ISSI IS31LT3910 Universal high brightness led driver Datasheet

IS31LT3910
UNIVERSAL HIGH BRIGHTNESS LED DRIVER
WITH TEMPERATURE COMPENSATION
GENERAL DESCRIPTION
The IS31LT3910 is a peak current mode control LED
driver IC. The IS31LT3910 operates in constant off-time
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.
November 2011
FEATURES

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
APPLICATIONS

DC/DC or AC/DC LED driver applications

General purpose constant current source

Signal and decorative LED lighting

backlighting LED driver
APPLICATION CIRCUIT
VinDC 8~450V
C1
D1
NTC
L1
Rin
LEDS
IS31LT3910
VIN
VREF
R1
GATE
Q1
LD
Cin
PWMD
TOFF
C2
CS
GND
RCS
COFF
Integrated Silicon Solution, Inc. — www.issi.com
Rev.A, 11/08/2011
1
IS31LT3910
PIN CONFIGURATIONS
Package
Pin Configurations (Top View)
VREF
PWMD
VIN
TOFF
SOP-8
LD
GND
CS
GATE
PIN DESCRIPTION
Pin
Pin No.
VREF
1
PWMD
2
LD
3
GND
4
GATE
5
CS
6
TOFF
7
VIN
8
Description
This pin provides reference a voltage of 1.2V, no bypass
capacitor is needed.
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.
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.
Ground return for all internal circuitry. This pin must be electrically
connected to ground.
This pin is the output gate driver for an external N-channel power
MOSFET.
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.
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.
This pin is the input of an 8V to 450V voltage supply through a
resistor, clamped at 7.1V internally, it must be bypassed with a
capacitor to GND.
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, 11/08/2011
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
Integrated Silicon Solution, Inc. — www.issi.com
Rev.A, 11/08/2011
3
IS31LT3910
ABSOLUTE MAXIMUM RATINGS
Parameter
Value
VIN pin voltage to GND
CS, LD, PWMD, GATE, TOFF, VREF pin voltage to GND
VIN pin Input Current Range (Note1)
Junction Temperature Range
Storage Temperature Range
ESD Human Model
RθJA
-0.3V to 8V
-0.3V to 6V
1mA to 10mA
-40°C to 150°C
-65°C to 150°C
2000V
80°C/W
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 (Note 1), RIN =2K, unless otherwise noted)
Symbol
Parameter
Conditions
Min. Typ.
VINDC
Input DC supply voltage range
VIN_clamp
IIN
UVLO
△UVLO
VEN(lo)
VEN(hi)
REN
VIN clamp voltage
Operation current range
Undervoltage lockout threshold
Undervoltage lockout hysteresis
Pin PWMD input low voltage
Pin PWMD input high voltage
Pin PWMD pull-up resistance
Current sense pull-in threshold
voltage
Linear Dimming pin voltage low
threshold
Linear Dimming pin voltage high
threshold
Current sense blanking interval
VCS,TH
VLD
TBLANK
tDELAY
Delay to output
TOFF
tRISE
tFALL
VREF
VREFLOAD
Off Time
GATE output rise time
GATE output fall time
REF pin voltage
Load regulation of reference voltage
Connect a decent resistor
from DC supply voltage to
VIN pin (Note 2)
(Note 3)
VIN=6V GATE floating
VIN rising
VIN falling
8
6.6
0.32
6.0
7.1
0.5
6.5
500
Max.
Unit
450
V
7.6
0.64
6.9
2
75
100
125
V
mA
V
mV
V
V
KΩ
215
240
265
mV
1.2
400
VCS=VCS,TH+50mV after
TBLANK
TOFF pin Floating
CGATE=500pF
CGATE=500pF
0.05
mV
0.24
mV
480
550
30
420
1.12
IREF=0~500uA,PWMD=5.0V
510
19
29
1.20
0.5
ns
ns
600
1.30
5
ns
ns
ns
V
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.
Integrated Silicon Solution, Inc. — www.issi.com
Rev.A, 11/08/2011
4
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 offtime of the oscillator inside. The oscillator produces
pulses at regular intervals. These pulses set the SR flipflop 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:
0.24V(orVLD)
RCS 
1.15  ILED(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).
IIN  0.5mA  QG  fS
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.A, 11/08/2011
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 turn-offs 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:
TOFF _ TIME ( s )  0.51  10 6  (1 
C OFF
)
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
5
IS31LT3910
(See more detail in Temperature Compensation section).
To use the internal 240mV, the LD pin must be
connected to PWMD pin.
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
 0.24
R1  RNTC
Assuming a 30% ripple in the inductor, the temperature
compensated continuous current may be computed as:
Iout 
RNTC  Vref
( R1  RNTC )  Rs * 1.15
IC INPUT RESISTOR (RIN) AND HOLD CAPACITOR
(CIN)
RIN 
VINDC  VIN 230  7.0

 223K
I IN
1
parallel two 430 k  /0.5W resistor for lifetime
consideration. C IN is chosen to be 10uF/ 40V capacitor
TOFF TIME REGULATION CAPACITOR (COFF) AND
TOFF TIME
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 25~30KHz). In IS31LT3910,
TOFF _ TIME  0.51 10  6  (1 
C OFF
)
10 pF
So, apply Coff =150pF, then Toff=8.16us
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 1uF should
be used.
Since the output average IO_AVG = 160mA, Assume 50%
current ripple, then
Make sure the value of R1 is more than 1K.
1
1
I Ripple   50%  160mA  40mA
2
2
When the LD pin voltage reduces to less than 50mv, the
chip is shutdown .
I O _ PEAK  I O_AVG 
After the ambient temperature returns to a safe
temperature,the current will return to the set value.
RCS 
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)
1
I Ripple  200mA
2
0.24V
 1.2
200mA
THE INDUCTOR (L1) CHOSEN
The inductor value depends on the ripple current in the
LEDs. Toff=8.16us
L
VO  TOFF 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.
Integrated Silicon Solution, Inc. — www.issi.com
Rev.A, 11/08/2011
6
IS31LT3910
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.
MOSFET (Q1) AND DIODE (D1)
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:
The peak voltage seen by the MOSFET is equal to the
maximum input voltage. Using a 50% safety rating,
I diode  I PEAK  150%
VFET  1.5  VINDC
For this example, 600V/1A fast recovery diode is
recommended.
The maximum RMS current through the MOSFET
depends on the maximum current. Hence, the current
rating of the MOSFET is:
Integrated Silicon Solution, Inc. — www.issi.com
Rev.A, 11/08/2011
7
IS31LT3910
PACKAGE INFORMATION
SOP-8
Integrated Silicon Solution, Inc. — www.issi.com
Rev.A, 11/08/2011
8
Similar pages