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IS31LT3910-GRLS2-TR Datasheet(PDF) 7 Page - Integrated Silicon Solution, Inc

Part # IS31LT3910-GRLS2-TR
Description  UNIVERSAL HIGH BRIGHTNESS LED DRIVER WITH TEMPERATURE COMPENSATION
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Manufacturer  ISSI [Integrated Silicon Solution, Inc]
Direct Link  http://www.issi.com
Logo ISSI - Integrated Silicon Solution, Inc

IS31LT3910-GRLS2-TR Datasheet(HTML) 7 Page - Integrated Silicon Solution, Inc

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IS31LT3910
Integrated Silicon Solution, Inc. – www.issi.com
7
Rev. B, 05/12/2014
in Temperature Compensation section).
To use the internal 240mV, the LD pin must be
connected to PWMD pin.
PWM DIMMING
PWM Dimming can be achieved by driving the
PWMD pin with a low frequency square wave signal.
When the PWM signal is low, the GATE driver is
disabled; and when the PWMD signal is high, the
GATE driver is enabled. Since the PWMD signal
does not turn off the other parts of the IC, the
response of the IS31LT3910 to the PWMD signal is
almost instantaneous. The rate of rise and fall of the
LED current is thus determined solely by the rise
and fall times of the inductor current.
To disable PWM dimming function, leave the PWMD
pin floating.
TEMPERATURE COMPENSATION
IS31LT3910 provides thermal protection for your
LEDs. Refer to application circuit, adding a NTC
themistor close to the LEDs string will realize the
temperature compensation of LEDs current. If the
temperature of the LEDs rises, the resistance of the
NTC thermistor decreases until the voltage of the LD
pin falls below 0.24V. Then the average current is
controlled by the LD pin and the temperature
compensation function starts. The formula is given
as below:
V
R
R
V
R
NTC
REF
NTC
24
.
0
1
Assuming a 30% ripple in the inductor, the
temperature compensated continuous current may
be computed as:
15
.
1
)
(
1
S
NTC
REF
NTC
OUT
R
R
R
V
R
I
Make sure the value of R1 is more than 1kΩ.
When the LD pin voltage reduces to less than 50mV
the chip is shutdown.
After the ambient temperature returns to a safe
temperature, the current will return to the set value.
Example:
DC input voltage: VINDC =230V
Output LED strings: VO =134.4V (42 LEDs in series,
3.2V for each one),
IO_AVG =160mA (8 parallels LEDs, 20mA for each one)
IC INPUT RESISTOR (RIN) AND HOLD
CAPACITOR (CIN)
k
I
V
V
R
IN
IN
INDC
IN
223
1
0
.
7
230
parallel two 430kΩ/0.5W resistor for lifetime
consideration. CIN is chosen to be 10µF/40V
capacitor
TOFF TIME REGULATION CAPACITOR (COFF) AND
TOFF TIME
For high output voltage, low output current
application, we need shorter tOFF time to obtain the
smaller application inductor. For high output current
application, it is suggested that the frequency is set
to not more than 50kHz (typical 25kHz~30kHz). In
IS31LT3910,
)
10
1
(
10
51
.
0
6
_
pF
C
t
OFF
TIME
OFF
So, apply COFF =150pF, then tOFF =8.16µs.
CURRENT SENSE RESISTOR (RCS)
Design for low current ripple will also improve
current accuracy, but it will require a large value of
inductor. High current ripple allows a lower cost
inductor. So we need to consider these two factors
when selecting an inductor.
A capacitor placed in parallel with the array of LEDs
can be used to reduce the LED current ripple while
keeping the same average current. A typical value is
1µF should be used.
Since the output average IO_AVG = 160mA, Assume
50% current ripple, then
mA
mA
I
Ripple
40
160
%
50
2
1
2
1
mA
I
I
Ripple
PEAK
O
200
2
1
I
O_AVG
_
2
.
1
200
24
.
0
mA
V
R
CS
THE INDUCTOR (L1) CHOSEN
The inductor value depends on the ripple current in
the LEDs. tOFF =8.16µs
mH
I
t
V
L
Ripple
OFF
O
7
.
13
80
16
.
8
4
.
134
The inductor chosen should have a saturation
current higher than the peak output current and a
continuous current rating above the required mean
output current.
The DC resistance (DCR) of the inductor is also
essential when choosing an inductor. Bigger DCR
will lead to more heat. The value of the inductor will
reduce as its temperature rises, leading to higher
current ripple, which in turn, reduces the average
output current.


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