ISSI IS31LT3354

IS31LT3354
40V LED DRIVER WITH EXTERNAL SWITCH
SEPTEMBER 2011
GENERAL DESCRIPTION
The IS31LT3354 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 required LED voltage.
The device operates from an input supply between 6V
and 40V and provides an externally adjustable output
current of up to 2A or even higher, which is determined
by the external MOSFET and inductor.
The IS31LT3354 includes 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 of 0.2V or lower to the ADJ pin
turns the output off and switches the chip into a low
current standby state.
The chip is assembled in SOT23-5 package.
FEATURES
 Simple low parts count
 Wide input voltage range: 6V to 40V
 Output Current only limited by external
component selection
 High efficiency (up to 98% )
 Typical 1200:1 dimming ratio
 Typical 3% output current accuracy
 Single pin on/off and brightness control using
DC voltage or PWM
 Up to 1MHz switching frequency
 Inherent open-circuit LED protection
 Thermal shutdown protection circuitry
APPLICATIONS
 Low voltage halogen replacement LEDs
 Automotive lighting
 Low voltage industrial lighting
 LED back-up lighting
 Illuminated sign
TYPICAL APPLICATION CIRCUIT
Rs
Vin(DC)=6V~40V
C3
D1
5
C1
C2
0.1uF
4
VIN
3
ADJ
I SENSE
IS31LT3354
SN3354
GATE
GND
up to
10 LEDs
L1
1
AP2306
2
Figure 1 Typical IS31LT3354 Application Schematic
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
1
IS31LT3354
PIN CONFIGURATION
Package
Pin Configuration (Top view)
SOT23-5
PIN DESCRIPTION
Pin Name
NO. Description
GATE
1
Output gate driver for an external NMOSFET
GND
2
Ground (0V)
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
ADJ
3
* 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.
Connect resistor RS from this pin to VIN to define nominal average output current IOUTnom
ISENSE
4
=0.1/RS
Input voltage (6V to 40V). Decouple to ground with 1μF or higher X7R ceramic capacitor close
VIN
5
to device
ORDERING INFORMATION
INDUSTRIAL RANGE: -40°C TO +85°C
Order Part No.
Package
QTY/Reel
IS31LT3354–STLS2-TR
SOT23-5, Lead-free
3000
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Rev. A, 09/01/2011
2
IS31LT3354
ABSOLUTE MAXIMUM RATINGS (NOTE 1)
Symbol Parameter
Rating
VIN
Input voltage
-0.3V to +50V
VISENSE
ISENSE voltage
VGATE
GATE pin voltage
-0.3V to +6V
VADJ
Adjust pin input voltage
-0.3V to +6V
Ptot
Power dissipation
600mW @SOT23-5
TOP
Operating temperature
-40 to 85°C
TST
Storage temperature
-55 to 150°C
Tj MAX
Junction temperature
150°C
RJA
Junction to ambient
108°C/W @SOT23-5
ESD Susceptibility (human body model)
4kV
VIN+0.3V to VIN-5V ,VIN>5V
VIN+0.3V to -0.3V,VIN<5V
ELECTRICAL CHARACTERISTICS
TA = 25°C (Note 2)
Symbol
Parameter
VIN
Input voltage
IINQoff
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
VSENSEHYS
Sense threshold hysteresis
ISENSE
ISENSE pin input current
VSENSE = 0.1V
VREF
Internal reference voltage
Measured on ADJ pin
with pin floating
VADJ
External control voltage range on ADJ pin for
dc brightness control
VADJoff
DC voltage on ADJ pin to switch chip from
active (on) state to quiescent (off) state
VADJ falling
0.15
VADJon
DC voltage on ADJ pin to switch chip from
quiescent (off) state to active (on) state
VADJ rising
0.2
RADJ
Resistance between ADJ pin and VREF
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Rev. A, 09/01/2011
Conditions
Min.
Typ.
Max.
