lx1996 Microsemi Corporation, lx1996 Datasheet - Page 16

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

Copyright © 2007
Rev. 1.0, 2007-12-14
I
Peak inductor current is determined by the resistor connected
between SRC pin and GND, R
the following equation:
Typically, Peak current is determined by the size of the FET
desired. Peak current will also determine the amount of energy
stored in the inductor during a single switching cycle.
The V
determine the peak current level for each boost condition (V
V
DC maximum threshold, V
constant voltage increment is equal to 112.5mV/15 = 7.5mV.
If we select R
Maximum inductor peak current:
If higher output power is needed, the value of R
lowered accordingly. For example: If 50mΩ is used:
Each current step will be:
Maximum inductor peak current:
The current sense signal is very sensitive; therefore, the LX1996
SRC pin must be connected directly to the junction of the sense
resistor, R
practical. The return side of the sense resistor must tie directly
to the PGND pin, and then tie the PGND pin to the ground
plane.
NDUCTOR
OUT
, and I
CS
I
I
Note: 50mA term is due to propagation delay and is
based on 4.7uH inductor and V
I
I
detect signal is fed into a 4 bit up/down counter to
STEP
PEAK (MAX)
STEP
PEAK (MAX)
TM
SRC
C
OUT
URRENT
.= 7.5mV/75mΩ = 100mA
SRC
= 7.5mV/50mΩ = 150mA
, and the NFET Source, as close to R
IL
). There are 15 total current steps available. The
PK
= 75mΩ, then step current:
=
= 100mA x 15 +50mA = 1.55A
= 150mA x 15 + 50mA = 2.3A
112.5mV
S
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
ENSE
R
SRC
SCR
R
SRC.
ESISTOR
, is equal to 112.5mV, and the
+
VCC
It’s value is determined by
L
BOOST
×
S
IN
20ns
ELECTION
= 12V.
A P P L I C A T I O N I N F O R M A T I O N
mA
Analog Mixed Signal Group
SENSE
(R
Microsemi
SRC
®
must be
SRC
)
IN
as
,
T
The following circuit is a typical application using the OT pin
for temperature profiling:
Use NTC thermistor with resistance of 100k at 25°C. B constant
can be from 3000 to 4500.
Thermal profile is set by the internal circuit with the following
equation:
At PRG pin, 2V is the 100% value and 0.8V is the shut off
point (40%).
Multi-string LED Back-light Controller
HERMAL
1. Find out the NTC resistance at 85°C (R
manufacturing chart or formula:
2. Calculate R1 value to set the shut off temperature at
85°C by: R1 = 2.4 x R
NTC is at 85°C.
V
PRG
=(V
P
Where R
ROFILER
P
OT
RODUCTION
-0.8)*1.2, V
THERM
C
OMPONENT
85.
5V
is the thermistor resistor
OT
D
This will set VOT = 1.47V when
=(R
R1
NTC
ATASHEET
THERM
S
OT
ELECTION
/(R
THERM
85
+ R1))*5;
LX1996
) by
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