IS31LT3910

IS31LT3910
UNIVERSAL HIGH BRIGHTNESS LED DRIVER
WITH TEMPERATURE COMPENSATION
May 2014
GENERAL DESCRIPTION
FEATURES
The IS31LT3910 is a peak current mode control LED
driver IC. The IS31LT3910 operates in constant offtime mode. It allows efficient operation of High
Brightness (HB) LEDs with voltage sources ranging
from 8VDC to 450DC or 110VAC/220VAC. The
IS31LT3910 includes a PWM dimming input that can
accept an external control signal with a duty ratio of
0 - 100% and a frequency of up to a few kilohertz. It
also includes a 0 - 240mV linear dimming input
which can be used both for linear dimming and
temperature compensation of the LED current.

The IS31LT3910 is ideally suited for buck LED
drivers. Since the IS31LT3910 operates in peak
current mode control, the controller achieves good
output current regulation without the need for any
loop compensation. It achieves good PWM dimming
response because the response time is limited only
by the rate of rise and fall of the inductor current,
enabling very fast rise and fall time.
Wide input range from 8VDC to 450DC or
110VAC/220 VAC
Temperature compensation to regulate LED
current
Application from a few mA to more than 1A
output
Constant off-time operation
Linear and PWM dimming capability
Switch mode controller for single switch LED
drivers
Requires few external components for operation
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APPLICATIONS

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DC/DC or AC/DC LED driver applications
General purpose constant current source
Signal and decorative LED lighting
backlighting LED driver
TYPICAL APPLICATION CIRCUIT
VINDC
8V ~ 450V
8
7
CIN
NTC
D1
RIN
COFF
VCC
C1
L1
TOFF
GATE
2
1
R1
3
5
M1
IS31LT3910
PWMD
VREF
LD
CS
GND
6
4
RCS
C2
Figure 1 Typical Application Circuit
Integrated Silicon Solution, Inc. – www.issi.com
Rev. B, 05/12/2014
1
IS31LT3910
PIN CONFIGURATIONS
Package
Pin Configurations (Top View)
SOP-8
PIN DESCRIPTION
No.
Pin
Description
1
VREF
This pin provides reference a voltage of 1.2V, no bypass
capacitor is needed.
2
PWMD
This is the PWM dimming input of the IC. When this pin is pulled
to GND, the gate driver is turned off. When the pin is pulled high,
the gate driver operates normally.
3
LD
This pin is the linear dimming input and sets the current sense
threshold as long as the voltage at the pin is less than 240mV
(Typ.). It can also used as temperature compensation threshold
voltage.
4
GND
Ground return for all internal circuitry. This pin must be electrically
connected to ground.
5
GATE
This pin is the output gate driver for an external N-channel power
MOSFET.
6
CS
This pin is the current sense pin used to sense the MOSFET
current by means of an external sense resistor. When this pin
exceeds the lower of either the internal 240mV or the voltage at
the LD pin, the gate output goes low.
7
TOFF
This pin sets the off time of the power MOSFET. If left floating
then the off time will be 510ns. When a capacitor is connected
between TOFF and GND, the off time is increased.
8
VIN
This pin is the input of an 8V ~ 450V voltage supply through a
resistor, clamped at 7.1V internally, it must be bypassed with a
capacitor to GND.
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Rev. B, 05/12/2014
2
IS31LT3910
ORDERING INFORMATION
Industrial Rage: -40°C to +85°C
Order Part No.
Package
QTY/Reel
IS31LT3910-GRLS2-TR
SOP-8, Lead-free
2500
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products at any time without notice. ISSI assumes no liability arising out of the application or use of any information, products or services
described herein. Customers are advised to obtain the latest version of this device specification before relying on any published information
and before placing orders for products.
Integrated Silicon Solution, Inc. does not recommend the use of any of its products in life support applications where the failure or malfunction
of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness.
