IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
1
1.0MHZ BOOST CONVERTER WITH 35V INTERNAL NMOS
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.
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
APPLICATIONS
TV monitor backlighting
PDA, handheld computer
GPS receiver
TYPICAL APPLICATION CIRCUIT
OVP
FB
LX
IS31LT3505
L
1
10μH
R
SET
6
2
10
7
VP
R
1
R
2
10 μF
C
OUT
22μF
C
3
1μF
VDD
GND
C
1
1μF
4
9
1, 3 , 5
EN
C
2
1μF
300Ω100kΩ
D
1
(Schottky,ss26)
10nF
C
IN
22μF
6.0V~30 V
Figure 1 Application Circuit (Constant Current to Drive White LEDs)
Copyright©2011IntegratedSiliconSolution,Inc.Allrightsreserved.ISSIreservestherighttomakechangestothisspecificationanditsproductsatany
timewithoutnotice.ISSIassumesnoliabilityarisingoutoftheapplicationoruseofanyinformation,productsorservicesdescribedherein.Customersare
advisedtoobtainthelatestversionofthisdevicespecificationbeforerelyingonanypublishedinformationandbeforeplacingordersforproducts.
IntegratedSiliconSolution,Inc.doesnotrecommendtheuseofanyofitsproductsinlifesupportapplicationswherethefailureormalfunctionofthe
productcanreasonablybeexpectedtocausefailureofthelifesupportsystemortosignificantlyaffectitssafetyoreffectiveness.Productsarenot
authorizedforuseinsuchapplicationsunlessIntegratedSiliconSolution,Inc.receiveswrittenassurancetoitssatisfaction,that:
a.)theriskofinjuryordamagehasbeenminimized;
b.)theuserassumeallsuchrisks;and
c.)potentialliabilityofIntegratedSiliconSolution,Incisadequatelyprotectedunderthecircumstances
OCTOBER 2011
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
2
PIN CONFIGURATION
Package Pin Configuration (Top View)
MSOP-10
PIN DESCRIPTION
Pin Name I/O Description
1 PGND - Power ground.
3,5 GND - Ground.
2 VP I/O
Internal 5V regulator. A power supply for the internal NMOS
gate driver and the internal control circuitry.
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.
ORDERING INFORMATION
INDUSTRIAL RANGE: -40°C TO +85°C
Order Part No. Package QTY/Reel
IS31LT3505-SLS2-TR MSOP-10, Lead-free 2500
1
2
3
47
8
9
10
AGND
VDD
EN FB
NC
LX
VP
PGND
5AGND 6OVP
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 09/01/201 1 3
ABSOLUTE MAXIMUM RATINGS
Parameter Value
Supply voltage, VDD -0.3V ~ +6.0V
Voltage at LX pin -0.3V ~ +40V
All other pins -0.3V ~ +6.0V
Operate temperature range -40°C ~ +85°C
Storage temperature range -65°C ~ +150°C
Junction temperature range -40°C ~ +150°C
Lead temperature (Soldering, 10s) 260°C
RJA 60°C/W
ESD HBM 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 Parameter Condition Min. Typ. Max. Unit
VDD Supply voltage 6 30 V
UVLO Undervoltage threshold VP falling 2.9 V
∆UVLO Undervoltage threshold
hysteresis 100 mV
IDD Supply current Continuous switching 2 mA
No switching 1.1
ISD Shutdown current VEN = 0V 15 μA
VP Internal regulator 6V<VDD<30V, CVP=10µF 4.5 5 5.5 V
VEN ON EN on threshold VEN rising 1.4 V
VEN OFF EN off threshold VEN falling 0.4 V
Fosc Operation frequency 1 MHz
DMAX Maximum duty cycle 90 %
RDS_ON Internal NMOS on-resistance 0.8 1.2 Ω
ISW_LK Internal NMOS leakage current VSW = 35V 1 μA
ISW_LIMIT Internal NMOS current limit Duty = 90% 1.8 2.1 2.4 A
VOVP_TH Over voltage threshold 0.9 V
VFB Feedback voltage 0.285 0.3 0.315 V
TOTP Over temperature threshold 150 °C
TOTP-HYS Over temperature threshold
hysteresis 50 °C
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 09/01/201 1 4
TYPICAL OPERATING CHARACTERISTICS
