ISSI IS31LT3505

IS31LT3505
1.0MHZ BOOST CONVERTER WITH 35V INTERNAL NMOS
OCTOBER 2011
FEATURES
 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
DESCRIPTION
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 1 MHz. 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.
IS31LT3505 is available in MSOP-10. It operates
from 6V to 30V over the temperature range of -40°C
to +85°C.
APPLICATIONS
 TV monitor backlighting
 PDA, handheld computer
 GPS receiver
TYPICAL APPLICATION CIRCUIT
D1
(Schottky,ss26)
L1
10μ H
6.0V~30V
300Ω 100kΩ
4
C IN
22μ F
9
C1
1μF
LX
EN
VDD
OVP
IS31LT3505
C2
1μF
1, 3, 5
GND
VP
FB
10
R1
6
2
10nF
10μF
C3
1μF
C OUT
22μ F
R2
7
R SE T
Figure 1 Application Circuit (Constant Current to Drive White LEDs)
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
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
Pin
Name
I/O
Description
1
PGND
-
Power ground.
3,5
GND
-
Ground.
2
VP
I/O
4
EN
I
Enable control input. Do not let this pin floating.
6
OVP
I
Over-voltage protection of output.
7
FB
I
Feedback voltage of output.
8
NC
-
No connection, must floating.
9
VDD
-
Supply voltage.
10
LX
O
The drain of the internal NMOS.
Thermal
Pad
-
Connect to Ground.
Internal 5V regulator. A power supply for the internal NMOS
gate driver and the internal control circuitry.
ORDERING INFORMATION
INDUSTRIAL RANGE: -40°C TO +85°C
Order Part No.
Package
QTY/Reel
IS31LT3505-SLS2-TR
MSOP-10, Lead-free
2500
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Rev. A, 09/01/2011
2
IS31LT3505
ABSOLUTE MAXIMUM RATINGS
Parameter
Value
Supply voltage, VDD
Voltage at LX pin
All other pins
Operate temperature range
Storage temperature range
Junction temperature range
Lead temperature (Soldering, 10s)
RJA
ESD HBM
-0.3V ~ +6.0V
-0.3V ~ +40V
-0.3V ~ +6.0V
-40°C ~ +85°C
-65°C ~ +150°C
-40°C ~ +150°C
260°C
60°C/W
4kV
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 = -40°C ~ +85°C, VDD = 12V (unless otherwise noted), Typical values are at TA = 25°C.
Symbol
VDD
UVLO
∆UVLO
Parameter
Condition
Supply voltage
Undervoltage threshold
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=10µF
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
VFB
Feedback voltage
TOTP
Over temperature threshold
Over temperature threshold
hysteresis
TOTP-HYS
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 09/01/2011
1.8
2.1
1.2
Ω
1
μA
2.4
A
0.9
0.285
0.3
V
0.315
V
150
°C
50
°C
3
IS31LT3505
TYPICAL OPERATING CHARACTERISTICS
360
100
355
90
Efficiency(%)
Iout(mA)
350
345
340
80
70
60
335
Vout=30V,RSET=0.88,L=10
Vout=30V,RSET=0.88,L=10uH
50
330
10
15
20
25
10
30
15
20
Vin(V)
30
Figure 3 Efficiency vs. Vin
Figure 2 Iout vs. Vin
360
100
355
90
Efficiency(%)
350
Iout(mA)
25
Vin(V)
345
340
80
70
60
335
Vin=12VDC,RSET=0.88,L=10u
Vin=12VDC,RSET=0.88,L=10u
50
330
12
15
18
21
24
27
12
30
15
18
21
27
30
Vout(V)
Vout(V)
Figure 5 Efficiency vs. Vout
Figure 4 Iout vs. Vout
730
100
720
90
Efficiency(%)
710
Iout(mA)
24
700
80
70
690
60
680
Vout=30V,RSET=0.42,L=10u
Vout=30V,RSET=0.42,L=10u
50
670
18
20
22
24
26
28
18
20
22
24
26
28
Vin(V)
Vin(V)
Figure 6 Iout vs. Vin
Figure 7 Efficiency vs. Vin
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Rev. A, 09/01/2011
4
IS31LT3505
730
720
710
Efficiency(%)
90
Iout(mA)
100
700
690
80
70
60
680
Vin=24VDC,RSET=0.42,L=10uH
Vin=24VDC,RSET=0.42,L=10uH
670
25
26
27
28
29
50
30
25
26
27
28
Vout(V)
Figure 8
Iout vs. Vout Figure 9 Efficiency vs. Vout 29
30
Vout(V)
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. A, 09/01/2011
5
IS31LT3505
Vin=12VDC, RSET=0.88, L=10uH, Vout=28V
Figure 12 Operation waveform
Figure 13 OVP waveform
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Rev. A, 09/01/2011
6
IS31LT3505
Input and Output Capacitor
The output capacitor is decided by the output voltage
ripple. A low ESR electric capacitor (22uF or larger)
and a 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 (22uF or larger) and a 1µF/50V ceramic
capacitor in parallel as output 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.
