LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
1/9
Released 1.0
GENERAL DESCRIPTION
The LSP7503 is a step-up DC/DC converter
specifically designed to drive LEDs with a
constant current. The device can drive two or
more LEDs in series from a Li-Ion battery. Series
connection of the LEDs provides identical LED
currents resulting in uniform brightness. The 0.2V
feedback voltage minimizes power loss in the
current setting resistor for better efficiency.
The LSP7503 can be operated at 640KHz or
1.3MHz allowing for small filter solution and low
noise. An external compensation pin gives the
user flexibility in setting loop compensation,
which allows the use a low-ESR ceramic output
capacitors. Internal Soft-start function results in
small inrush current and can be programmed with
an external capacitor.
The LSP7503 device includes under-voltage
lockout, and current limiting protection preventing
damage in the event of an output overload.
FEATURES
1.6A, 0.23Ω, Internal Switch
Input Range: +2.6V to +5.5V
Low Shutdown Current: 0.1uA
Adjustable Frequency: 640kHz or 1.3MHz
0.2V Feedback Volt age
Small 8-Pin MSOP Green Package
TYPICAL APPLICATIONS
Cellular Phones, PDA.
Handheld Computers, UMPC
Digit al Cameras, Photo Frames.
MP3 Players
GPS Receivers
PIN ASSIGNMENT
1
2
3
4
8
7
6
5
FB
VDD
FREQ
SS
SW
SHDN
COMP
MSOP-8
(Top View)
GND
PIN DESCRIPTION
Pin Name Function
1 COMP Compensation pin for error amplifier
2 FB
Feedback pin with a typical reference voltage of 0.2V for setting LED driving
current.
SET
LED RV
I2.0
=
3 SHDN Shut Down pin. When SHDN is low, the LSP7503 will turn off.
4 GND Ground pin.
5 SW Switch Pin.
6 VDD Power Supply pin.
7 FS
Frequency select pin. The oscillator frequency is set to 640kHz when FREQ is low,
and 1.3MHz when FREQ is high
8 SS Soft-Start control pin.
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
2/9
Released 1.0
TYPICAL APPLICATION CIRCUIT
ABSOLUTE MAXIMUM RATINGS
Parameters Rating Unit
SW to GND 18 V
Input Voltage: SHDN
__________
/ VDD / FREQ to GND 6 V
SS to GND -0.3 ~ VDD + 0.3 V
SW pin maximum current 2.3 A
Operating temperature -20 ~ +85 °C
Maximum Operating Junction Temperature, TJ 150
°C
Storage Temperature Range -45 to 125 °C
Lead Temperature (Soldering, 10 seconds) 260 °C
Note: Exceeding these ratings could cause damage to the device.
All voltages are with respect to Ground. Currents are positive into, negative out of the specified terminal.
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
3/9
Released 1.0
THERMAL IMPEDIENCE
Thermal Resistance from Junction to Ambient, θ JA 180°C /W
Junction Temperature Calculation: TJ = TA + (PD × θ JA).
The θJA numbers are guidelines for the thermal performance of the device/pc-board system.
Connect the ground pin to ground using a large pad or ground plane for better heat dissipation.
All of the above assume no ambient airflow.
Maximum Power Calculation:
TJ(MAX) – TA(MAX)
PD(MAX)= θJA
TJ (°C): Maximum recommended junction temperature
TA (°C): Ambient temperature of the application
θJA (°OC /W): Junction-to-Ambient thermal resistance of the package, and other heat dissipating
materials.
