ISSI IS31LT3380

IS31LT3380
40V/1.2A LED DRIVER WITH SWITCH DIMMING
OCTOBER 2011
GENERAL DESCRIPTION
The IS31LT3380 is a continuous mode inductive
step-down converter, designed for driving a single LED or
multiple series connected LEDs efficiently from a voltage
source higher than the LED voltage. The chip operates
from an input voltage between 8.5V and 40V and
provides an output current up to 1.2A.
The IS31LT3380 includes a high-side output current
sensing circuit, which uses an external resistor to set the
nominal average output current. The IS31LT3380
includes an integrated output switch which has a very low
conducting impedance to ensure high system efficiency.
The IS31LT3380 has switch dimming function. The chip
detects external switch action to adjust output current,
allowing for dimming functionality to be achieved without
changing the original lighting system circuitry.
The switch dimming is implemented in either two-level
mode or three-level mode. The output current of every
level and the total number of levels are customer
selected by setting the corresponding input conditions of
DIM1 and DIM2 pin.
The output current is set at the initial value the first time
that power is supplied to the chip. After the initial power
up sequence, the chip adjusts the output current
according to the external switch action. After the lowest
current level, the current cycles back to the initial value if
more switch action is detected. If the power is switched
off for longer than 2 seconds, the device will return to it’s
initial state, and the output current will be set to the initial
value at the next time that power is applied.
FEATURES
 Multi-modes switch dimming
 Up to 1.2A output current
 High efficiency (up to 98%)
 Wide input voltage range: 8.5V to 40V
 Internal 40V power switch
 Simple low parts count
 Typical 5% output current accuracy
 Up to 1MHz switching frequency
 Inherent LED open-circuit/short-circuit protection
 Thermal shutdown protection circuitry
 SOP8 package
APPLICATIONS
 MR16, MR11 LED spotlight
 LED street lighting
 PAR LED bulb
 Other LED lighting
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
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 09/01/2011
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IS31LT3380
APPLICATION CIRCUIT
Figure 1-A
Figure 1-B
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Rev. A, 09/01/2011
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IS31LT3380
PIN CONFIGURATIONS
Package
Pin Configurations
SOP-8
PIN DESCRIPTION
Pin Name
NO.
Description
VIN
1
Input voltage (8.5V to 40V). Decouple to ground with 0.1μF X7R ceramic capacitor close to device. While
using the switch dimming function, connect a 220Uf or higher capacitor close the device.
ISENSE
2
Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom =0.1/RS
DIM1
6
Set the current-levels number of and current ratio of each level of switch dimming function as below:
DIM1=“floating” DIM2=“floating”，not dimming；
DIM1=“floating” DIM2=“GND”，three-levels，100%---50%---20%；
DIM2
5
DIM1=“GND” DIM2=“floating”，three-levels，100%---60%---30%；
DIM1=“GND” DIM2=“GND”，two-levels，100%---30%。
GND
7
Ground (0V)
LX
8
Drain of power switch
NC
3,4
Not connect. Float required
ORDERING INFORMATION
Industrial Range: -40°C to 105°C
Order Number
Package
QTY/Reel
IS31LT3380-GRLS3-TR
SOP-8, Lead-free
2500
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Rev. A, 09/01/2011
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IS31LT3380
ABSOLUTE MAXIMUM RATINGS (NOTE1)
Symbol
Parameter
Rating
VIN
Input voltage
-0.3V to +50V
VISENSE
ISENSE voltage
VLX
LX output voltage
-0.3V to +50V
VDIM
DIM1 ,DIM2 pin input voltage
-0.3V to +6V
ILX
Switch output current
1.5A
Ptot
Power dissipation
0.5W
TOP
Operating temperature
-40 °C to 105°C
TST
Storage temperature
-55 to 150°C
Tj MAX
Junction temperature
150°C
RθJA
Junction to ambient
100°C/W
ESD Rating (HBM)
3kV
Vin+0.3V to Vin-5V ,Vin>5V
Vin+0.3V to -0.3V,Vin<5v
ELECTRICAL CHARACTERISTICS
(VIN=12V, TA=25°C unless otherwise stated) (NOTE 2)
Symbol
VIN
Parameter
Conditions
Min.
Typ.
Max.