Figure 2 Iout vs. Vin
Figure 4 Iout vs. Vout
Figure 6 Iout vs. Vin
Figure 3 Efficiency vs. Vin
Figure 5 Efficiency vs. Vout
Figure 7 Efficiency vs. Vin
330
335
340
345
350
355
360
10 15 20 25 30
Iout(mA)
Vin(V)
Vout=30V,RSET=0.88,L=10uH
330
335
340
345
350
355
360
12 15 18 21 24 27 30
Iout(mA)
Vout(V)
Vin=12VDC,RSET=0.88,L=10u
670
680
690
700
710
720
730
18 20 22 24 26 28
Iout(mA)
Vin(V)
Vout=30V,RSET=0.42,L=10u
50
60
70
80
90
100
10 15 20 25 30
Efficiency(%)
Vin(V)
Vout=30V,RSET=0.88,L=10
50
60
70
80
90
100
12 15 18 21 24 27 30
Efficiency(%)
Vout(V)
Vin=12VDC,RSET=0.88,L=10u
50
60
70
80
90
100
18 20 22 24 26 28
Efficiency(%)
Vin(V)
Vout=30V,RSET=0.42,L=10u
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
5
670
680
690
700
710
720
730
25 26 27 28 29 30
Iout(mA)
Vout(V)
Vin=24VDC,RSET=0.42,L=10uH
50
60
70
80
90
100
25 26 27 28 29 30
Efficiency(%)
Vout(V)
Vin=24VDC,RSET=0.42,L=10uH
250
260
270
280
290
300
310
320
6 101418222630
ReferenceVoltage(mV)
Vin(V)
Figure 8 Iout vs. Vout
Figure 9 Efficiency vs. Vout
Figure 10 V
FB
voltage vs. Vin
Figure 11 Soft-start waveform
Vin=12VDC,R
SET
=0.88,L=10uH,Vout=28V
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
6
Figure 12 Operation waveform
Figure 13 OVP waveform
Vin=12VDC,R
SET
=0.88,L=10uH,Vout=28V
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev. A, 09/01/201 1 7
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 soft-
start 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.
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.).
SET
FBDC
FB
LED RRVVR
V
I4
3)(
(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.).
SET
FBPWM
FB
LED RRR VDutyVR
V
I54
3)(
(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
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
8
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).
V
OVP
= 0.9V ×(R
1
+R
2
)/R
2
(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 R
SET
ground to signal ground (pin 5)
connection should be as short as possible to have
an accurate LED current.
The capacitor C
VDD
and C
VP
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.
OVP
FB
LX
IS31LT3505
L1
10μH
RSET
6
2
10
7
VP
R1
R2
R3
10kΩ
R4
156kΩ
DC Control 0V - 5V
10μF
COUT
22μF
C3
1μF
VDD
GND
C1
1μF
6.0V~30V
4
9
1, 3, 5
EN
C2
1μF
300Ω100kΩ
CIN
22µF
D1
(Schottky,ss26)
10nF
Figure 14 Application Circuit (Constant Current to Drive White LEDs With DC Dimming)
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
9
OVP
FB
LX
IS31LT3505
L
1
10μH
R
SET
6
2
10
7
VP
R
1
R
2
R
3
10kΩ
R
4
56kΩ
R
5
100kΩ1μF
PWM Signal
0V
5V
Micro
Controller
10μF
C
3
1μF
C
OUT
22μF
VDD
GND
C
1
1μF
6.0V~30V
4
9
1, 3, 5
EN
C
2
1μF
30 0 Ω100kΩ
C
IN
22µF
D
1
(Schottky ,ss26)
10nF
Figure 15 Application Circuit (Constant Current to Drive White LEDs With PWM Dimming)
OVP
FB
LX
VDD
GND
IS31LT3505
10 μF
L
1
10µH
C
1
1μF
R
SET
6.0V~30V
4
9
1, 3, 5
6
2
10
7
EN
VP
C
2
1μF
C
IN
22µF C
3
1μF
R
1
R
2
R
3
D
1
(Schottky,ss26) V
OUT
= V
FB
×(R
3
+R
SET
)/R
SET
Load
300Ω100kΩ
C
OUT
22μF
10 nF
Figure 16 Application Circuit (Constant Voltage to Drive Other Devices)
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
10
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
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
11
TAPE AND REEL INFORMATION
MSOP-10
IS31LT3505
Integrated Silicon Solution, Inc. – www.issi.com
Rev.
A
, 09/01
/
2011
12
PACKAGE INFORMATION
MSOP-10