ELECTRICAL CHARACTERISTICS
VDD=SHDN
__________ =3V, FREQ=GND; TA=25°C, (Unless otherwise noted)
Parameter Symbol Conditions Min Typ Max Unit
Input Voltage Range VDD 2.6 5.5 V
VDD Under voltage Lockout UVLO
When VDD is rising, typical
hysteresis is 40mV; SW remains off
below this level
2.25 2.38 2.52 V
Quiescent Current IDD VFB =0.25V, Not switching
VFB =0.15V, switching 0.21
1.2
0.35
5.0
mA
mA
Shutdown Current ISC EN = GND 0.1 10 uA
FB Reference Voltage VFB 0.2 V
FB Input Current Ibias FB=VREF 1 40 nA
FB Voltage Line
Regulation 2.6V＜VDD＜5.5V - 0.1 0.15 %/V
Error Amp
Transconductance Gm ICOMP=±5uA 70 105 240 uA/V
Error Amp Gain Av - 1500 - V/V
FREQ=GND 540 640 740
Oscillated Frequency Fosc FREQ=VDD 1100 1320 1600 kHz
FREQ=GND 79 85 92
Maximum Duty Cycle DMAX FREQ=VDD 85
%
Current Limit ILIM V
DD=1V, D=0.65 1.2 1.6 2.3 A
ON-Resistance RON Isw=1.2A 0.23 0.5 Ω
Leakage Current ISWOFF V
SW=12V 0.01 20 uA
Reset Switch Resistance 300 Ω
Soft Start Charge current Iss Vss=1.2V 1.5 4 7 uA
Input Low Voltage VIL SHDN, FREQ; VDD=2.6V to 5.5V. 0.3VDD V
Input High Voltage VIH SHDN, FREQ; VDD=2.6V to 5.5V. 0.7VDD V
Hysteresis SHDN, FREQ; 0.1VDD V
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
4/9
Released 1.0
ELECTRICAL CHARACTERISTICS (Continued)
VDD=SHDN
__________ =3V, FREQ=GND; TA=25°C, (Unless otherwise noted)
Parameter Symbol Conditions Min Typ Max Unit
FREQ Pull-Down Current IFREQ 1.8 5.0 9.0 uA
SHDN Input Current ISHDN 0.001 1 uA
Note: Guaranteed by design, not 100% tested in production.
FUNCTIONAL BLOCK DIAGRAM
1
Driver
FB
Ramp
Summer
PWM
Comparator
Error
A
mp
63mΩ
Sync S
LOGIC
CONTROL
R
∑
Switch
Oscillator
＋
－ －
＋
0.2V Reference
2
7
3
6
4
8
5
GND
SHDN
VDD
SW
SS
COMP
FRE
Q
Current
Sense
Slope
Compensation
APPLICATION INFORMATION
1. Setting the LED Driving Current
FB pin is used to set the LED driving current, ILED, of the LSP7503 driver. A resistor RSET connected to FB pin,
the LED driving current is determined by:
SETSET
FB
LED RV
R
V
I2.0
==
Where, the feedback pin voltage, VFB, is fixed at 0.2V.
2. Selection of Output Capacitor
It is recommended to select output capacitors that the capacitance is high enough and the ESR (Effective
Series Resistance) is low enough. These (high capacitance & low ESR) are very important for the Boost Converter
to be able to meet the VOUT ripple specification.
Ceramic capacitors often have low ESR and can meet the Boost Converter requirements as long as the
capacitance values are enough. Note that the capacitance values of all kinds of ceramic capacitor drop when there
are DC voltages bias on them. The higher the DC bias, the lower the effective capacitance. The Zxx series
capacitors (ex: Z5U) often drop more capacitance than what Yxx series capacitors (ex: Y5V) will drop. And Yxx
series are usually worse than Xxx series (ex: X5R). Therefore, it is better to use X5R/X7R type of ceramic capacitors
and don’t use Yxx series (ex: Y5V) or Zxx series (ex: Z5U) types of capacitor. Although they could be cheaper than
X5R or X7R, the permanence of Yxx series and Zxx series are not as good as X5R/X7R and are easier to have
problems like audio noise problem.
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
5/9
Released 1.0
The lifetime of a ceramic capacitor is shorter if the DC-bias is close to its maximum DC rating. For example, to a
VOUT=13.4V application (ex: 4pcs LED in series), a 25VDC capacitor should have a longer lifetime than a 16VDC
capacitor does, even when other characteristics of these two capacitors are similar.