Unit
40
V
95
110
μA
450
600
μA
96
101
mV
6
80
91
±15
8
%
10
μA
1.2
0.3
V
1.2
V
0.2
0.25
V
0.25
0.3
V
500
KΩ
3
IS31LT3354
ELECTRICAL CHARACTERISTICS
TA = 25°C, VIN = 12V (Note 3)
Symbol Parameter
Isource
Gate sourcing current
Isink
Gate sinking current
Conditions
Min.
Typ.
80
100
Max.
Unit
mA
mA
DPWM(LF)
Brightness control range at low
frequency PWM signal
PWM frequency =100Hz PWM
amplitude=5V,Vin=15V,
L=27uH, Driving 1 LED
1200:1
DPWM(HF)
Brightness control range at low
frequency PWM signal
PWM frequency =10KHz PWM
amplitude=5V,Vin=15V,
L=27uH, Driving 1 LED
13:1
fSW
Operating frequency
ADJ pin floating
L=100μH (0.82Ω) IOUT=350mA
@ VLED=3.4V Driving 1 LED
154
KHz
TONmin
Minimum switch ‘ON’ time
200
ns
TOFFmin
Minimum switch ‘OFF’ time
Recommended maximum
operating frequency
Recommended duty cycle range
of output switch at fSWmax
Internal comparator propagation
delay
200
ns
fSWmax
DSW
TPD
1
0.3
0.7
MHz
0.9
50
ns
TSD
Thermal shutdown temperature
140
°C
TSD-HYS
Thermal shutdown hysteresis
20
°C
Notes:
1. 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.
2. All parts are production tested at TA = 25°C. Other temperature limits are guaranteed by design
3. Guaranteed by design
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Rev. A, 09/01/2011
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IS31LT3354
TYPICAL PERFORMANCE CHARACTERISTICS
For typical application circuit and Tamb=25°C unless otherwise stated.
2200
2150
1LED
Iout(mA)
2100
2LEDs
2050
3LEDs
2000
4LEDs
5LEDs
1950
6LEDs
1900
7LEDs
L=47uH, Rs=0.05
MOS=AP2306
1850
8LEDs
1800
0
5
10
15
20
25
30
Vin(V)
Efficiency vs. No. of LEDs
Output current variation with Supply Voltage
100%
2200
2150
2100
80%
Vin
18V
24V
70%
30V
60%
Iout(mA)
Efficiency
90%
2050
Vin
2000
24V
30V
1950
18V
1900
L=47uH, Rs=0.05
MOS=AP2306
L=47uH, Rs=0.05
MOS=AP2306
1850
50%
1800
0
5
10
15
Vout(V)
20
25
30
Efficiency vs. Output Voltage
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Rev. A, 09/01/2011
0
5
10
15
20
25
30
Vout(V)
Output current variation with Output Voltage
5
500
120
400
100
80
300
Iin(uA)
Iin (uA)
IS31LT3354
200
60
40
100
20
0
0
0
5
10
15
20
25
30
35
40
Vin (V)
0
5
10
15
20
25
30
35
40
Vi n( V)
Shutdown Current vs. Vin (Quiescent)
Supply Current vs. Vin (Operating)
1.201
Vref(V)
1.2005
1.2
1.1995
1.199
1.1985
5
10
15
20
25
30
35
40
Vin(V)
Vref vs. Vin
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IS31LT3354
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:
Nominal average
RS (Ω)
output current (mA)
0.05
2000
0.083
1200
0.15
667
0.3
333
Vsense is divided into two ranges to improve current
accuracy, please refer to bin information on page 3.
The above values assume that the ADJ pin is floating
and at a nominal voltage of VREF =1.2V.
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.
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 PWM signal must have the
driving ability to drive internal 500KΩ pull-up resistor.
5v
0v
SN3354
ADJ IS31LT3354
GND
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:
IS31LT3354
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.
SN3354
ADJ IS31LT3354
DC
GND
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%.
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Rev. A, 09/01/2011
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.
