IS31LT3360
40V/1.2A LED DRIVER WITH INTERNAL SWITCH
GENERAL DESCRIPTION
The IS31LT3360 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 supply between 6V and 40V and provides
an externally adjustable output current of up to1.2A.
The IS31LT3360 includes an integrated output switch
and a high-side output current sensing circuit, which uses
an external resistor to set the nominal average output
current.
Output current can be adjusted linearly by applying an
external control signal to the ADJ pin. The ADJ pin will
accept either a DC voltage or a PWM waveform. This will
provide either a continuous or a gated output current.
Applying a voltage less than 0.2V to the ADJ pin turns the
output off and switches the chip into a low current
standby state.
The chip is assembled in SOT89-5 package.
OCTOBER 2011
FEATURES
Up to 1.2A output current
High efficiency (up to 98% )
Wide input voltage range: 6V to 40V
Internal 40V power switch
Simple low parts count
Typical 3% output current accuracy
Single pin on/off and brightness control using DC
voltage or PWM
Up to 1MHz switching frequency
Inherent LED open-circuit/short-circuit protection
Thermal shutdown protection circuitry
Typical 1200：1 dimming ratio
APPLICATIONS
LED MR16, MR11 spotlight
LED street light
PAR light
Other LED lighting
APPLICATION CIRCUIT
IS31LT3360
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, 10/19/2011
1
IS31LT3360
PIN CONFIGURATIONS
Package
Pin Configurations
SOT89-5
PIN DESCRIPTION (NOTE 1)
Pin Name
NO.
Description
LX
1
Drain of power switch
GND
2
Ground (0V)
ADJ
3
Multi-function On/Off and brightness control pin:
* Leave floating for normal operation.(VADJ = VREF = 1.2V giving nominal average output
current IOUT nom =0.1/RS )
* Drive to voltage below 0.2V to turn off output current
* Drive with DC voltage (0.3V<VADJ <1.2V) to adjust output current from 25% to 100% of
IOUTnom
* Drive with PWM signal to adjust output current.
* When driving the ADJ pin above 1.2V, the current will be clamped to 100% brightness
automatically.
ISENSE
4
Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom
=0.1/RS
VIN
5
Input voltage (6V to 40V). Decouple to ground with 0.1μF X7R ceramic capacitor as close
to device as possible
Thermal
pad
Connect to GND
Note 1: Refer to Electrical Characteristics table for minimum and maximum specification.
ORDERING INFORMATION
Industrial Range: -40°C to +105°C
Order Number
Package
QTY/Reel
IS31LT3360-SDLS3
SOT89-5, Lead-free
2500
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Rev. A, 10/19/2011
2
IS31LT3360
ABSOLUTE MAXIMUM RATINGS (NOTE 2)
Symbol
Parameter
Rating
VIN
Input voltage
-0.3V to +50V
VISENSE
ISENSE voltage
VLX
LX output voltage
-0.3V to +50V
VADJ
Adjust pin input voltage
-0.3V to +6V
ILX
Switch output current
1.5A
Ptot
Power dissipation
0.5W
TOP
Operating temperature
-40 to 105°C
TST
Storage temperature
-55 to 150°C
Tj MAX
Junction temperature
150°C
RθJA
Junction to ambient
100°C/W
ESD Rate(human body mode)
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 3)
Symbol
VIN
Parameter
Conditions
Input voltage
Quiescent supply current with output off
ADJ pin grounded
IINQon
Quiescent supply current with output
switching
ADJ pin floating
VSENSE
Mean current sense threshold voltage
Sense threshold hysteresis
ISENSE pin input current
VSENSE =VIN -0.1
VREF
Internal reference voltage
Measured on ADJ pin
with pin floating
VADJoff
VADJon
RADJ
ILXmean
90
97
ISENSE
VADJ
External control voltage range on ADJ pin
for dc brightness control
DC voltage on ADJ pin to switch chip
from active (on) state to quiescent (off)
state
DC voltage on ADJ pin to switch chip
from quiescent (off) state to active (on)
state
Typ.
