IS31LT3350-V2STLS2-TR ISSI, IS31LT3350-V2STLS2-TR Datasheet - Page 9

• Manufacturer Part Number: IS31LT3350-V2STLS2-TR
• Description: LED Lighting Drivers 40V LED Drvr w/ intrnl switch
• Manufacturer: ISSI

Specifications of IS31LT3350-V2STLS2-TR

• Input Voltage: 6 V to 40 V
• Operating Frequency: 154 kHz
• Maximum Supply Current: 80 uA
• Output Current: 400 mA
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOT-23-5
• Factory Pack Quantity: 3000

IS31LT3350
If the source is a DC supply, the capacitor is decided by
ripple of the source, the value is given by:
I
power supply. Ton is the “ON” time of MOSFET. The
value is normally 2 times of the minimum value.
If the source is an AC supply, the typical output voltages
ripple from a nominal 12V AC transformer can be ±10%.
If the input capacitor value is lower than 200μF, the AC
input
waveform is distorted, sometimes the lowest value will be
lower than the forward voltage of LED strings. This will
lower the average current of the LEDs. So it is
recommended to set the value of the capacitor bigger
than 200uF.
For maximum stability over temperature and voltage,
capacitors with X7R, X5R, or better dielectric are
recommended. Capacitors with Y5V dielectric are not
suitable for decoupling in this application and should not
be used.
Inductor selection
Recommended inductor values for the IS31LT3350 are in
the range of 47μH to 220μH.
Higher values of inductance are recommended at higher
supply voltages and low output current in order to
minimize errors due to switching delays, which result in
increased ripple and lower efficiency. Higher values of
inductance also result in a smaller change in output
current over the supply voltage range. (See graphs). The
inductor should be mounted as close to the chip as
possible with low resistance connections to the LX and
V
The chosen coil should have a saturation current higher
than the peak output current and a continuous current
rating above the required mean output current. It is
recommended to use inductor with saturation current
bigger than 1.2A for 700mA output current and inductor
with saturation current bigger than 500mA for 350mA
output current.
The inductor value should be chosen to maintain
operating duty cycle and switch 'on/off' times within the
specified limits over the supply voltage and load current
range.
The following equations can be used as a guide.
LX Switch 'On' time
Note: T
LX Switch 'Off' time
Note: T
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 10/18/2011
T
T
C
F
IN
ON
OFF
is the value of output current，
min
pins.



V
V

I
ONmin
OFFmin
IN
F
U
LED
*

MAX
T
V
>200ns

>200ns
on
LED
V
D


L
I
I

AVG
AVG
L
I

(
I
(
R
rL
S


R
rL
S

)

U
R
MAX
LX
)
is the ripple of
Where:
L is the coil inductance (H)
r
I
∆I is the coil peak-peak ripple current (A) {Internally set to
0.3 × I
V
V
R
V
current (V)
Example:
For V
and V
T
T
This gives an operating frequency of 546kHz and a duty
cycle of 0.34
Optimum performance will be achieved by setting the
duty cycle close to 0.5 at the nominal supply voltage. This
helps to equalize the undershoot and overshoot and
improves temperature stability of the output current.
Diode selection
For maximum efficiency and performance, the rectifier
(D1) should be a fast low capacitance Schottky diode
with low reverse leakage at the maximum operating
voltage and temperature.
If alternative diodes are used, it is important to select
parts with a peak current rating above the peak coil
current and a continuous current rating higher than the
maximum output load current. It is very important to
consider the reverse leakage of the diode when operating
above 85°C. Excess leakage will increase the power
dissipation in the device.
The higher forward voltage and overshoot due to reverse
recovery time in silicon diodes will increase the peak
voltage on the LX output. If a silicon diode is used, care
should be taken to ensure that the total voltage appearing
on the LX pin including supply ripple, does not exceed
the specified maximum value.
Reducing output ripple
Peak to peak ripple current in the LED can be reduced, if
required, by shunting a capacitor C
as shown below:
avg
L
ON
OFF
IN
LED
D
LX
is the coil resistance (Ω)
is the diode forward voltage at the required load
is the required LED current (A)
is the supply voltage (V)
is the switch resistance (Ω)
= (47e-6 × 0.105)/(12 - 3.4 - 0.612) = 0.62μs
is the total LED forward voltage (V)
= (47e-6 × 0.105)/(3.4 + 0.36 + 0.322)= 1.21μs
IN
D
=12V, L=47μH, r
=0.36V
avg
}
L
=0.64Ω, V
LED
led
=3.4V, I
across the LED(s)
avg
=333mA
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