LM3502
Step-Up Converter for White LED Applications
LM3502 Step-Up Converter for White LED Applications
August 2006
General Description
The LM3502 is a white LED driver for lighting applications.
For dual display or large single white LED string backlighting
applications, the LM3502 provides a complete solution. The
LM3502 contains two internal white LED current bypass
FET(Field Effect Transitor) switches that are ideal for controlling dual display applications. The white LED current can
be adjusted with a PWM signal directly from a microcontroller without the need of an RC filter network.
With no external compensation, cycle-by-cycle current limit,
over-voltage protection, and under-voltage protection, the
LM3502 offers superior performance over other application
specific standard product step-up white LED drivers.
Typical Application
Features
n Drive up to 4, 6, 8 or 10 white LEDs for Dual Display
Backlighting
>
n
80% efficiency
n Output Voltage Options: 16V , 25V , 35V, and 44V
n Input Under-Voltage Protection
n Internal Soft Start Eliminating Inrush Current
n 1 MHz Constant Switching Frequency
n Wide Input Voltage: 2.5V to 5.5V
n Small External Components
n Low Profile Packages:
-10 Bump MicroSMD
-16 Pin LLP
<
1 mm Height
Applications
n Dual Display Backlighting in Portable Devices
n Cellular Phones and PDAs
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FIGURE 1. Blacklight Configuration with 10 White LEDs
© 2006 National Semiconductor Corporation DS201317 www.national.com
Connection Diagrams
LM3502
10-Bump Thin MicroSMD
Package (TLP10)
16-Lead Thin Leadless Leadframe
Package (SQA16A)
TOP VIEW
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Pin Descriptions/Functions
Bump # Pin # Name Description
A1 9 Cntrl Shutdown Control Connection
B1 7 Fb Feedback Voltage Connection
TOP VIEW
C1 6 V
D1 4 V
OUT2
OUT1
D2 2 and 3 Sw Drain Connection of The Power NMOS Switch (Figure 2: N1)
D3 15 and 16 PGND Power Ground Connection
C3 14 AGND Analog Ground Connection
B3 13 V
A3 12 En2 NMOS FET Switch Control Connection
A2 10 En1 PMOS FET Switch Control Connection
1 NC No Connection
5 NC No Connection
8 NC No Connection
11 NC No Connection
DAP DAP Die Attach Pad (DAP), must be soldered to the printed circuit board’s ground plane for
Drain Connections of The NMOS and PMOS Field Effect Transistor (FET) Switches
(Figure 2: N2 and P1)
Over-Voltage Protection (OVP) and Source Connection of The PMOS FET Switch
(Figure 2: P1)
Supply or Input Voltage Connection
IN
enhanced thermal dissipation.
20131703
Cntrl (Bump A1): Shutdown control pin. When V
1.4V, the LM3502 is enabled or ON. When V
Cntrl
is ≥
Cntrl
is ≤ 0.3V,
the LM3502 will enter into shutdown mode operation. The
LM3502 has an internal pull down resistor on the Cntrl pin,
thus the device is normally in the off state or shutdown mode
of operation.
Fb (Bump B1): Output voltage feedback connection. The
white LED string network current is set/programmed using a
resistor from this pin to ground.
(Bump C1): Drain connections of the internal PMOS
V
OUT2
and NMOS FET switches. (Figure 2: P1 and N2). It is recommended to connect 100nF at V
and LM3502-44 versions if V
(Bump D1): Source connection of the internal PMOS
V
OUT1
OUT2
for the LM3502-35V
OUT2
is not used.
FET switch (Figure 2: P1) and OVP sensing node. The
output capacitor must be connected as close to the device
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as possible, between the V
pin and ground plane. Also
OUT1
connect the Schottky diode as close as possible to the
pin to minimize trace resistance and EMI radiation.
V
OUT1
Sw (Bump D2): Drain connection of the internal power
NMOS FET switch. (Figure 2: N1) Minimize the metal trace
length and maximize the metal trace width connected to this
pin to reduce EMI radiation and trace resistance.
PGND (Bump D3): Power ground pin. Connect directly to
the ground plane.
AGND (Bump C3): Analog ground pin. Connect the analog
ground pin directly to the PGND pin.
(Bump B3): Supply or input voltage connection pin. The
V
IN
capacitor should be as close to the device as possible,
C
IN
between the V
pin and ground plane.