Products are not authorized for use in such applications unless Integrated Silicon Solution, Inc. receives written assurance to its satisfaction,
that:
a.) the risk of injury or damage has been minimized;
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Rev. B, 05/12/2014
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IS31LT3910
ABSOLUTE MAXIMUM RATINGS
VIN pin voltage to GND
CS, LD, PWMD, GATE, TOFF, VREF pin voltage to GND
VIN pin Input Current Range (Note 1)
Junction temperature range, TJ
Storage temperature range, TSTG
RθJA
ESD (HBM)
ESD (CDM)
-0.3V ~ +8V
-0.3V ~ +6V
1mA ~ 10mA
-40°C ~ +150°C
-65°C ~ +150°C
80°C/W
8kV
500V
Note: Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions
is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
The specifications are at TA=25°C and VINDC =10V RIN =2kΩ, unless otherwise noted (Note 1).
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
450
V
Input DC supply voltage range
Connect a decent resistor
from DC supply voltage to
VIN pin (Note 2)
VIN clamp voltage
(Note 3)
6.6
7.1
7.6
V
Operation current range
VIN =6V, GATE floating
0.32
0.48
0.64
mA
UVLO
Undervoltage lockout threshold
VIN rising
6.0
6.5
6.9
V
△UVLO
Undervoltage lockout hysteresis
VIN falling
VINDC
VIN_CLAMP
IIN
8
500
mV
VENL
Pin PWMD input low voltage
0.8
VENH
Pin PWMD input high voltage
2
REN
Pin PWMD pull-up resistance
75
100
125
kΩ
VCS_TH
Current sense pull-in threshold
voltage
230
240
250
mV
VLD
Linear Dimming pin voltage low
threshold
0.05
mV
VHD
Linear Dimming pin voltage high
threshold
0.24
mV
tBLANK
Current sense blanking interval
400
V
V
480
550
ns
Delay to output
VCS=VCS_TH+50mV after
tBLANK
tOFF
Off Time
TOFF pin Floating
tRISE
GATE output rise time
CGATE =500pF
19
ns
tFALL
GATE output fall time
CGATE =500pF
29
ns
VREF
REF pin voltage
tDELAY
VREF_LOAD
Load regulation of reference
voltage
30
420
1.12
IREF =0~500µA,VPWMD =5.0V
510
ns
600
ns
1.20
1.30
V
0.5
5
mV
Note 1: All parameters are tested at 25°C. Specifications over temperature are guaranteed by design.
Note 2: VINDC is the power supply to LED, and there should be an appropriate resistor between VINDC and VIN .
Note 3: Beyond the input current range, VIN may not clamp at 7.1V.
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Rev. B, 05/12/2014
4
IS31LT3910
FUNCTIONAL BLOCK DIAGRAM
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Rev. B, 05/12/2014
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IS31LT3910
APPLICATION INFORMATION
The IS31LT3910 is optimized to drive buck LED
drivers using peak current mode control. This control
method provides fairly accurate LED current control
without the need for a high side current sensing or
the design of any additional loop compensation. The
IC uses very few external components and provides
for either Linear or PWM control of the LED current.
A capacitor connected to the TOFF pin programs the
off-time of the oscillator inside. The oscillator
produces pulses at regular intervals. These pulses
set the SR flip-flop in the IS31LT3910 which causes
the GATE driver to turn on. When the MOSFET
turns on, the current through the inductor starts
ramping up. This current flows through the external
sense resistor RCS and produces a ramp voltage at
the CS pin. The comparators are constantly
comparing the CS pin voltage to both the voltage at
the LD pin and the internal 240mV. Once the
blanking time is complete, the output of these
comparators is allowed to reset the flip flop. When
the output of either one of the two comparators goes
high, the flip flop is reset and the GATE output goes
low. If neither of the comparator goes high, the
GATE keeps high. Assuming a 30% ripple in the
inductor, the current sense resistor RCS can be set
by using:
RCS
0.24V(orVLD )

1.15  I LED (A)
A constant off-time peak current control scheme can
easily operate at duty cycles greater than 0.5 and
also gives inherent input voltage rejection making
the LED current almost insensitive to input voltage
variations.
INPUT VOLTAGE REGULATOR
When a voltage is applied through a suitable input
resistor to the VIN pin, the IS31LT3910 maintains a
constant 7.1V (Typ.) at the VIN pin. This voltage is
used to power the IC. The VIN pin must be
bypassed by a low ESR capacitor to provide a low
impedance path for the high frequency current of the
output GATE driver.