Unit
40
V
120
140
μA
450
600
μA
95
100
105
mV
5.05
5.85
6.65
V
Input voltage
8.5
IINQoff
Quiescent supply current with
output off
70
IINQon
Quiescent supply current with
output switching
VISENSE
Mean current sense threshold
voltage
UVLO
Undervoltage lockout threshold
△UVLO
Undervoltage lockout hysteresis
1.65
V
Sense threshold hysteresis
±15
%
8
μA
2
S
VSENSEHYS
ISENSE
Ts
Kmode1
ISENSE pin input current
VIN falling
VSENSE =VIN -0.1
the interval time between the
external switch action
In dimming mode 1,the ratio of
different output current level to the
initial current (DIM1=“floating”
DIM2=“GND”)
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Rev. A, 09/01/2011
Level 1
100%
Level 2
50%
Level 3
17%
20%
23%
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IS31LT3380
ELECTRICAL CHARACTERISTICS
(VIN=12V, TA=25°C unless otherwise stated) (NOTE 2) (continued)
Symbol
Parameter
Conditions
Level 1
100%
Kmode2
In dimming mode 2,the ratio of
different output current level to
the initial current (DIM1=“GND”
DIM2=“floating”)
Level 2
60%
Kmode3
Level 3
In dimming mode 3,the ratio of
different output current level to
the initial current (DIM1=“GND”
DIM2=“GND”)
Min.
28%
Level 1
Level 2
Typ.
30%
Max.
Unit
32%
100%
28%
30%
32%
ILXmean
Continuous LX switch current
1.2
ILX(leak)
LX switch leakage current
RLX
LX Switch ‘On’ resistance
TONmin
Minimum switch ‘ON’ time
LX switch ‘ON’
200
ns
TOFFmin
Minimum switch ‘OFF’ time
LX switch ‘OFF’
200
ns
fLXmax
Recommended maximum
operating frequency
TPD
Internal comparator propagation
delay
25
ns
TSD
Thermal shutdown temperature
150
°C
TSD-HYS
Thermal shutdown hysteresis
20
°C
0.3
A
1
μA
0.4
Ω
1
MHz
NOTES:
1. Work beyond maximum absolute voltage and current will cause permanent damage to the device. Long time working under absolute conditions
will lower the reliability of the dice.
2. Production testing of the chip is performed at 25°C. Functional operation of the chip and parameters specified are guaranteed by design,
characterization and process control in other temperature.
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Rev. A, 09/01/2011
5
IS31LT3380
TYPICAL OPERATING CONDITIONS
For typical application circuit and TA=25°C unless otherwise stated.
Supply Current vs. Vin (Operating)
Efficiency vs. No. of LEDs
L=47uH, Rs=0.10Ohm
Efficiency vs. No. of LEDs
L=47uH, Rs=0.30Ohm
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Rev. A, 09/01/2011
Efficiency vs. No. of LEDs
L=47uH, Rs=0.15Ohm
Output current variation with output Voltage
L=47uH, Rs=0.15Ohm
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IS31LT3380
TYPICAL OPERATING CONDITIONS
Temperature VS Vsense voltage
L=47uH, Rs=0.10Ohm
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Rev. A, 09/01/2011
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IS31LT3380
SWITCH DIMMING WAVEFORM
LED open waveform
Vin=24VDC，L=47uH, Rs=0.30Ohm
LED short waveform
Vin=24VDC，L=47uH, Rs=0.30Ohm
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Rev. A, 09/01/2011
8
IS31LT3380
APPLICATION INFORMATION
Setting nominal average output current with
external resistor R S
The nominal average output current in the LED(s) is
determined by the value of the external current sense
resistor (RS) connected between VIN and ISENSE and is
given by:
IOUT nom = 0.1/RS
The table below gives values of nominal average output
current for several preferred values of current setting
resistor (RS) in the typical application circuit shown on
page 2:
RS (Ω)
Nominal average output
current (mA)
0.082
1200
0.15
667
0.3
333
The resistance tolerance of Rs should be 1%, and the
power rating must be high enough to continuously handle
the average output current. Good over-temperature
performance of the resistor is also recommended.
Multi-modes Switch dimming
The IS31LT3380 detects external switch action to adjust
output current.
The input condition of the DIM1 and DIM2 pins set the
number of current-levels and current ratio for the switch
dimming function as shown below:
DIM1
DIM2
Dim-ratio
floating
floating
Not dimming
floating
GND
100%---50%---20%---100%
GND
floating
100%---60%---30%---100%
GND
GND
100%---30%---100%
For instance, suppose the DIM1 pin is floating and the
DIM2 pin is connected to GND. The first time the switch
is turned on, the initial current is IOUTnom.