IN
En2 (Bump A3): Enable pin for the internal NMOS FET
switch (Figure 2: N2) during device operation. When V
En2
is
Pin Descriptions/Functions
(Continued)
≤ 0.3V, the internal NMOS FET switch turns on and the SUB
display turns off. When V
FET switch turns off and the SUB display turns on. The En2
pin has an internal pull down resistor, thus the internal
NMOS FET switch is normally in the on state of operation
with the SUB display turned off.
and V
If V
En1
will enter a low I
are ≤ 0.3V and V
En2
shutdown mode of operation where all the
Q
internal FET switches are off. If V
is ≥ 1.4V, the internal NMOS
En2
is ≥ 1.4V, the LM3502
Cntrl
is not used, En2 must
OUT2
En1 (Bump A2): Enable pin for the internal PMOS FET
switch (Figure 2: P1) during device operation. When V
≤ 0.3V, the internal PMOS FET switch turns on and the MAIN
display is turned off. When V
is ≥ 1.4V, the internal PMOS
En1
FET switch turns off and the MAIN display is turned on. The
En1 pin has an internal pull down resistor, thus the internal
PMOS FET switch is normally in the on state of operation
with the MAIN display turned off. If V
and V
is ≥ 1.4V, the LM3502 will enter a low IQshutdown
Cntrl
En1
mode of operation where all the internal FET switches are
off. If V
is not used, En2 must be grounded and use En1
OUT2
a long with Cntrl, to enable the device.
be grounded or floating and use En1 along with Cntrl, to
enable the device.
Ordering Information
Voltage Option Order Number Package Marking Supplied As
16 LM3502ITL-16 SANB 250 units, Tape-and-Reel
16 LM3502ITLX-16 SANB 3000 units, Tape-and-Reel
16 LM3502SQ-16 L00048B 250 units, Tape-and-Reel
16 LM3502SQX-16 L00048B 3000 units, Tape-and-Reel
25 LM3502ITL-25 SAPB 250 units, Tape-and-Reel
25 LM3502ITLX-25 SAPB 3000 units, Tape-and-Reel
25 LM3502SQ-25 L00049B 250 units, Tape-and-Reel
25 LM3502SQX-25 L00049B 3000 units, Tape-and-Reel
35 LM3502ITL-35 SARB 250 units, Tape-and-Reel
35 LM3502ITLX-35 SARB 3000 units, Tape-and-Reel
35 LM3502SQ-35 L00044B 250 units, Tape-and-Reel
35 LM3502SQX-35 L00044B 3000 units, Tape-and-Reel
44 LM3502ITL-44 SDLB 250 units, Tape-and-Reel
44 LM3502ITLX-44 SDLB 3000 units, Tape-and-Reel
44 LM3502SQ-44 L00050B 250 units, Tape-and-Reel
44 LM3502SQX-44 L00050B 3000 units, Tape-and-Reel
and V
are ≤ 0.3V
En2
En1
LM3502
is
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Absolute Maximum Ratings (Notes 6, 1)
If Military/Aerospace specified devices are required,
LM3502
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Pin −0.3V to +5.5V
V
IN
Sw Pin −0.3V to +48V
Fb Pin −0.3V to +5.5V
Cntrl Pin −0.3V to +5.5V
V
Pin −0.3V to +48V
OUT1
V
Pin −0.3V to V
OUT2
En1 −0.3V to +5.5V
En2 −0.3V to +5.5V
Continuous Power Dissipation Internally Limited
Maximum Junction Temperature
(T
J-MAX)
Storage Temperature Range −65˚C to +150˚C
OUT1
+150˚C
ESD Rating (Note 2)
Human Body Model:
Machine Model:
2kV
200V
Operating Conditions (Notes 1, 6)
Junction Temperature (T
Ambient Temperature (T
Input Voltage, V
IN
Cntrl, En1, and En2 Pins 0V to 5.5V
) Range −40˚C to +125˚C
J
) Range −40˚C to +85˚C
A
Pin 2.5V to 5.5V
Thermal Properties (Note 4)
Junction-to-Ambient Thermal Resistance (θ
Micro SMD Package 65˚C/W
Leadless Leadframe Package 49˚C/W
)
JA
Preliminary Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for T
Limits in bold typeface apply over the full operating junction temperature range (−40˚C ≤ T
specified, V
= 2.5V.
IN
≤ +125˚C). Unless otherwise
J
= 25˚C.
