IS31LT3948-GRLS2-TR ISSI, IS31LT3948-GRLS2-TR Datasheet - Page 10

• Manufacturer Part Number: IS31LT3948-GRLS2-TR
• Description: LED Lighting Drivers PFM Mode Boost LED Drvr
• Manufacturer: ISSI

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