IS31LT3948-GRLS2-TR ISSI, IS31LT3948-GRLS2-TR Datasheet - Page 10
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IS31LT3948-GRLS2-TR
Manufacturer Part Number
IS31LT3948-GRLS2-TR
Description
LED Lighting Drivers PFM Mode Boost LED Drvr
Manufacturer
ISSI
Datasheet
1.IS31LT3948-GRLS2-TR.pdf
(14 pages)
Specifications of IS31LT3948-GRLS2-TR
Input Voltage
5 V to 100 V
Maximum Supply Current
400 uA
Maximum Operating Temperature
+ 85 C
Mounting Style
SMD/SMT
Package / Case
SOP-8
Factory Pack Quantity
2500
IS31LT3948
Example
Input: Vin = 12~24V
Output: Iout = 350mA, Vout≈30~40V (9~12LEDs,
Vf=3.3V)
To calculate the worst case parameter, use the
minimum input voltage, the maximum output voltage,
and maximum output current. So choose: Vin = 12V,
Iout = 350mA, Vout≈40V (12LEDs, Vf=3.3V)
1. R1 and C1&C2
Assume Iin = 2.5mA
Choose C1 as 220µF/35V
2. R2 to set minimal-TOFF
The recommended value is 1µs
Choose R2 = 24kΩ
3. Rfb to set output current and C3
Choose C3 = 220µF/63V (Low ESR electrolytic
capacitor)
4. R6, R7, R8 and C4
Take Duty=100%, V
equation, then we have:
Which Simplifies to:
15.66
The lowpass filter formed by R8 & C4 must have a
corner frequency much lower than the PWM frequency.
As the corner frequency of the filter decreases, the
response time of the LED current to changes in PWM
increases. Choose a corner frequency 50 times lower
than f
Assuming f
= 0.1µF, we find R8 ≥ 400kΩ.
Choose C4 = 0.1µF, R8 = 400kΩ.
Choose a nominal value for R7, then compute R6.
Choose R7 = 10kΩ, then R6 = 26.2kΩ
Take Duty=0, V
equation, then we have:
Integrated Silicon Solution, Inc. – www.issi.com
Rev.B, 12/24/2012
T
0
R
Rfb
Iout
R
R6 、 R7 、 R8 can be calculate by:
OFF
1
8
0
(min)
PWM
C
Vin
3 .
V
4
V
Iout
R6
fbTH
.
Iin
fbTH
R
PWM
C2 as 10µF/16V
40
Vcc
2
6
R
50
f
is 200Hz (or higher), and choosing C4
5 (
PWM
PWM
. 0
10
R
7
86
6
3
12
100
= 5V and Iout=350mA into the
k
PWM
R
(
8
V
. 0
R
PWM
%
86
= 5V and Iout=0 into the
EXT
. 0
Duty
Rfb
) 3
1
/(
s
R
7
V
fbTH
R
) 8
)
/(
R
7
R
) 8
So R
R
4. R3 to set input peak current
Assume:
: assumed power conversion efficiency (the
recommended value is 0.9)
Choose R3=0.123Ω, Ipeak=1.95A
5. L1 to set frequency
Input average current:
The current ripple in the inductor:
According to T
This
Then the assumed switching frequency:
The recommended switching frequency:
20KHz < f < 200KHz, according to the switching
frequency is inversely proportional to the inductor value,
choose L=100 µH. Therefore:
The saturation current of the inductor must exceed the
input peak current (
T
T
Iout
I
R
I
R
f
OFF
fb
Ripple
ON
peak
cs
f
L
1
'
0
will be different from the no dimming application.)
5 .
3 .
/ 1
fb
(
IN
R
=0.91 (With the RC filter PWM dimming, the
I
f
V
gives
)
V
DS
V
V
T
40
peak
'
26
12
in
2
CSTH
out
ON
fbTH
(
100
I
ON
1
22
2 .
( IN
peak
5 .
)
I
. 0
I
0
)
avg
V
peak
T
off
(
9 .
35
5 (
IN
L>22µH.
OFF
D
R
R
(
I
>
IN
63
)
. 0
(
avg
6
CS
I
T
IN
)
Ripple
V
OFF
123
0
I
)
KHz
(
. 1
Rfb
1
IN
I
in
peak
(
%
5 .
Ripple
(
V
95
285
)
0
(min)
R
I
I
PWM
4 .
( IN
avg
avg
L
I
A
L
. 0
1
avg
I
KHz
(
:
)
(
5 .
Assuming
avg
IN
IN
).
) 3
(
.
R
L
IN
)
)
(
Duty
DS
Rfb
IN
/(
)
V
400
)
(
V
ON
out
V
1
R
in
V
out
3 .
)
L
in
A
V
I
10
R
out
I
1
fbTH
L=22µH
out
CS
)
s
)
)
/(
. 0
1
10
R
2
3 .
35
5 .
7
A
A
and
s
R
) 8