Electrolytic capacitors have higher ESR than what ceramic capacitors do. If electrolytic capacitors are used as
output capacitors, the ESR should be low enough to meet the VOUT ripple voltage requirement:
RippleI VoltagePeaktoPeakRippleV
ESR
OUT
OUT )(
<<
For example, if the VOUT ripple voltage of a 5V output DC/DC converter should be smaller than 250mVPeak-to-Peak
and if the ripple current is 0.5A, an capacitor whose ESR is << (0.25V/0.5A)≒500mΩ should be chosen. A 680uF of
ESR<500 mΩ capacitor can be used in this case.
Note that the ESR of electrolytic capacitor is highly dependent on the temperature - the lower the temperature
the higher the ESR and vice versa. This temperature dependence causes VOUT ripple problem and system stability
problem sometimes. It may need to use tantalum capacitors or other capacitors that the ESR are temperature
independent for applications that temperature ranges are wide.
It is important to ensure the ripple current rating of the output capacitor is enough or the capacitor might burn
out during operation. To most electrolytic capacitors, the body temperatures should not be higher than environment
temperature plus 10°C. If the body temperature of the capacitor is too high, the ripple current could be higher than
the rating of the capacitor. For example, if the air temperature that close to the input capacitor is 45°C, it is better
that the body temperature is << (45°C + 10°C) = 55°C.
3. Selection of Input Capacitor
It is recommended to put a ceramic capacitor(s) of several uF to 10uF as input capacitor of the Boost Converter.
The capacitor(s) is better to be X7R/X5R type.
4. Selection of Inductor
It is recommended to use ferrite core as the chock material. Don’t use iron powder core because the core loss
will be too high for applications that the operation frequency is larger than 300KHz, although the cost of an iron
powder core could be cheaper. The DC-R of the chock wire should be as low as possible to reduce the power loss.
Below is an equation about the inductor value:
)3()()(
,
,
2
,
η
⋅⋅
⋅
−
⋅=
SWMAXOUT
MININOUT
OUT
MININ fI
VV
V
V
L
Where,
EfficiencyTypical
HzFrequencySwitchingf
ACurrentOutputMaximumI
VVoltageOutputTypicalV
VVoltageInputMinimumV
HValueInductorL
SW
MAXOUT
OUT
MININ
:
)(:
)(:
)(:
)(:
)( :
,
,
η
Using a higher value inductor can reduce the power loss of the Boost converter. Anyway, a higher value
inductor often is bigger in size or has higher DC-R, and the higher DC-R may increase the inductor power loss.
Shielding inductor has better EMI performance but the DC-R is often higher than non-shielding inductors of the
same size.
It is recommended to adopt an inductor value that the DC/DC converter will not transfer from
Discontinue-Current-Mode (DCM) to Continue-Current-Mode (CCM) or vise versa when VIN or IOUT change. Such
mode changing will cause the duty cycle of the Boost DC/DC converter becomes unstable.
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
6/9
Released 1.0
5. Selection of Flywheel Diode
An Schottky diode that the voltage rating larger than 20V and the current rating larger than IOUT is
recommended. Adopt a Schottky diode of lower dropout voltage can improve the system efficiency. Please also
check the leakage current specification of the Schottky diode at the same time.
Note that the maximum working temperatures of many Schottky diodes are only 120°C. Please double check
the working temperature of the Schottky diode to ensure it is within the specification.
6. Minimum & Maximum Duty Cycle Limitation
PWM ICs often have trouble to convert a VOUT from a VIN if the duty cycle is too small (close to 0%) or too big
(close to 100%). Small duty cycle happens when VOUT/VIN is low and big duty cycle happens when VOUT/VIN is High.
DC/DC converter designers need to carefully examine whether the DC/DC converter under design has such duty
cycle limit problem, especially when the nominal VOUT/VIN is already <1.2 or >5.
Note that factors like VIN deviation, component value deviation, temperature change, switching frequency
deviation…etc, can push the duty cycle to be much higher/lower than what we expect from the nominal VOUT/VIN
values. For example, the duty of a 3.3V input, 12V output DC/DC converter seems to be around 72.5%, but the
actual duty cycle could be up to 80% if we include the voltage drops of output Schottky, VIN trace drop, VIN ripple
voltage drop, inductor line drop …etc.