See the section on PWM dimming for more details of
the various modes of control using high frequency and
low frequency PWM signals.
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 95μA nominal.
7
IS31LT3354
Inherent open-circuit LED protection
Inductor selection
If the connection to the LED(s) is open-circuited, the
coil is isolated from the switch, so neither the chip nor
the switch will be damaged, unlike in many boost
converters, where the back EMF may damage the
switch by forcing the drain above its breakdown
voltage.
Recommended inductor values for the IS31LT3354
are in the range 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. (See
graphs). The inductor should be mounted as close to
the chip as possible with low resistance connections to
the GATE 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 an inductor with saturation
current bigger than 1.2A for 700mA output current and
inductor with saturation current bigger than 500mA for
350mA output current.
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.
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 DC supply, the capacitor is decided by
ripple of the source, the value is given by:
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 MOSFET the
value is normally 2 times of the minimum value.
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 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.
Switch MOSFET selection
The IS31LT3354 demands a power N-MOSFET as a
switch. The voltage and current rating of the MOSFET
must be higher than the application output voltage and
the inductor peak current. The VGS(th) of MOSFET
should be lower than 3V and the RDSon should be as
low as possible for maximum efficiency and
performance. AP2306 and AP2310 are recommended.
NOTE: For the recommended MOSFETs, the
maximum load current is about 2A. For high current
applications, the operating input voltage, the LED
current, and the switching frequency will determine the
operating temperature of the MOSFET. Switching
frequency can be lowered by choosing a larger value
of inductance, however, the MOSFET specifications
must be carefully analyzed first. The key
specifications to consider are RDSON and CDS, both
should be as low as possible.
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Rev. A, 09/01/2011
Switch 'On' time
TON 
V IN  V LED
LI
 I AVG ( R S  rL  R LX )
Note: TONmin>200ns
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)
RLX is the switch resistance (Ω)
VD is the diode forward voltage at the required load
current (V)
8
IS31LT3354
Example:
Operation at low supply voltage
For VIN=12V, L=47μH, rL=0.64Ω, VLED=3.4V, Iavg
=333mA and VD =0.36V
TON = (47e-6 × 0.105)/(12 - 3.4 - 0.612) = 0.62μs
TOFF = (47e-6 × 0.105)/(3.4 + 0.36 + 0.322)= 1.21μs
This gives an operating frequency of 546kHz and a
duty cycle of 0.34
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.
The internal regulator disables the driving to the switch
until the supply has risen above the startup threshold
set internally which makes the power MOSFET’s
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.
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 switch. If a silicon diode is used,
care should be taken to ensure that the total voltage
appearing across the switch including supply ripple,
does not exceed the specified maximum value.
Reducing output ripple
Thermal considerations
The IS31LT3354 utilizes an external MOSFET to
switch the inductor current, and thus dissipates very
little power. The thermal characteristics of the
MOSFET dominate in typical application circuits for the
IS31LT3354. Care should be taken to ensure a large
copper ground plane and a good thermal conductivity
between the MOSFET and the ground plane.
Note that the switch power dissipation increases with
increasing supply voltage. This is caused primarily by
two things, the resulting increase in switching
frequency and the higher voltage across the switch
during the off time. This may result from the use of
unsuitable coils, or excessive parasitic output
capacitance on the switch output.
Peak to peak ripple current in the LED can be reduced,
if required, by shunting a capacitor Cled across the
LED(s) as shown below:
Rs
Cled
D1
5
C1
C2
0.1uF
4
VIN
3
ADJ
I SENSE
IS31LT3354
SN3354
GATE
GND
L1
1
AP2306
2
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.
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IS31LT3354
Layout considerations
External MOSFET Drain
The Drain of the external MOSFET 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.
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
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.
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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 raise the ADJ
pin voltage and cause excessive output current. A
ground ring placed around the ADJ pin will minimize
changes in output current under these conditions.
10
IS31LT3354
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 19
Classification Profile
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IS31LT3354
PACKAGING INFORMATION
SOT23-5
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