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.
ILED = VFB/RSET
(1)
In order to have an accurate LED current, precision
resistors are preferred (1% is recommended).
Dimming Control
IS31LT3505 can modulate the brightness of LEDs by
controlling the DC voltage or the PWM duty cycle
(Figure 14,15).
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.
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Rev. A, 09/01/2011
The output LED voltage will decrease when the output
current becomes lower. Therefore, it must be ensure
that 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 increases, 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 
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.).
VOUT =VFB× (R3 +RSET)/RSET
(4)
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 be connectted to a 10nF ceramic
7
IS31LT3505
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).
VOVP = 0.9V×(R1+R2)/R2
(5)
Setting Other Components
There is a R, C between power supply positive
terminal to VDD pin. A 300 resistor for R and 1µF
ceramic capacitor for C are recommended. (Note:
When the input voltage is lower than 8V, the
recommended value of R is 50)
The VP pin, output of the internal regulator, must be
connected to a 10µF bypass capacitor.
If the 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.



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
layout is not carefully done, the regulator could show
instability as well as EMI problems.
 Wide traces should be used for connection of the
high current loop.
 When laying out the 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.
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 crossing 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.
D1
(Schottky,ss26)
L1
10μH
6.0V~30V
300Ω 100kΩ
4
C IN
22µF
9
C1
1μ F
LX
EN
VDD
OVP
IS31LT3505
C2
1μ F
1, 3, 5
GND
VP
FB
10
R1
6
2
10nF
10μF
C3
1μF
C OUT
22μF
R2
7
R4
156kΩ
R3
10k Ω
R SE T
DC Control 0V - 5V
Figure 14 Application Circuit (Constant Current to Drive White LEDs With DC Dimming)
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Rev. A, 09/01/2011
8
IS31LT3505
D1
(Schottky ,ss26)
L1
10μH
6.0V~30V
300Ω 100k Ω
4
CIN
22µF
9
C1
1μF
EN
OVP
VDD
IS31LT3505
C2
1μF
1, 3, 5
5V
GND
VP
FB
10
R1
6
2
C3
1μF
10nF
COUT
22μF
R2
10μF
7
PWM Signal
0V
Micro
Controller
LX
R4
56kΩ
R3
10kΩ
R5
100kΩ
1μF
RSE T
Figure 15 Application Circuit (Constant Current to Drive White LEDs With PWM Dimming)
D1
(Schottky,ss26)
L1
10µH
6.0V~30V
V OUT = V FB ×(R 3 +R SE T)/R SE T
300Ω 100kΩ
4
CIN
22µF
9
C1
1μ F
LX
EN
VDD
OVP
IS31LT3505
VP
10
R1
6
2
C2
1μF
C3
1μF
10nF
C OUT R 3
22μ F
Load
R2
10μF
1, 3, 5
GND
FB
7
R SE T
Figure 16 Application Circuit (Constant Voltage to Drive Other Devices)
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Rev. A, 09/01/2011
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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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Rev. A, 09/01/2011
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IS31LT3505
TAPE AND REEL INFORMATION
MSOP-10
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Rev. A, 09/01/2011
11
IS31LT3505
PACKAGE INFORMATION
MSOP-10
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Rev. A, 09/01/2011
12