IS31LT3505

IS31LT3505
1.0MHZ BOOST CONVERTER WITH 35V INTERNAL NMOS
July 2014
GENERAL DESCRIPTION
FEATURES
The IS31LT3505 is a constant current step-up
converter with internal NMOS. The step-up converter
topology allows series connection of the white LEDs
so the LED currents are identical for uniform
brightness as well as constant output voltage to drive
other devices. The output current of each channel
can be set by an external resistor and dimming the
brightness of LEDs with the PWM signal or DC
voltage. The IS31LT3505 operates with a switching
frequency up to 1MHz. A low 0.3V feedback voltage
minimizes power loss in the current setting resistor
for better efficiency. With OVP circuit, the chip and
the system can be safe even if the load is not
connected.
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IS31LT3505 is available in MSOP-10. It operates
from 6V to 30V over the temperature range of -40°C
to +85°C.
6V to 30V supply voltage
High efficiency: 90% typical
PWM dimming control
Fast 1.0MHz switching frequency
Internal high power 35V NMOS
Internal soft start
Adjustable LED Open Protection
Over-temperature protection
MSOP-10 package
APPLICATIONS

TV monitor backlighting

PDA, handheld computer

GPS receiver
TYPICAL APPLICATION CIRCUIT
Figure 1 Typical Application Circuit (Constant Current to Drive White LEDs)
Note: These components with * symbol are recommended to fix the value. Or it will decrease the performance of whole system. Integrated Silicon Solution, Inc. – www.issi.com
Rev. D, 07/15/2014
1
IS31LT3505
PIN CONFIGURATION
Package
Pin Configuration (Top View)
MSOP-10
PGND
1
10
LX
VP
2
9
VDD
AGND
3
8
NC
EN
4
7
FB
AGND
5
6
OVP
PIN DESCRIPTION
No.
Pin
1
PGND
2
VP
Internal 5V regulator. A power supply for the internal NMOS
gate driver and the internal control circuitry.
3,5
AGND
Signal ground. All external components ground must be
connected to this pin.
4
EN
6
OVP
7
FB
Feedback voltage of output.
8
NC
No connection.
9
VDD
Supply voltage.
10
LX
Thermal Pad
Description
Power ground.
Enable control input. Do not let this pin floating.
Over-voltage protection of output.
The drain of the internal NMOS.
Connect to ground.
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IS31LT3505
ORDERING INFORMATION
INDUSTRIAL RANGE: -40°C TO +85°C
Order Part No.
Package
QTY/Reel
IS31LT3505-SLS2-TR
MSOP-10, Lead-free
2500
Copyright © 2014 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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IS31LT3505
ABSOLUTE MAXIMUM RATINGS
Supply voltage, VDD
Voltage at EN, LX pin
All other pins
Operate temperature range
Storage temperature range
Junction temperature range
JA
ESD (HBM)
-0.3V ~ +40V
-0.3V ~ +40V
-0.3V ~ +6.0V
-40°C ~ +85°C
-65°C ~ +150°C
-40°C ~ +150°C
60°C/W
2.5kV
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 condition beyond those indicated in the operational sections of the specifications is
not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
TA = 25°C, VDD = 12V, unless otherwise noted.
Symbol
VDD
Parameter
Condition
Supply voltage
UVLO
Undervoltage threshold
∆UVLO
Undervoltage threshold
hysteresis
IDD
Supply current
ISD
Min.
Typ.
6
VP falling
Continuous switching
Max.
Unit
30
V
2.9
V
100
mV
2
No switching
1.1
Shutdown current
VEN = 0V
15
VP
Internal regulator
6V<VDD<30V, CVP=100nF
4.5
VEN ON
EN on threshold
VEN rising
1.4
VEN OFF
EN off threshold
VEN falling
5
mA
μA
5.5
V
V
0.4
V
Fosc
Operation frequency
1
MHz
DMAX
Maximum duty cycle
90
%
RDS_ON
Internal NMOS on-resistance
0.8
ISW_LK
Internal NMOS leakage current
VSW = 35V
ISW_LIMIT
Internal NMOS current limit
Duty = 90%
VOVP_TH
Over voltage threshold
1.8
2.1
1.2
Ω
1
μA
2.4
A
0.9
VFB
Feedback voltage
TOTP
Over temperature threshold
150
°C
TOTP-HYS
Over temperature threshold
hysteresis
50
°C
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Rev. D, 07/15/2014
0.285
0.3
V
0.315
V
4
IS31LT3505
TYPICAL OPERATING CHARACTERISTICS
100
360
355
350
Iout(mA)
345
340
90
Efficiency(%)
335
330
60
Vout=30V,RSET=0.88,L=10uH
50
15
20
Figure 2
25
30
10
Iout vs. Vin
Figure 3
345
335
Vin=12VDC,RSET=0.88,L=10uH
21
70
60
Vin=12VDC,RSET=0.88,L=10uH
50
18
80
330
15
Vin(V)
24
27
30
12
Vout(V)
730
700
690
27
30
80
70
60
680
24
100
Efficiency(%)
21
90
710
Iout(mA)
18
720
15
Vout(V)
Figure 5 Efficiency vs. Vout
Figure 4 Iout vs. Vout
Efficiency vs. Vin 90
350
12
30
100
340
25
Efficiency(%)
Iout(mA)
20
Vin(V)
360
15
355
70
Vout=30V,RSET=0.88,L=10uH
10
80
Vout=30V,RSET=0.42,L=10uH
Vout=30V,RSET=0.42,L=10uH
670
50
18
20
22
24
26
Vin(V)
Figure 6
Iout vs. Vin
28
18
20
22
24
26
28
Vin(V)
Figure 7 Efficiency vs. Vin Integrated Silicon Solution, Inc. – www.issi.com
Rev. D, 07/15/2014
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IS31LT3505
100
730
720
Iout(mA)
Efficiency(%)
710
90
700
690
70
60
680
Vin=24VDC,RSET=0.42,L=10uH
Vin=24VDC,RSET=0.42,L=10uH
50
670
80
25
26
27
28
25
30
26
27
28
29
30
Vout(V)
Vout(V)
Figure 8