7. Open Circuit Protection.
The Zener diode DOVP and Resistor R5 are used to protect the converter while any LED failure results in the
open circuit of output.
LAYOUT GUIDELINES
PCB layout is an important stage for power circuit, especially the switching type DC/DC converter that providing
high current/voltage and using high switching frequency. If PCB layout is not carefully done, the Boost converter
may be unstable or cause serious EMI problems.
Use wide, short, and straightforward traces for high current paths. About the input capacitors, two or more
ceramic capacitors of several uF or bigger are recommended to be used. Place one of them very close to the VIN pin
of IC and ground, and at least one another very close to the inductor.
It is very important to keep the loop of the SW pin, Schottky diode, output capacitor, and the GND pin of
LSP7503 as small as possible, and also minimize the length of the traces between these components, as shown in
the following Fig. 1. This is because the di/dt at these traces is very high and according to the formula of
dt
di
Lv ⋅=
the related voltage spikes will be very high if the trace inductance is high. Such voltage spikes not just cause EMC
problems, but may interfere or even damage the IC sometimes.
The most important is, it is better NOT to use via holes in the loop described above, because via holes have
high inductance.
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
7/9
Released 1.0
A710
VDD
SS
FB
GND
DF
L
SW
FREQ
COMP
RP
CP2
COUT
CIN
CP1 RSET
VIN VOUT
CSS
SHDN
ON
OFF
The loop should be
as small as possible.
Fig. 1
Second, keep all the analog components and signal traces, for example the FB sense trace, far away from the
noisy areas, that is, the areas near inductor, LSP7503 switch pin, and Schottky diode. If the FB sense trace is close
to the noisy area, large noise may be coupled into FB pin and cause ILED value not accurate or unstable. Please refer
to Fig. 2.
A710
VDD
SS
FB
GND
DF
L
SW
FREQ
COMP
RP
CP2
COUT
CIN
CP1 RSET
VIN VOUT
CSS
SHDN
ON
OFF
A
nalog components &
traces should be far
away from noisy area.
Noisy area.
Fig. 2
A big ground plane (form input to out put) can help almost all the performance of the chip. Beside the ground
trace on the top layer, please use another layer as the ground layer.
LSP7503
L
S
P7
503
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
8/9
Released 1.0
ORDERING INFORMATION
MARKING INFORMATION
MSOP – 8 Pin
Package:
MS : MSOP8L Output Voltage:
Blank: Adj Packing:
A: Tape & Real Temperature Grade:
E: -40～125℃
LSP7503X X X X
1 2 3 4
8 7 6 5
L S C
7 5 0 3
Y W X
Logo
Part Number
Internal Code
Date Code
Y: Year(9=2009)
W: Week
Code#123456789ABCD
Week12345678910111213
Code# E F G H J K L M N P O R S
Week 14 15 16 17 18 19 20 21 22 23 24 25 26
Code# T U V W X Y Z a b c d e f
Week 27 28 29 30 31 32 33 34 35 36 37 38 39
Code#gh Imnpqs t uvwx
Week 40 41 42 43 44 45 46 47 48 49 50 51 52
LSP7503
Step Up Converter for White LED Back Lighting
20090720_1.0
9/9
Released 1.0
PACKAGE INFORMATION
MSOP – 8 Pin
K
D G
M
J
C
P
A
F
B
INCHES MILLIMETERS
MIN TYP MAX MIN TYP MAX
A 0.114 0.118 0.122 2.90 3.00 3.10
B 0.114 0.118 0.122 2.90 3.00 3.10
C 0.040 0.044 1.02 1.12
D 0.012 0.30
F 0.016 0.021 0.031 0.40 0.53 0.80
G - 0.026 - - 0.65 -
J 0.006 0.15
K 0.000 - 0.006 0.00 - 0.15
M 0º - 8º 0º - 8º
P 0.185 0.193 0.201 4.70 4.90 5.10
