LX1996ILQ MICROSEMI [Microsemi Corporation], LX1996ILQ Datasheet - Page 12

• Manufacturer Part Number: LX1996ILQ
• Description: Multi-string LED Back-light Controller
• Manufacturer: MICROSEMI [Microsemi Corporation]

Copyright © 2007
Rev. 1.0, 2007-12-14
B
The LX1996 uses a variable peak current variable off time
hysteretic PFM converter topology where inductor peak current
is digitally controlled. It is a current mode step up converter in
PFM burst mode. The LX1996 controls the inductor peak and
the off time to minimize the switching frequency variation over
a wide input power supply range. It regulates the boost output
voltage based on the lowest voltage among the CS pins. This
converter stays in burst PFM mode for the entire load range and
has extremely fast load response. It automatically increases or
decreases the inductor peak current depending on the output
load to minimize the output voltage ripple while maintaining the
output in regulation. Maximum coil peak current can be up to
1.55A with a 75mΩ sense resistor. For a higher power system,
a user can use a smaller resistance current sense resistor.
Conversely, a lower power system can use a larger resistance
current sense resistor.
1MHz. A 6.8uH inductor is recommended for systems that
require direct digital dimming. A larger inductor may be used
for a higher efficiency if the system runs only in analog
dimming mode.
VLDO R
Regulated 5V is generated from VIN. It provides power to the
internal 5V circuits, an external NFET, and an optional external
light sensor (LX1972) or SMBus chip (LX1800). The total
maximum output current is limited to 1mA from the VLDO pin.
A capacitor must be closely connected across the VLDO pin
and GND for noise decoupling and phase compensation. A
value of 4.7μF is required. The VLDO Regulator shuts off
while the chip is in sleep mode (EN=low) to minimize the sleep
mode current. The VLDO output voltage is monitored by the
under voltage lockout circuit.
LED C
CS1 through CS6 are the LED current sinks.
information on these pins is used to regulate the boost converter
output. Boost converter output regulation is based on the lowest
CS pin voltage. This regulation scheme insures that the LED
string with the largest total V
regulation, while maximizing the total efficiency for all other
strings.
proportional to CS current. For example, typical V
800mV at I
uses active current sources for the best string current matching
between strings. The high impedance outputs have excellent
voltage compliances, therefore, V
critical. The current sources can be paralleled for higher current
rating.
OOST
URRENT
C
ONVERTER
EGULATOR
CS pin voltage (at the lowest CS pin) is linearly
TM
CS
= 20mA, and 80mV at I
S
INKS
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
Typical switching frequency is over
F
F
matching of the LEDs is less
has ample headroom for
CS
= 2mA. The LX1996
O P E R A T I O N T H E O R Y
Analog Mixed Signal Group
Microsemi
CS
®
Voltage
will be
LED current can be set with an external resistor using the
following formula:
V
Dimming. In combined Analog and Digital Dimming, V
2V with DI ≥ 2V. When DI < 2V, then V
D
The LX1996 offers 6 methods of dimming using only 2 pins
(See Table 1).
D
The LED current source outputs are directly turned on and off
by the PWM signal. A high on PWM input pin turns on the
output. The current pulse magnitude is independent of PWM
duty cycle and is set by the PRG resistor with a fixed 2V
reference at the PRG pin.
C
The LED current source outputs are directly turned on and off
by the PWM signal. A high on PWM input pin turns on the
output.
voltage input at the DI pin. Current is set by the PRG resistor,
with a reference voltage at the PRG pin that varies with the
voltage input at the DI pin in a 1:1 ratio. The minimum and
maximum reference voltage at the PRG pin is clamped at 0.2
and 2 Volts, respectively.
A
Analog to digital dimming mode converts a DC voltage on the
DI input into a PWM output. DI input will be in the range of
0.4 to 2.4V. An internal PWM signal is generated to control the
LED output, just as in direct digital dimming. Two external
components are required to make this mode work; C
R
dimming oscillator, and the ultrasonic pulse skipping timer. R
must be a fixed 100kΩ. The internal oscillator frequency is set
by the value of the C
equation:
Multi-string LED Back-light Controller
T
PRG
IMMING
IRECT
OMBINED
NALOG TO
is a reference current setting resistor for the internal
= 2V for either Direct Digital Dimming or Direct Analog
D
F = 5e
The current pulse magnitude is dependent on the
IGITAL
C
A
ONTROL
D
NALOG AND
P
IGITAL
-6
RODUCTION
/C
D
T
IMMING
, C
T
D
I
T
capacitor
LED
IMMING
in Farad, F in Hertz.
D
=
IGITAL
(PWM)
D
R
V
PRG
PRG
ATASHEET
,
and is found by the following
×
D
50
IMMING
PRG
= DI.
(PWM)
LX1996
T
and R
PRG
Page 12
=
T
T
.