Iout vs. Vout
29
Figure 9 Efficiency vs. Vout
320
Reference Voltage(mV)
310
300
290
280
270
260
250
6
10
14
18
22
26
30
Vin(V)
Figure 10 VFB voltage vs. Vin
Vin=12VDC, RSET=0.88, L=10uH, Vout=28V
Figure 11 Soft-start waveform
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Rev. D, 07/15/2014
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IS31LT3505
Vin=12VDC, RSET=0.88, L=10uH, Vout=28V
Figure 12 Operation waveform
Vin=12VDC, RSET=0.88, L=10uH, Vout=28V, Vovp=33V
Figure 13 OVP waveform
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Rev. D, 07/15/2014
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IS31LT3505
FUNCTIONAL BLOCK DIAGRAM
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IS31LT3505
APPLICATION INFORMATION
INPUT AND OUTPUT CAPACITOR
The output capacitor is decided by the output voltage
ripple. A low ESR ceramic capacitor (recommended
22µF) and a 0.1µF/50V ceramic capacitor in parallel
will provide sufficient output capacitance for most
applications. The input capacitor is used to reduce the
input voltage ripple and noise. A low ESR electric
capacitor (22µF or larger) and a 1µF/50V ceramic
capacitor in parallel as input capacitor is
recommended. Place the input and output capacitors
close to the IS31LT3505 to reduce the ripple.
INDUCTOR
Inductor value involves trade-offs in performance.
Larger inductors reduce inductor ripple current and
larger inductors also bring in unwanted parasitic
resistor that degrade the performance. Select an
inductor with a rating current over input average
current and the saturation current over the Internal
NMOS current limit. A 10µH inductor with saturation
current over 2A is sufficient for the most applications.
Note: The DC voltage (PWM duty cycle) is inversely
proportional to the LED current. That is when DC
voltage is maximum (the PWM signal is 100% duty
cycle), the output current is minimum, ideally zero,
and when DC voltage is minimum (the PWM signal is
0% duty cycle), the output current is maximum.
The output LED voltage will decrease when the output
current becomes lower. Therefore, it must to ensure
the output voltage always higher than the input voltage
during the dimming.
DC VOLTAGE CONTROL
Figure 14 shows that the intensity of the LEDs can be
adjusted by the DC voltage. As the DC voltage
increases, the current pass through R3 increasingly
and the voltage drop on R3 increase, i.e. the LED
current decreases. The LED current can be calculated
by the Formula (2). The internal feedback voltage VFB
is 0.3V (Typ.).
VFB 
I LED 
DIODE
To achieve high efficiency, a Schottky diode must be
used. Ensure that the diode's average and peak
current rating exceed the output LED current and
inductor peak current. The diode's reverse breakdown
voltage must exceed the over voltage protection
voltage (VOVP). Therefore, A SS26 Schottky diode is
sufficient for the most applications.
SOFT-START
The function of soft-start is made for suppressing the
inrush current to an acceptable value at startup. The
IS31LT3505 provides a built-in soft-start function by
clamping the input current and increasing step-by-step
so that the output voltage will rise gradually in the softstart period.
LED CURRENT CONTROL
The IS31LT3505 regulates the LED current by setting
the external resistor connecting to feedback and
ground. The internal feedback reference voltage is
0.3V (Typ.). The LED current can be set from the
Formula (1) easily.
I LED
V
 FB
RSET
(1)
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended).
DIMMING CONTROL
R3  (VDC  VFB )
R4
RSET
(2)
When the DC voltage is from 0V to 5V, the value of R3
should be 10kΩ. Refer to Figure 14.
PWM SIGNAL CONTROL
A filtered PWM signal acts as the DC voltage to
regulate the output current. The recommended
application circuit is shown as Figure 15. In this circuit,
the output ripple depends on the frequency of PWM
signal. For smaller output voltage ripple, the
recommended frequency of 5V PWM signal should be
above 2kHz. To the fixed frequency of PWM signal
and change the duty cycle of PWM signal can get
different output current. The LED current can be
calculated by the Formula (3). The internal feedback
voltage VFB is 0.3V (Typ.).
VFB 
I LED 
R3  (VPWM  Duty  VFB )
R4  R5
RSET
(3)
When it’s the 5V PWM signal, the value of R3 should
be 10kΩ. Refer to Figure 15.
SETTING THE OUTPUT VOLTAGE
When IS31LT3505 drives other devices (Figure 16)
with the constant voltage, the output voltage is set
through the Formula (4). The internal feedback voltage
VFB is 0.3V (Typ.).
IS31LT3505 can modulate the brightness of LEDs by
controlling the DC voltage or the PWM duty cycle
(Figure 14, 15).
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Rev. D, 07/15/2014
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IS31LT3505
VOUT 
VFB  R3  RSET 
RSET
(4)
layout is not carefully done, the regulator could show
instability as well as EMI problems.