The input current drawn from the VIN pin is a sum of
the 0.5mA (Typ) current drawn by the internal circuit
and the current drawn by the GATE driver (which in
turn depends on the switching frequency and the
GATE charge of the external MOSFET).
I IN  0.5mA  QG  f S
In the above equation, fS is the switching frequency
and QG is the GATE charge of the external MOSFET
(which can be obtained from the datasheet of the
MOSFET).
Integrated Silicon Solution, Inc. – www.issi.com
Rev. B, 05/12/2014
CURRENT SENSE
The current sense input of the IS31LT3910 goes to
the non-inverting inputs of two comparators. The
inverting terminal of oe comparator is tied to an
internal 240mV reference whereas the inverting
terminal of the other comparator is connected to the
LD pin. The outputs of both these comparators are
fed into an OR GATE and the output of the OR
GATE is fed into the reset pin of the flip-flop. Thus,
the comparator which has the lowest voltage at the
inverting terminal determines when the GATE output
is turned off.
The outputs of the comparators also include a typical
480ns blanking time which prevents spurious turnoffs of the external MOSFET due to the turn-on
spike normally present in peak current mode control.
In rare cases, this internal blanking time might not be
enough to filter out the turn-on spike. In these cases,
an external RC filter needs to be added between the
external sense resistor (RCS) and the CS pin.
Please note that the comparators are relatively fast
with a typical 80ns response time. A proper layout
minimizing external inductances will prevent false
triggering of these comparators.
OSCILLATOR
The oscillator in the IS31LT3910 is controlled by a
single capacitor connected at the TOFF pin. The
equation governing the tOFF time of oscillation period
is given by:
t OFF _ TIME ( s )  0.51 10 6  (1 
COFF
)
10 pF
LINEAR DIMMING
The Linear Dimming pin is used to control the LED
current. An external voltage ranging from 50mV to
240mV can be applied to the LD pin to adjust the
LED current during operation. There are two cases
when it may be necessary to use the Linear
Dimming pin.
In some cases, it may not be possible to find the
exact RCS value required to obtain the LED current
when the internal 240mV is used. In these cases, an
external voltage divider from the VIN pin can be
connected to the LD pin to obtain a voltage (less
than 240mV) corresponding to the desired voltage
across RCS.
Linear dimming may be desired to adjust the current
level to reduce the brightness of the LEDs.
Connecting a resistor between the VREF pin and the
LD pin, and also connecting an NTC thermistor
between the LD pin and ground (refer to Application
Circuit), the IS31LT3910 is able to realize the
temperature compensation function (See more detail
6
IS31LT3910
in Temperature Compensation section).
To use the internal 240mV, the LD pin must be
connected to PWMD pin.
TOFF TIME REGULATION CAPACITOR (COFF) AND
TOFF TIME
PWM DIMMING
PWM Dimming can be achieved by driving the
PWMD pin with a low frequency square wave signal.
When the PWM signal is low, the GATE driver is
disabled; and when the PWMD signal is high, the
GATE driver is enabled. Since the PWMD signal
does not turn off the other parts of the IC, the
response of the IS31LT3910 to the PWMD signal is
almost instantaneous. The rate of rise and fall of the
LED current is thus determined solely by the rise
and fall times of the inductor current.
To disable PWM dimming function, leave the PWMD
pin floating.
TEMPERATURE COMPENSATION
IS31LT3910 provides thermal protection for your
LEDs. Refer to application circuit, adding a NTC
themistor close to the LEDs string will realize the
temperature compensation of LEDs current. If the
temperature of the LEDs rises, the resistance of the
NTC thermistor decreases until the voltage of the LD
pin falls below 0.24V. Then the average current is
controlled by the LD pin and the temperature
compensation function starts. The formula is given
as below:
RNTC  VREF
 0.24V
R1  RNTC
Assuming a 30% ripple in the inductor, the
temperature compensated continuous current may
be computed as:
I OUT 
parallel two 430kΩ/0.5W resistor for lifetime
consideration. CIN is chosen to be 10µF/40V
capacitor
For high output voltage, low output current
application, we need shorter tOFF time to obtain the
smaller application inductor. For high output current
application, it is suggested that the frequency is set
to not more than 50kHz (typical 25kHz~30kHz). In
IS31LT3910,
t OFF _ TIME  0.5110 6  (1 
COFF
)
10 pF
So, apply COFF =150pF, then tOFF =8.16µs.
