IS31LT3948-GRLS2-TR ISSI, IS31LT3948-GRLS2-TR Datasheet - Page 7

• 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
2) RC filter PWM dimming:
A filtered PWM signal can be used as an adjustable
DC voltage for LED dimming control. The filtered PWM
signal becomes DC voltage which is summed together
with the FB voltage to regulate the output current. Fix
the frequency of the PWM signal and change the duty
cycle to adjust the LED current. The LED current can
be calculated by the following equation:
The PWM duty cycle is inversely proportional to the
LED current. That is, when the PWM signal is 100%
duty cycle, the output current is minimum, ideally zero,
and when the PWM signal is 0% duty cycle, the output
current is maximum.
See details value in the Example section.
Note: When the V
consideration must be given to ensure that V
remains greater than V
Input Peak Current control
IS31LT3948 limits the peak inductor current, and thus
peak input current through the feedback of R3
connected from source of NMOS to ground. The
required average input current is based on the boost
ratio, Vout/Vin, and the designed value for average
LED current. The required average input current can
be calculated as:
: assumed power conversion efficiency (the
recommended value is 0.9)
Generally, setting the peak inductor current to 1.5
times the average input current is sufficient to maintain
a good regulation of the output current.
V
V
Integrated Silicon Solution, Inc. – www.issi.com
Rev.B, 12/24/2012
Iout
CSTH
CSTH

: If 0.5<V
=0.24V. ADJ floating, V
V
I
fbTH
peak
IS31LT3948
(

IN
I
R
)
ADJ
avg
6


(
<2.4V, V
OUT
IN
1
(
V
5 .
Figure 11
)
PWM
/V


IN
IN
V
I

avg
V
out
ratio is less than 2, careful
at the minimum dimming level.
Duty
CSTH
Rfb
in
(

IN

CSTH
I
)

= V
out


V
V
=0.24V.
ADJ
fbTH
R
CSTH
CS
/10. If V
)
/(
(5)
R
7

ADJ
OUT
R
(6)
) 8
>2.4V,
(4)
Input Capacitor
The input capacitor of the IS31LT3948 will supply the
transient input current of the power inductor. Value of
100μF or higher is recommended to prevent excessive
input voltage ripple.
Setting T
IS31LT3948 operates in a pulsed frequency
modulation mode. The boost control loop is a constant
off-time architecture. The off time is programmable and
set by an external resistor connected between the T
pin and GND. In most application, the recommended
time is:
Note: The minimum
Inductor Selection
Inductor value directly determines the switching
frequency of the converter. To the fixed condition and
the larger inductor value the lower switching frequency.
The higher frequency will reduce the value of inductor,
but will increase the switching loss on NMOS.
The switching frequency can be calculated blow.
Switching frequency:
The current ripple in the inductor:
NMOS on time:
NMOS off time:
Note: the selection of inductor must ensure that the
T
not output the required current.
Where:
V
T
T
OFF
OFF
in
ON
: Input voltage (V)
T
10
larger than the
(min)
8
6
4
2
0
OFF

0
I
V
Ripple
T
in
OFF(min)

is 1µs. The governing equation for the off
OFF

25
V
out
I

(min)
avg
2

50
(

IN
V


)
D
I
40

T
75
T
peak
I

I
OFF
OFF
Ripple
(
f
Ripple
V

R
(
 / 1
in
10
IN
L
100
(min)
(min)


)

12



R
I
L
R (K)
,or else the converter can
T
L
avg
125
I
DS

is 1µs.
ON
avg
(
R
(
IN
ON
(

EXT
IN
)
150
)
T

)

OFF

R
R
L
175
CS

)
(7)
200
(9)
7
(8)
(11)
225
(10)
OFF
250