J
Symbol Parameter Conditions Min Typ Max Units
V
IN
I
Q
V
Fb
I
CL
I
Fb
F
S
R
DS(ON)
Input Voltage 2.5 5.5 V
Non-Switching
Switching
Shutdown
Shutdown
Low I
Q
>
0.25V
Fb
Fb = 0V, Sw Is Floating
Cntrl = 0V
Cntrl = 1.5V, En1 = En2 = 0V
0.5
1.9
0.1
6
1
3
3
15
Feedback Voltage 0.18 0.25 0.3 V
NMOS Power Switch
Current Limit
Feedback Pin Bias
Current (Note 8)
−16, Fb = 0V
−25, Fb = 0V
−35, Fb = 0V
−44, Fb = 0V
Fb = 0.25V
250
400
450
450
400
600
750
750
650
800
1050
1050
64 500 nA
Switching Frequency 0.8 1 1.2 MHz
NMOS Power Switch
ON Resistance
= 500 mA
I
Sw
0.55 1.1 Ω
(Figure 2: N1)
R
PDS(ON)
PMOS ON Resistance
of V
OUT1/VOUT2
Switch
= 20 mA, En1 = 0V, En2 = 1.5V
I
PMOS
5 10 Ω
(Figure 2: N1)
R
NDS(ON)
NMOS ON Resistance
OUT2
/Fb Switch
of V
= 20 mA, En1 = 1.5V, En2 = 0V
I
NMOS
2.5 5 Ω
(Figure 2: N2)
D
I
I
I
MAX
Cntrl
Sw
V
OUT1
Maximum Duty Cycle Fb = 0V 90 95 %
Cntrl Pin Input Bias
Current (Note 3)
Sw Pin Leakage
Current (Note 3)
(OFF) V
OUT1
Current (Note 3)
Pin Leakage
Cntrl = 2.5V
Cntrl = 0V
Sw = 42V, Cntrl = 0V
V
= 14V, Cntrl = 0V (16)
OUT1
= 23V, Cntrl = 0V (25)
V
OUT1
= 32V, Cntrl = 0V (35)
V
OUT1
= 42V, Cntrl = 0V (44)
V
OUT1
7
0.1
0.01 5 µA
0.1
0.1
0.1
0.1
14
3
3
3
3
mA
mA
µA
µA
mA
µA
µA
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LM3502
Preliminary Electrical Characteristics (Notes 6, 7) Limits in standard typeface are for T
Limits in bold typeface apply over the full operating junction temperature range (−40˚C ≤ T
specified, V
= 2.5V. (Continued)
IN
≤ +125˚C). Unless otherwise
J
= 25˚C.
J
Symbol Parameter Conditions Min Typ Max Units
I
(ON) V
V
OUT1
I
V
OUT2
UVP Under-Voltage
OVP Over-Voltage
V
En1
Pin Bias
OUT1
Current (Note 3)
V
Pin Leakage
OUT2
Current (Note 3)
Protection
Protection (Note 5)
PMOS FET Switch
Enabling Threshold
V
= 14V, Cntrl = 1.5V (16)
OUT1
= 23V, Cntrl = 1.5V (25)
V
OUT1
= 32V, Cntrl = 1.5V (35)
V
OUT1
= 42V, Cntrl = 1.5V (44)
V
OUT1
Fb = 0V, Cntrl = 0V, V
OUT2
= 42V
On Threshold
Off Threshold 2.2
On Threshold (16)
Off Threshold (16)
On Threshold (25)
Off Threshold (25)
On Threshold (35)
Off Threshold (35)
On Threshold (44)
Off Threshold (44)
14.5
14.0
22.5
21.5
32.0
31.0
40.5
39.0
Off Threshold (Display Lighting)
On Threshold (Display Lighting) 1.4
40
50
50
85
80
100
100
140
0.1 3 µA
2.4
2.5
2.3
15.5
15
24
23
34
33
42
41
0.8
16.5
16.0
25.5
24.5
35.0
34.0
43.5
42.0
0.3
0.8
(Figure 2: P1)
V
En2
NMOS FET Switch
Enabling Threshold
Off Threshold (Display Lighting)
On Threshold (Display Lighting) 1.4
0.8
0.8
0.3
(Figure 2: N2)
V
T
I
En1
I
En2
Cntrl
SHDW
Device Enabling
Threshold
Shutdown Delay Time 8 12 16 ms
En1 Pin Input Bias
Current
En2 Pin Input Bias
Current
Off Threshold
OnThreshold 1.4
En1 = 2.5V
En1=0V
En2 = 2.5V
En2=0V
0.8
0.8
0.1
0.1
0.3
7
7
14
14
µA
V
V
V
V
V
µA
µA
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical characteristic specifications do not apply when
operating the device outside of its rated operating conditions.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin.
Note 3: Current flows into the pin.
Note 4: The maximum allowable power dissipation is a function of the maximum junction temperature, T
and the ambient temperature, T
calculated using: P
on this topic, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and Application Note 1112 (AN1112) for microSMD chip scale package.
Note 5: The on threshold indicates that the LM3502 is no longer switching or regulating LED current, while the off threshold indicates normal operation.
Note 6: All voltages are with respect to the potential at the GND pin.
Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.
Note 8: Current flows out of the pin.
