lx1996 Microsemi Corporation, lx1996 Datasheet - Page 15

• Manufacturer Part Number: lx1996
• Description: Multi-string Led Back-light Controller
• Manufacturer: Microsemi Corporation

Copyright © 2007
Rev. 1.0, 2007-12-14
Ultra Sonic Pulse Skipping (USPS)
At low LED current level, the boost frequency may go to
audible range. With an USPS option (analog dimming mode
only), the audible noise is completely eliminated.
When this mode is enabled, maximum switching interval is
limited to 40μS, thus keeping the switching frequency beyond
audible range. USPS is an option for analog dimming only.
PROTECTION MODES
T
The LX1996 includes a thermal protection circuit. When the IC
senses temperature over +140°C (typ), the LX1996 turns off the
LED output current and other circuits. The output recovers
when the die temperature drops down to 100°C. The thermal
protection does not latch off the IC.
O
A simple resistor divider between the boost output and ground
is used to program the LX1996’s OVP limit. The over-voltage
threshold is set to 2V at the OVP pin. When OVP limit
threshold is met, the LX1996 will regulate the boost output to
the OVP 2V threshold. OVP limit should be set to a voltage
approximately 2 Volts above the maximum total V
individual diode strings.
LED O
In case of an open string, the open string CS voltage will drop to
zero and the boost voltage will rise up to its programmed OVP
limit or the CS pin maximum voltage limit. When the boost
output reaches the OVP limit or the voltage of any CS pin
reaches 25V, the open string is latched off from the regulation
loop. At the moment the open string latches off, The boost
converter will stop switching until the lowest CS pin voltage
gets down to 0.8V. The change in boost output voltage should
have no impact to the remaining active LED string currents
since they are provided by high impedance current sinks. Once
the open condition is corrected, cycling power or the EN input
will reset the latched off CS string.
In case of the LED short condition, LX1996 output works
normally until the CS voltage rises to 15V (up to four shorted
LEDs, typical). When the CS pin voltage reaches 15V, the
shorted string is latched off and is disconnected from the boost
converter’s regulation loop. If the latched off string CS pin
voltage reaches 25V, the circuit will hold the CS pin voltage at
this level.
LED C
For LED Bar temperature sensing, the LED current profiler
circuit receives a voltage on the OT pin from the output of a
HERMAL
VER
V
URRENT
PEN
OLTAGE
TM
S
/
SHORT PROTECTION
HUTDOWN
T
P
EMPERATURE
ROTECTION
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
P
ROFILER
O P E R A T I O N T H E O R Y
Analog Mixed Signal Group
F
of the
Microsemi
®
voltage divider network. This network consists of a thermistor
and a bias resistor connected between +5V and GND. The LED
current profiler circuit will reduce the LED current setting by
reducing the PRG pin voltage. With our recommended external
components, derating begins at 60°C (recommended by most
LED manufacturers). The LED current will reduce proportional
to the temperature, and shut off when the sensing temperature
reaches 85°C. The LED current will recover when the sensing
temperature drops to 75°C. See example chart below (recovery
not shown).
Multi-string LED Back-light Controller
25m A
20m A
15m A
10m A
5m A
0°C
P
RODUCTION
20°C
B = 4500
B = 4000
B = 3000
LED C urrent Vs. Tem perature
40°C
D
ATASHEET
60°C
80°C
LX1996
Page 15
100°C