ISSI IS31LT3918-GRLS2 High voltage led lighting driver with switch dimming Datasheet

IS31LT3918
HIGH VOLTAGE LED LIGHTING DRIVER WITH SWITCH DIMMING
OCTOBER 2011
GENERAL DESCRIPTION
IS31LT3918 LED driver IC is a peak current detection
buck converter which operates in constant off time
mode. It operates over a very wide input voltage supply
range of 6VDC to 450VDC or 110VAC/220VAC.
IS31LT3918 incorporates the special feature of switch
dimming by detecting OFF-ON cycles of the main
power switch. When the switch is cycled within a 2
second period (typical) the device automatically
switches the dimming level to the next step. As a result,
dimming can be achieved without replacing any wiring
in the original system. There are multiple modes of
switch dimming that the user may configure 2 steps or
3 steps, as well as different levels of dimming via the
external pins DIM1 and DIM2.
IS31LT3918 can also realize LED dimming using an
external PWM signal. It can accept a PWM signal from
0% to 100% duty cycle. The LED current may also be
adjusted linearly by applying an analog input voltage in
the range of 0.5V to 2.5V.
IS31LT3918 adopts a peak current mode control
architecture, which eliminates the need for any
additional loop compensation while maintaining a good
degree of constant output current regulation.
FEATURES

User configurable switch dimming levels

3% output current accuracy

Over current, voltage and temperature protection

High efficiency (typical up to 95%)

