IS31LT3350-V2STLS2-TR ISSI, IS31LT3350-V2STLS2-TR Datasheet - Page 10

• 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
A value of 1μF will reduce nominal ripple current by a
factor three (approx.). Proportionally lower ripple can be
achieved with higher capacitor values. Note that the
capacitor will not affect operating frequency or efficiency,
but it will increase start-up delay, by reducing the rate of
rise of LED voltage.
Operation at low supply voltage
The internal regulator disables the drive to the switch until
the supply has risen above the startup threshold set
internally which makes power MOSFET on-resistance
small enough. Above this threshold, the chip will start to
operate. However, with the supply voltage below the
specified minimum value, the switch duty cycle will be
high and the chip power dissipation will be at a maximum.
Care should be taken to avoid operating the chip under
such conditions in the application, in order to minimize
the risk of exceeding the maximum allowed die
temperature. (See next section on thermal
considerations).
Note that when driving loads of two or more LEDs, the
forward drop will normally be sufficient to prevent the chip
from switching below approximately 6V. This will
minimize the risk of damage to the chip.
Thermal considerations
When operating the chip at high ambient temperatures,
or when driving maximum load current, care must be
taken to avoid exceeding the package power dissipation
limits. Two packages are available for different
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 10/18/2011
C1
VIN
ADJ
V IN
IS31LT3350
Rs
D1
I SENSE LX
LED
L 1
GND
Cled
application because of thermal consideration. SOT23-5 is
only available for 350mA current output application. The
SOT89-5 package’s thermal resistance is much lower so
that it can handle 700mA current and suitable for 700mA
application.
Note that the chip power dissipation will most often be a
maximum at minimum supply voltage. It will also increase
if the efficiency of the circuit is low. This may result from
the use of unsuitable coils, or excessive parasitic output
capacitance on the switch output.
Layout considerations
LX pin
The LX pin of the chip is a fast switching node, so PCB
traces should be kept as short as possible. To minimize
ground 'bounce', the ground pin of the chip should be
soldered directly to the ground plane.
Coil and decoupling capacitors
It is particularly important to mount the coil and the input
decoupling capacitor close to the chip to minimize
parasitic resistance and inductance, which will degrade
efficiency. It is also important to take account of any trace
resistance in series with current sense resistor R
ADJ pin
The ADJ pin is a high impedance input, so when left
floating, PCB traces to this pin should be as short as
possible to reduce noise pickup. ADJ pin can also be
connected to a voltage between 1.2V~5V. In this case,
the internal circuit will clamp the output current at the
value which is set by ADJ=1.2V.
High voltage traces
Avoid running any high voltage traces close to the ADJ
pin, to reduce the risk of leakage due to board
contamination. Any such leakage may raise the ADJ pin
voltage and cause excessive output current. A ground
ring placed around the ADJ pin will minimize changes in
output current under these conditions.
S
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