IS31LT3352-V2GRLS2-TR ISSI, IS31LT3352-V2GRLS2-TR Datasheet - Page 11

• Manufacturer Part Number: IS31LT3352-V2GRLS2-TR
• Description: LED Lighting Drivers 40V LED Drvr w/ External Switch
• Manufacturer: ISSI

Specifications of IS31LT3352-V2GRLS2-TR

• Input Voltage: 6 V to 40 V
• Operating Frequency: 154 kHz
• Maximum Supply Current: 80 uA
• Output Current: 750 mA
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOP-8
• Factory Pack Quantity: 2500

IS31LT3352
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. 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.
Temperature compensation of output current
High luminance LEDs often need to be supplied with a
temperature compensated current in order to maintain
stable and reliable operation at all drive levels. The LEDs
are usually mounted remotely from the chip. For this
reason, the temperature coefficients of the internal
circuits for the IS31LT3352 have been optimized to
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 09/01/2011
Vin
V
IN
IS31LT3352
D 1
Rs
I
SENSE
LED
L1
LX
C
LED
minimize the change in output current when no
compensation is employed. If output current
compensation is required, it is possible to use an external
temperature sensing network - normally using Negative
Temperature Coefficient (NTC) thermistors and/or
diodes, mounted very close to the LED(s). The output of
the sensing network can reduce output current with
increasing temperature through internal circuit.
As shown in the figure below, the temperature
compensation curve is decided by R1, NTC thermistor
R2 and resistor R3. When LED(s) temperature increases,
thermistance of R2 starts to reduce. As R2 reduces to the
point that R2’s thermistance plus R3 resistance equaling
to R1 resistance, temperature compensation function
starts to work and Iout starts to reduce.
The Iout current with temperature compensation’s
equation is:
In the case that 0.3< VADJI <1.2V:
I
In the case that VADJI >1.2V:
I
R2 and R3 decide the temperature compensation slope,
if R3 is just 0ohm, slope is only decided by thermistor
R2’s parameter B-constant. And larger R3’s resistance
results in slope more even.
If the temperature compensation threshold is selected,
larger R2 and R3 selected need larger R1 to match and
vice versa. Too large R1 make Rth pin more sensitive to
noise, too small R1 will make IC current consumption
larger. From 1K to 100K of R1 is recommended.
An IS31LT3352 calculator is available from the ISSI to
assist with temperature compensation design and here
are some detail examples as below:
R2(NTC)
OUTdc
OUTdc
R3
= 0.083*V
= 0.1*(R2+R3)/R1*RS
R1
ADJI
(R2+R3)/R1*RS
R
R
NTC
TH
IS31LT3352
ADJO
GND
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