After the 1st dimming-switch, IOUT(1) = 50% * IOUTnom；
After the 2nd dimming-switch, IOUT(2) = 20% * IOUTnom；
After the 3rd dimming-switch, IOUT(3) = 100%* IOUTnom；
This process recycles.
If a normal-switch is detected or DIM1 and DIM2 are both
floating, the output current goes back to the initial state of
100%.
Since the chip needs to count the time for more than 2
seconds after the switch is OFF, during the OFF time one
capacitor bigger than 220uF is required to keep the chip
work in low quiescent current mode.
Inherent open-circuit LED protection
If the connection to the LED(s) is open-circuited, the
circuit is open, so the chip will not be damaged, unlike in
many boost converters, where the back EMF may
damage the internal switch by forcing the drain above its
breakdown voltage.
Capacitor selection
A low ESR capacitor should be used for input decoupling,
as the ESR of this capacitor appears in series with the
supply source impedance and lowers overall efficiency.
This capacitor has to supply the relatively high peak
current to the coil and smooth the current ripple on the
input supply.
If the source is a DC supply, the capacitor is decided by
ripple of the source, the value is given by:
There are two types of switch action: a normal switch,
which means the off-time between each subsequent
switching is longer than 2s; and a dimming-switching,
which means the off-time between each subsequent
switch actions is less than 2s.
The nominal average output current is given by:
IOUT = K*0.1/R S [K is the current ratio to initial
current]
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Rev. A, 09/01/2011
C min 
I F * Ton
U MAX
IF is the value of output current， U MAX is the ripple of
power supply. Ton is the “ON” time of the MOSFET,
normally set at about twice the “OFF” time.
If the source is an AC supply, the output voltage ripple
from a nominal 12V AC transformer can be ±10%. If the
input capacitor value is lower than 200μF, the AC input
waveform will be distorted, and sometimes the lowest
input voltage can be lower than the forward voltage of the
LED strings. This lowers the average current of the
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IS31LT3380
LEDs. Therefore, it is recommended to select 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 IS31LT3380 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. The inductor
should be mounted as close to the chip as possible with
low resistance connections to the LX and VIN pins.
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,etc.
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
TON 
V IN  V LED
LI
 I AVG ( R S  rL  R LX )
Note: TONmin>200ns
RLX is the switch resistance (Ω)
VD is the diode forward voltage at the required load
current (V)
Example:
For VIN=12V, L=47μH, rL=0.26Ω, VLED=3.4V, Iavg =333mA
and VD =0.36V， RLX=0.27Ω
TON = (47e-6 × 0.105)/(12 - 3.4 - 0.274) = 0.59μs
TOFF = (47e-6 × 0.105)/(3.4 + 0.36 + 0.188)= 1.25μs
This gives an operating frequency of 543kHz and a duty
cycle of 0.32
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 C3 across the LED(s)
as shown below:
LX Switch 'Off' time
TOFF 
V LED
LI
 V D  I AVG (rL  R S )
Note: TOFFmin>200ns
Where:
L is the coil inductance (H)
rL is the coil resistance (Ω)
Iavg is the required LED current (A)
∆I is the coil peak-peak ripple current (A) {Internally set to
0.3 × Iavg}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
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A C3 value of 1μF will reduce nominal ripple current by a
factor of approximately three. 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.
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IS31LT3380
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 be a
maximum at the 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
VIN/GND pin
The GND of the power supply usually has some distance
between it and the chip GND pin, causing parasitic
resistance and inductance, resulting in ground voltage
bounce when the MOSFET switches. To minimize
ground bounce, the ground pin of the chip should be
soldered directly to the ground plane. Connecting a
0.1uF capacitor between the VIN and GND pins as close
to the chip as possible minimizes the effects of ground
bounce.
LX pin
The LX pin of the chip is a fast switching node, so PCB
traces should be kept as short as possible.
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 RS.
DIM pin
The DIM pin is a high impedance input, when it is left
floating; this pin is pull up to 3.3V by internal circuit. Avoid
running any high voltage traces close to the DIM pins.
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Rev. A, 09/01/2011
11
IS31LT3380
PACKAGE INFORMATION
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Rev. A, 09/01/2011