Max.
Unit
40
V
120
160
μA
450
600
μA
6
IINQoff
VSENSEHYS
Min.
100
103
mV
±15
%
8
μA
1.2
V
0.3
1.2
V
VADJ falling
0.15
0.2
0.25
V
VADJ rising
0.2
0.25
0.3
V
Resistance between ADJ pin and VREF
500
KΩ
Continuous LX switch current
1.2
A
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Rev. A, 10/19/2011
3
IS31LT3360
ELECTRICAL CHARACTERISTICS
(VIN=12V, TA=25°C unless otherwise stated) (NOTE3) (continued)
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
1
μA
0.4
Ω
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’
F=100Hz，Vin=15V，1LED，
L=27uH
200
ns
Ddim
fLXmax
DLX
TPD
TSD
TSD-HYS
Typical dimming ratio
0.27
Recommended maximum
operating frequency
Recommended duty cycle
range of output switch at
fLXmax
Internal comparator
propagation delay
Thermal shutdown
temperature
Thermal shutdown hysteresis
1200:1
1
0.3
0.7
MHz
0.9
50
ns
150
°C
20
°C
NOTES:
2. Operation of the device at value exceeding the absolute maximum ratings may cause permanent damage to the device and reduce overall
reliability.
3. 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 at other temperature.
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Rev. A, 10/19/2011
4
IS31LT3360
Icc(uA)
Icc(uA)
TYPICAL OPERATING CONDITIONS
For typical application circuit and TA=25°C unless otherwise stated.
600
500
400
250
200
150
300
100
200
50
100
0
0
5
10
15
20
25
30
35
5
40
10
15
20
25
30
35
Vin(V)
Vin(V)
Quiescent Shutdown Current vs. Vin
Rs=0.10Ω
1LED
3LED
7LED
10LED
5
10
15
20
25
30
35
Effiency(%)
Effiency(%)
Operating Supply Current vs. Vin
1
0.95
0.9
0.85
0.8
0.75
0.7
0.65
0.6
1
0.95
0.9
0.85
0.8
0.75
0.7
0.65
0.6
Rs=0.15Ω
1LED
3LED
7LED
10LED
5
40
10
15
20
25
30
35
40
Vin(V)
Vin(V)
Efficiency vs. No. of LEDs
L=47uH, Rs=0.10Ohm
Efficiency vs. No. of LEDs
L=47uH, Rs=0.15Ohm
0.7
1
Rs=0.30Ω
1LED
3LED
7LED
10LED
0.95
0.9
0.85
0.8
0.75
0.69
1LED
3LED
7LED
10LED
0.66
0.65
0.64
0.7
0.63
0.62
0.65
0.61
0.6
Rs=0.15Ω
0.68
0.67
Iout(A)
Effiency(%)
40
0.6
5
10
15
20
25
30
35
40
Vin(V)
Efficiency vs. No. of LEDs
L=47uH, Rs=0.30Ohm
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Rev. A, 10/19/2011
5
10
15
20
25
30
35
40
Vin(V)
Output current variation with output Voltage
L=47uH, Rs=0.15Ohm
5
IS31LT3360
Vsense(mV)
TYPICAL OPERATING CONDITION
40VDC
99.4
99.2
99.0
98.8
98.6
98.4
98.2
98.0
97.8
-40 -30 -20 -10 0
10
20
30
40
50
60
70
80
90
100 110
温度(℃)
Temperature VS Vsense voltage
L=47uH, Rs=0.10Ohm
Dimming Rate，current rise time=7.85us
F=100Hz
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 10/19/2011
square waveform in ADJ pin
6
IS31LT3360
LED open circuit protection
Vin=24VDC，L=47uH, Rs=0.30Ohm
LED short circuit protection
Vin=24VDC，L=47uH, Rs=0.30Ohm
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 10/19/2011
7
IS31LT3360
APPLICATION INFORMATION
Setting nominal average output current with external
resistor RS
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 1:
RS (Ω)
Nominal average
output current (mA)
0.082
1200
0.15
667
0.3
333
Output current adjustment by PWM control
Directly driving ADJ input
A Pulse Width Modulated (PWM) signal with duty cycle
DPWM can be applied to the ADJ pin, as shown below, to
adjust the output current to a value below the nominal
average value set by resistor RS, the signal range is from
0V~5V. The logic “high” is higher than 1.2V, the logic
“low” is lower than 0.2V. The PWM signal must have the
driving ability to drive the internal 500KΩ pull-up resistor.