Wide input voltage range: 6VDC~450VDC or
85Vac~ 265Vac

Linear and PWM dimming

Very few external components
APPLICATIONS

DC/DC or AC/DC constant current LED driver

Signal and decorative lighting

Backlight LED driver
TYPICAL APPLICATION CIRCUIT
LED+
Switch K
D1
D2
85Vac- 265Vac
C3
D5
R1
C1
LED-
R5
Q2
D3
D4
Fuse
C2
L1
DZ
D6
R2
R6
VIN
DIM1
Q1
Gate
CS
IS3918
IS31LT3918
C6
C4
DIM2
ADJ
GND
TOFF
R4
R3
Copyright © 2011 Integrated Silicon Solution, Inc. All rights reserved. ISSI reserves the right to make changes to this specification and its 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;
b.) the user assume all such risks; and
c.) potential liability of Integrated Silicon Solution, Inc is adequately protected under the circumstances
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Rev. A, 09/01/2011
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IS31LT3918
PIN CONFIGURATION
Package
Pin Configurations
SOP-8
PIN DESCRIPTION
Pin Name
Pin Number
DIM1
1
DIM2
2
ADJ
3
GND
GATE
CS
4
5
6
Toff
7
Vin
8
Description
These two pins configure the dimming levels as follows:
DIM1=“floating”DIM2=“floating”, no dimming (100% only);
DIM1=“floating”DIM2=“GND” , 100%-30%-100%
DIM1=“GND”DIM2=“floating”, 100%-50%-100%
DIM1=“GND”DIM2=“GND”, 100%-50%-20%-100%
Linear and PWM dimming input pin.
Linear dimming range: 0.5V to 2.5V.
If VADJ < 0.5V, GATE output is off.
If 0.5V ≤ VADJ ≤ 2.5V, VCSTH = VADJ/10.
If VADJ > 2.5V, VCSTH = 0.25V.
When this pin is floating, there is an internal pull up to 4.5V (typical) and VCSTH = 0.25V.
PWM dimming frequency range: 200Hz -1kHz.
Ground pin. All internal currents return through this pin.
This pin connects to the external NMOS’s gate
Current detect pin, uses an external resistor to sense the peak inductor current.
This pin sets the off time for the switch by connecting a resistor between this pin and
GND.
8V – 450V supply voltage is connected to this pin via an external resistor. It is internally
clamped and must be bypassed using a capacitor to GND.
ORDERING INFORMATION
INDUSTRIAL RANGE: -40°C TO +85°C
Order Part No.
Package
QTY/Reel
IS31LT3918–GRLS2-TR
SOP-8, Lead-free
2500
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IS31LT3918
ABSOLUTE MAXIMUM RATINGS
Parameter
Range
Unit
Vin pin to GND
-0.3 – 6.0
V
DIM1,DIM2,CS, ADJ, GATE, Toff pin to GND
-0.3 - 6.0
V
10
mA
Junction temperature
-40 – 150
℃
Device storage temperature
-65 - 150
℃
ESD(Human Body model)
3500
V
Vin pin input current (Note1)
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 (Note 2), RIN =10K, unless otherwise noted)
Symbol
Parameter
Conditions
Supply voltage connected to
Min
Typ
Max
Unit
4.3
5
5.5
V
Vclamp
Vin PIN clamp voltage
UVLO
Undervoltage lockout
△UVLO
UVLO hysteresis
IIN
Quiescent Current
Vin=5V
300
400
uA
IIN,UV
Input current in UVLO
Vin=4V
90
120
uA
VCSTH
Current sense threshold
ADJ=5V
245
250
255
mV
TBLANK
Current sense blanking time
VCS=VCSTH+50mV
445
500
650
ns
Toff
Off time
REXT=250KΩ
9.8
10
10.2
us
Vin via an appropriate resistor
Vin rising
PWM input voltage high threshold
VADJ
(Note 3)
4.8
V
400
mV
2.5
PWM input voltage low threshold
0.25
Linear dimming input voltage range
0.5
0.5
V
0.75
V
2.5
V
ISOURCE
GATE source current
GATE=0
75
90
mA
ISINK
GATE sink current
GATE=5V
75
90
mA
TP
Over temperature protection threshold
△TP
VOCP
Toff_reset
TMAX
Over temperature protection hysteresis
Over current protection CS voltage
threshold
Over current protection Toff delay time
Maximum switch off time for switch
dimming
ADJ=5V,CS rising
0.35
150
o
C
20
o
C
0.4
0.45
V
500
us
2
s
Notes:
1. Beyond the input current range, Vin pin may not clamp at 5V.
2. VIN is the input voltage. When VIN>5V, input voltage connected to Vin pin should via a appropriate resistor.
3. When VADJ>2.5V, Iout is 100% output current. When VADJ<0.5V, Iout is shutdown. When 0.5V ≤VADJ≤2.5V, Iout is linear dimming.
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Rev. A, 09/01/2011
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IS31LT3918
Input Voltage Regulation
APPLICATION INFORMATION
IS31LT3918 is a peak current control LED driver IC.
It
The VIN pin is internally clamped at 5V (typical).
does not require any high side current sensing or the
When supplying a voltage larger than 5V, an external
design of any closed loop control, but provides a very
resistor must be used between the input voltage and
accurate constant LED driving current. IS31LT3918
the VIN pin. Bypass the VIN pin using a low ESR
includes an dimming input allowing either a PWM or an
capacitor to provide a high frequency path to GND. The
analog dimming signal.
current required by the device is 0.3mA plus the
switching current of the external switch. The switching
An external resistor connected to the Toff pin
frequency of the external NMOS affects the amount of
determines the internal oscillator’s constant off time.
current required, as does the NMOS’s gate charge
The off time adds to the on time, controlled by the
requirement (found on the NMOS data sheet).
internal switching control logic, to set the oscillation
IIN  0.3mA  QG  fS
frequency. The inductor current increases when the
switch is on. This current also flows through the
In the above equation, fS is the switching frequency, QG
external current sense resistor RCS, and when the
is the external NMOS gate charge (from the NMOS
voltage across RCS reaches the current sense threshold,
datasheet).
VCSTH or 1/10 of the ADJ input voltage, whichever is
lower, the switch turns off. The current through the
Current Detection
inductor will continue to flow through the LEDs, but will
The voltage input to the CS pin is provided to two
decrease linearly during the switch off time. After the
internal comparators. One of the comparators uses a
programmed off-time, the switch will turn on again. A
fixed 250mV reference, while the other uses a scaled
short blanking time of 500ns (typical) is implemented to
value of the ADJ pin voltage as reference. The outputs
block the voltage spike encountered across RCS,
of the comparators are ORed, thus causing the lower of
caused by the parasitic capacitance of the switch
the two thresholds to trigger the switch control logic. At
discharging. After the blanking time the control logic
the moment the switch control logic changes the gate
again compares the CS input voltage to the current
signal to low, the TOFF timer is started. The external
sense threshold.
switch will remain off for the length of time programmed,
and once the TOFF time is expired, the switch control
Choose the acceptable level of ripple current, K,then
logic again toggles the gate signal, this time from low to
calculate the value of the current sense resistor:
high, and the external switch turns on. As the external
RCS 
VCSTH
(1  K / 2) I LED
switch turns on, the parasitic capacitance on the drain
of the switch must discharge through the switch
channel causing a spike of current which can be quite
VCSTH: If VADJ < 0.5V, GATE output is off.
If 0.5V ≤ VADJ ≤ 2.5V, VCSTH = VADJ/10.
If VADJ > 2.5V, VCSTH = 0.25V.
When ADJ pin is floating, there is an internal pull
up to 4.5V (typical) and VCSTH = 0.25V.
K: acceptable current ripple, the recommended value
large, but only lasts for a very short period of time. To
range is 1~1.8.
sufficient to prevent false triggering of the CS threshold
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.
prevent this current from causing a false trip of the
current sense comparators, the signal is blocked from
the internal comparators for 500ns (typical).
In some
special cases, the 500ns blanking time may not be
logic. Under these circumstances, an additional RC
filter may be added to the CS input pin to help filter out
the voltage spike. Carefully layout of the PCB to
minimize parasitic capacitance, trace resistance and
inductance greatly aids in the elimination of false
triggering.
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IS31LT3918
Oscillator
output current is:
IS31LT3918’s TOFF pin controls the off time of the
a) 100% at power on.
internal oscillator. Oscillator off time is determined by
b) The first switch dimming action causes the
current to change to 30%.
the following equation:
c) A second switch dimming action causes the
Toff ( s )  40  10 12  REXT
current to return to 100%.
REXT:resistor connected between TOFF and GND
d) A third switch dimming action has the same
Switch Dimming
e) Subsequent switch dimming actions causes
effect as the first switch dimming action.
the cycle to continue.
IS31LT3918 detects the external switch action of the
main power switch, and can automatically adjust the
3.
When DIM1 is GND and DIM2 is floating, the
level of the output current based on the action of the
dimming sequence is as described in (2) above,
main power switch.
except that the current sequence is
100%-50%-100%.
The action of the external power switch can be divided
4.
When both DIM1 and DIM2 are connected to GND,
into two types. The first is “normal switch operation” in
the dimming sequence is as described in (2) above,
which the switch is toggled from ON to OFF, remaining
except that the current sequence is
OFF for longer than 2 seconds (typical). The second is
100%-50%-20%-100%.
“switch dimming action” in which the switch is toggled
If the switch is operated normally, that is, switched on
from ON to OFF and back ON within 2 seconds
once after being in the OFF position for a long time, or if
(typical).
both the DIM1 and DIM2 pins are floating, then the
When the device is in normal switch operation, it merely
output current always starts up at the initial value of
powers on in the first state when the power switch is
100%.
toggled to ON, and the device turns off when the
Note: Because the main power switch is used to initiate
external power switch is changed to OFF.
the switch dimming function, the device must have a
Switch dimming output current levels are configured by
large enough external capacitor on VIN to maintain
connecting the DIM1 and DIM2 pins as indicated in the
device operation for 2 seconds.
table below:
Applications Examples for specific values.
Please refer to the
DIM1
DIM2
Dimming levels
Linear Dimming
Floating
Floating
No Dimming
An external voltage, 0.5V to 2.5V, connected to the ADJ
Floating
GND
2 levels:100%-30%-100%
pin can adjust the LED current. Two possible situations
GND
Floating
2 levels:100%-50%-100%
might be used are:
GND
GND
3 levels:100%-50%-20%-100%
If it is not possible to change the value of RCS to obtain
the desired value of LED current, an external voltage
When operating in switch dimming mode, normally the
reference can be connected to the ADJ pin to adjust the
device will always power up at 100% output current.
voltage sense level across RCS, equivalent to changing
The operation of the power switch and the configuration
the value of RCS.
of the DIM1 and DIM2 pins control the dimming process
Connecting a resistor between the VIN and ADJ pin,
as follows:
then connecting a thermistor from the ADJ pin to GND
1.
When DIM1 and DIM2 pins are both floating, there
can adjust the LED current based on temperature, thus
is no switch dimming, and the output current is 100%
realizing the temperature compensation feature.
of the programmed value when the power is on.
2.
When DIM1 is floating and DIM2 is GND, the
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IS31LT3918
PWM Dimming
PWM signal does not shut down the other circuit blocks
PWM dimming may be realized by applying a low
of the device, thus the response to the PWM signal is
frequency PWM waveform to the ADJ pin. When the
relatively fast, and primarily determined by the rise and
PWM signal is low, less than 0.5V, the IS31LT3918
fall time of the inductor current.
remains off;When the PWM signal is high, greater than
To disable PWM dimming, just leave the ADJ pin not
2.5V, the driver is enabled and operates normally.
connected.
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Rev. A, 09/01/2011
The
6
IS31LT3918
Application Example
DC Input Voltage: VINAC =220V
=R4 with 1% precision.
4. Inductor(L1)
Output:Vo=40V(12, 1W LEDs in series, Vf=3.3V)
The inductance of inductor L1 is dependent on the LED
ILED=0.35A
current, in this case 350mA.
1. Vin power supply circuit
Toff=15.6uS, thus:
Refer to the Typical Application Circuit. The circuit
consists of R1, C2, DZ, R5, Q1, Q2, D6, R2, C4, C6.
R1, C2 and DZ, provide a steady, approximately 12V to
the gate of Q2. MOS Q2 starts in conduction state,
L
We have already chosen
VO  TOFF VO  TOFF 40  15.6  10 6


