).
Therefore the total ripple voltage is
VRIPPLE = ∆VDROOP + ∆VOVERSHOOT + 10mV
The initial droop can be estimated as follows where the
0.5V value in the denominator is an estimate of the voltage
drop across the inductor and the FET RDS_ON:
Copyright © 2000
Rev. 1.0b, 2005-03-03
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
5 Page 
INTEGRATED PRODUCTS
LX1993
High Efficiency LED Driver
PRODUCTION
DU
PACKAGE DIMENSIONS
8-Pin Miniature Shrink Outline Package (MSOP)
A
B
HG
P
C
M
ND L K
Dim
MILLIMETERS
MIN MAX
INCHES
MIN MAX
A 2.85 3.05 .112 .120
B 2.90 3.10 .114 .122
C – 1.10 – 0.043
D 0.25 0.40 0.009 0.160
G 0.65 BSC
0.025 BSC
H 0.38 0.64 0.015 0.025
J 0.13 0.18 0.005 0.007
K 0.95 BSC
0.037 BSC
L 0.40 0.70 0.016 0.027
M 3°
3°
N 0.05 0.15 0.002 0.006
P 4.75 5.05 0.187 0.198
Note: Dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(0.006”) on any side. Lead dimension shall
not include solder coverage.
Copyright © 2000
Rev. 1.0b, 2005-03-03
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet LX1993.PDF ] | |
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