Driving the ADJ input from a microcontroller
Another possibility is to drive the chip from the open drain
output of a microcontroller. The diagram below shows
one method of doing this:
The above values assume that the ADJ pin is floating and
at a nominal voltage of VREF =1.2V.
Note that RS=0.082Ωis the minimum allowed value of
sense resistor under these conditions to maintain switch
current below the specified maximum value. It is possible
to use different values of RS if the ADJ pin is driven from
an external voltage.
Rs need to be chosen 1% accuracy resistor with enough
power tolerance and good temperature characteristic to
ensure stable output current.
The diode and resistor suppress possible high amplitude
negative spikes on the ADJ input resulting from the
drain-source capacitance of the FET. Negative spikes at
the input to the chip should be avoided as they may
cause errors in output current or erratic device operation.
Output current adjustment by external DC control
voltage
The ADJ pin can be driven by an external dc voltage
(VADJ), as shown, to adjust the output current to a value
above or below the nominal average value defined by RS.
Shutdown mode
Taking the ADJ pin to a voltage below 0.2V will turn off
the output and supply current will fall to a low standby
level of 120μA nominal.
Inherent open-circuit LED protection
The nominal average output current in this case is given
by:
IOUTdc = 0.083*VADJ /RS [for 0.3V< VADJ <1.2V]
Note that 100% brightness setting corresponds to VADJ =
VREF. When driving the ADJ pin above 1.2V, the current
will be clamped to 100% brightness automatically.
The input impedance of the ADJ pin is 500kΩ ±25%.
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 10/19/2011
If the connection to the LED(s) is open-circuited, the coil
is isolated from the LX pin of the chip, 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:
8
IS31LT3360
I F * Ton
U MAX
IF is the value of output current， U MAX is the ripple of
LX Switch 'Off' time
power supply. Ton is the “ON” time of MOSFET. The
value is higher than the minimum value. Actually it is
more than 47uF.
If the source is an AC supply, typical output voltages
ripple from a nominal 12V AC transformer can be ±10%.
If the input capacitor value is lower than 200μF, the AC
input
waveform is distorted, sometimes the lowest value will be
lower than the forward voltage of LED strings. This will
lower the average current of the LEDs. So it is
recommended to set 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.
Note: TOFFmin>200ns
C min
Inductor selection
Recommended inductor values for the IS31LT3360 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
LI
I AVG ( R S rL R LX )
Note: TONmin>200ns
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Rev. A, 10/19/2011
TOFF
V LED
LI
V D I AVG (rL R S )
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)
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
at high temperature. 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:
9
IS31LT3360
Layout considerations
VIN pin
The GND of power supply usually have some distance to
the chip GND pin, which cause parasitic resistance and
inductance. It causes ground voltage bounce when the
internal MOSFET is switching. Connect a 0.1uF capacitor
C2 as close to device as possible to minimize the
ground bounce.
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.
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.
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Rev. A, 10/19/2011
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 capacitor C1
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.
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 affect the ADJ pin
voltage and cause unexpected output current. The
IS31LT3360 has external protection circuitry to prevent
excessive output current if ADJ voltage raise above
1.2V. A ground ring placed around the ADJ pin will
minimize changes in output current under these
conditions.
10
IS31LT3360
PACKAGE INFORMATION
SOT89-5
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Rev. A, 10/19/2011
11