SETTING THE OVER VOLTAGE PROTECTION

The open string protection is achieved through the
over voltage protection (OVP). In some cases, if the
output voltage reaches the programmed OVP voltage
(VOVP), the protection will be triggered. To make sure
the chip functions properly, the OVP setting resistor
divider must be set with a proper value. The OVP
voltage should be 3V higher than normal operation
output voltage and the maximum should not exceed
35V. OVP pin should connect a 10nF ceramic
capacitor to GND to avoid unexpected noise coupling
into this pin and affecting the OVP function. The OVP
threshold is calculated through the Formula (5).

0.9V  R1  R2 
R2

VOVP 
(5)
SETTING OTHER COMPONENTS
There is an R, C between power supply positive
terminal to VDD pin. 51 resistor for R and 220nF
ceramic capacitor for C are the recommended.
The VP pin, output of the internal regulator, must be
connected to a 100nF bypass capacitor.
If EN pin is not used to enable and disable the
IS31LT3505, it should be connected to power supply
positive through a 100k resistor. The enable pin
needs to be terminated and should not be left floating.
These components should be fixed the value as
above description, or it will decrease the performance
of whole system.


Wide traces should be used for connection of the
high current loop.
When laying out signal ground (pin 5), it is
recommended to use the traces separate from
power ground (pin1) traces and connect them
together at the input capacitor negative terminal or
the large ground plane that will avoid the signal
ground shift. Both of signal and power ground
should be as wide as possible. Other components
ground must be connected to signal ground.
Especially the RSET ground to signal ground (pin 5)
connection should be as short as possible to have
an accurate LED current.
The capacitor CVDD and CVP should be placed as
close as possible to VDD and VP pin for good
filtering. The ground of CVDD and CVP must be
connected to the signal ground (pin 5).
LX pin is a fast switching node. The inductor and
diode should be placed as close as possible to the
switch pin and the connection between this pin to
the inductor and the schottky diode should be kept
as short and wide as possible. Avoid other traces
cross and routing too long in parallel with this
node to minimum the noise coupling into these
traces.
The feedback network (FB, OVP) should be as
short as possible and routed away from the
inductor, the schottky diode and LX pin. The
feedback pin and feedback network should be
shielded with a ground plane or trace to minimize
noise coupling into this circuit.
The thermal pad on the back of package must be
soldered to the large ground plane for ideal power
dissipation.
PCB LAYOUT CONSIDERATION
As for all switching power supplies, especially those
providing high current and using high switching
frequencies, layout is an important design step. If
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Rev. D, 07/15/2014
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IS31LT3505
51
100k
4
CIN
22µF
D1
(Schottky,ss26)
L1
10 H
6.0V~30V
9
C1
1 F
LX
EN
VDD
OVP
IS31LT3505
C2
220nF
1, 3, 5
GND
VP
FB
10
R1
6
2
10nF
100nF
C3
COUT
0.1 F 22 F
R2
7
R4
156k
R3
10k
RSET
DC Control 0V - 5V
Figure 14 Application Circuit (Constant Current to Drive White LEDs With DC Dimming)
Figure 15 Application Circuit (Constant Current to Drive White LEDs With PWM Dimming)
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IS31LT3505
D1
(Schottky,ss26)
L1
10µH
6.0V~30V
VOUT = VFB×(R3+RSET)/RSET
51Ω 100kΩ
4
CIN
22µF
9
C1
1 F
LX
EN
VDD
OVP
IS31LT3505
VP
10
R1
6
2
C2
220nF
C3
COUT R3
0.1 F 22 F
10nF
Load
R2
100nF
1, 3, 5
GND
FB
7
RSET
Figure 16 Application Circuit (Constant Voltage to Drive Other Devices)
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IS31LT3505
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 17 Classification Profile
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IS31LT3505
PACKAGE INFORMATION
MSOP-10
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