CURRENT SENSE RESISTOR (RCS)
Design for low current ripple will also improve
current accuracy, but it will require a large value of
inductor. High current ripple allows a lower cost
inductor. So we need to consider these two factors
when selecting an inductor.
A capacitor placed in parallel with the array of LEDs
can be used to reduce the LED current ripple while
keeping the same average current. A typical value is
1µF should be used.
Since the output average IO_AVG = 160mA, Assume
50% current ripple, then
1
1
I Ripple   50% 160mA  40mA
2
2
I O _ PEAK  I O_AVG 
RNTC  VREF
( R1  RNTC )  RS  1.15
RCS 
1
I Ripple  200mA
2
0.24V
 1.2
200mA
Make sure the value of R1 is more than 1kΩ.
THE INDUCTOR (L1) CHOSEN
When the LD pin voltage reduces to less than 50mV
the chip is shutdown.
The inductor value depends on the ripple current in
the LEDs. tOFF =8.16µs
After the ambient temperature returns to a safe
temperature, the current will return to the set value.
Example:
DC input voltage: VINDC =230V
Output LED strings: VO =134.4V (42 LEDs in series,
3.2V for each one),
IO_AVG =160mA (8 parallels LEDs, 20mA for each one)
IC INPUT RESISTOR (RIN) AND HOLD
CAPACITOR (CIN)
R IN 
V INDC  V IN 230  7.0

 223k
I IN
1
Integrated Silicon Solution, Inc. – www.issi.com
Rev. B, 05/12/2014
L
VO  t OFF 134.4  8.16

 13.7 mH
I Ripple
80
The inductor chosen should have a saturation
current higher than the peak output current and a
continuous current rating above the required mean
output current.
The DC resistance (DCR) of the inductor is also
essential when choosing an inductor. Bigger DCR
will lead to more heat. The value of the inductor will
reduce as its temperature rises, leading to higher
current ripple, which in turn, reduces the average
output current.
7
IS31LT3910
MOSFET (Q1) AND DIODE (D1)
The peak voltage seen by the MOSFET is equal to
the maximum input voltage. Using a 50% safety
rating,
VFET  1.5  VINDC
The maximum RMS current through the MOSFET
depends on the maximum current. Hence, the
current rating of the MOSFET is:
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Rev. B, 05/12/2014
I FET  I PEAK  150%
For this application, choose a MOSFET 600V, 1A to
2A. 2N60 is good choice.
The peak voltage rating of the diode is the same as
the FET. The current range of the diode is:
I Diode  I PEAK  150%
For this example, 600V/1A fast recovery diode is
recommended.
8
IS31LT3910
CLASSIFICATION REFLOW PROFILES
Profile Feature
Pb-Free Assembly
Preheat & Soak
Temperature min (Tsmin)
Temperature max (Tsmax)
Time (Tsmin to Tsmax) (ts)
150°C
200°C
60-120 seconds
Average ramp-up rate (Tsmax to Tp)
3°C/second max.
Liquidous temperature (TL)
Time at liquidous (tL)
217°C
60-150 seconds
Peak package body temperature (Tp)*
Max 260°C
Time (tp)** within 5°C of the specified
classification temperature (Tc)
Max 30 seconds
Average ramp-down rate (Tp to Tsmax)
6°C/second max.
Time 25°C to peak temperature
8 minutes max.
Figure 2 Classification Profile
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Rev. B, 05/12/2014
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IS31LT3910
PACKAGE INFORMATION
SOP-8
Note: All dimensions in millimeters unless otherwise stated.
Integrated Silicon Solution, Inc. – www.issi.com
Rev. B, 05/12/2014
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