 1mH
I Ripple
K  I LED
1.8  0.35
Where, Iripple is the design target for ripple current.
and begins to charge C4 via R2 and D6. When Vin
reaches about 5V, the device starts to operate.
Component Parameters: R1=0.5MΩ, C1=22uF,
5. Freewheeling diode (D5) and NMOS (Q1, Q2)
Choose Q2 to have a voltage rating at least as large as
C2=10uF, DZ=12V, D6=SS16, R2=3KΩ, C4=10uF,
the maximum input voltage with approximately 50%
C6=10uF, R5=150Ω。
margin.
V FET =1.5×V INDC
2. Off time(TOFF)
Off time is given by:
Toff ( s )  40  10 12  REXT
The current through the NMOS is based on the peak
LED current, choose FET current rating with 50%
margin.
To decide the off time, assume the desired switching
I FET =I PEAK *150%
frequency is 50kHz, and the duty cycle is 18.2% (the
duty cycle is decided by the ratio of the output voltage
Thus, select 600V, 2A, NMOS, such as: 2N60
and input voltage), then Toff is 16.36uS, REXT =409K  ,
choose the closest resistor, REXT =390k  =R3,
Toff=15.6uS.
Q1 peak voltage is dependent on the DC input voltage
to the device. The recommended NMOS is AP2306
(30V, 5A).
3. Current Sense Resistor(RCS)
The current sense resistor is given by:
RCS
VCSTH

 R4
(1  K / 2) I LED
K is the ripple current coefficient. Assuming a typical
The diode ratings are equal to that of the NMOS, Q2.
Note: The diode must be a superfast recovery diode
and the Reverse Recovery Time(TRR) should be less
than 50nS. Thus, select 600V, 1A, superfast recovery
diode, such as: ES1J
value for K of 1.8, RCS=0.376  , choose RCS=0.38 
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IS31LT3918
Appendix: Typical application circuit of DC voltage input using a single External NMOS
Vin DC 6-150V
LED+
D1
Cin
Cout
Rin
LEDL1
VIN
Gate
DIM1
CS
Q1
IS3918
IS31LT3918
DIM2
ADJ
GND
TOFF
Cin
Rcs
Roff
Note: In the above configuration, it is important to pay attention to the VGSON value for Q1.
4.5V (typ), thus requiring that a low threshold voltage NMOS be used.
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Rev. A, 09/01/2011
IS31LT3918 provides a maximum gate drive of
8
IS31LT3918
CLASSIFICATION REFLOW PROFILES
Profile Feature
Preheat & Soak
Temperature min (Tsmin)
Temperature max (Tsmax)
Time (Tsmin to Tsmax) (ts)
Average ramp-up rate (Tsmax to Tp)
Liquidous temperature (TL)
Time at liquidous (tL)
Peak package body temperature (Tp)*
Time (tp)** within 5°C of the specified
classification temperature (Tc)
Average ramp-down rate (Tp to Tsmax)
Time 25°C to peak temperature
Pb-Free Assembly
150°C
200°C
60-120 seconds
3°C/second max.
217°C
60-150 seconds
Max 260°C
Max 30 seconds
6°C/second max.
8 minutes max.
Classification Profile
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IS31LT3918
TAPE AND REEL INFORMATION
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Rev. A, 09/01/2011
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IS31LT3918
PACKAGE INFORMATION
SOP-8
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Rev. A, 09/01/2011
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