(MAX) = (TJ(MAX) – TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature. For more information
D
. See Thermal Properties for the thermal resistance. The maximum allowable power dissipation at any ambient temperature is
A
(MAX), the junction-to-ambient thermal resistance, θJA,
J
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Block Diagram
LM3502
FIGURE 2. Block Diagram
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20131704
Detailed Description of Operation
The LM3502 utilizes an asynchronous current mode pulsewidth-modulation (PWM) control scheme to regulate the
feedback voltage over specified load conditions. The DC/DC
converter behaves as a controlled current source for white
LED applications. The operation can best be understood by
referring to the block diagram in Figure 2 for the following
operational explanation. At the start of each cycle, the oscillator sets the driver logic and turns on the internal NMOS
power device, N1, conducting current through the inductor
and reverse biasing the external diode. The white LED current is supplied by the output capacitor when the internal
NMOS power device, N1, is turned on. The sum of the error
amplifier’s output voltage and an internal voltage ramp are
compared with the sensed power NMOS, N1, switch voltage.
Once these voltages are equal, the PWM comparator will
then reset the driver logic, thus turning off the internal NMOS
power device, N1, and forward biasing the external diode.
The inductor current then flows through the diode to the
white LED load and output capacitor. The inductor current
recharges the output capacitor and supplies the current for
the white LED load. The oscillator then resets the driver logic
again repeating the process. The output voltage of the error
amplifier controls the current through the inductor. This voltage will increase for larger loads and decrease for smaller
loads limiting the peak current in the inductor and minimizing
EMI radiation. The duty limit comparator is always operational, it prevents the internal NMOS power switch, N1, from
being on for more than one oscillator cycle and conducting
large amounts of current. The light load comparator allows
the LM3502 to properly regulate light/small white LED load
currents, where regulation becomes difficult for the
LM3502’s primary control loop. Under light load conditions,
the LM3502 will enter into a pulse skipping pulse-frequencymode (PFM) of operation where the switching frequency will
vary with the load.
The LM3502 has 2 control pins, En1 and En2, used for
selecting which segment of a single white LED string network is active for dual display applications. En1 controls the
main display (MAIN) segment of the single string white LED
network between pins V
OUT1
and V
sub display (SUB) segment of the single string white LED
. En2 controls the
OUT2
network between the V
and Fb. For a quick review of
OUT2
the LM3502 control pin operational characteristics, see Fig-
ure 3.
When the Cntrl pin is ≥ 1.4V, the LM3502 will enter in low I
state if both En1 and En2 ≤ 0.3V. At this time, both the P1
and N2 FETs will turn off. The output voltage will be a diode
drop below the supply voltage and the soft-start will be reset
limiting the peak inductor current at the next start-up.
The LM3502 is designed to control the LED current with a
PWM signal without the use of an external RC filter. Utilizing
special circuitry, the LM3502 can operate over a large range
of PWM frequencies without restarting the soft-start allowing
for fast recovery at high PWM frequencies. Figure 4
reprsents a PWM signal driving the Cntrl pin where t
L
defined as the low time of the signal. The following is true:
<
If t
•
12ms (typical): The device will stop switching
L
during this time and the soft-start will not be reset allowing LED current PWM control.
>
If t
•
12ms (typical): The device will shutdown and the
L
soft-start will reset to prevent high peak currents at the
next startup. Both the N2 and P1 switches will turn off.
The LM3502 has dedicated protection circuitry active during
normal operation to protect the integrated circuit (IC) and
external components. The thermal shutdown circuitry turns
off the internal NMOS power device, N1, when the internal
semiconductor junction temperature reaches excessive levels. The LM3502 has a under-voltage protection (UVP) comparator that disables the internal NMOS power device when
battery voltages are too low, thus preventing an on state
where the internal NMOS power device conducts large
amounts of current. The over-voltage protection (OVP) comparator prevents the output voltage from increasing beyond
the protection limit when the white LED string network is
removed or if there is a white LED failure. OVP allows for the
use of low profile ceramic capacitors at the output. The
current though the internal NMOS power device, N1, is
monitored to prevent peak inductor currents from damaging
the IC. If during a cycle (cycle=1/switching frequency) the
peak inductor current exceeds the current limit for the
LM3502, the internal NMOS power device will be turned off
for the remaining duration of that cycle.
LM3502
Q
is
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LM3502
20131705
FIGURE 3. Operational Characteristics Table
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FIGURE 4. Control Signal Waveform
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Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3.
Efficiency: η =P
OUT/PIN
= [(V
OUT–VFb
(Non-Switching) vs V
I
Q
)*I
]/[VIN*IIN]. TA= 25˚C, unless otherwise stated.)
OUT
IN
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Switching Frequency vs Temperature
20131708
LM3502
IQ(Switching) vs V
IN
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IQ(Switching) vs Temperature
10 LED Efficiency vs LED Current 8 LED Efficiency vs LED Current
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Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3.
Efficiency: η =P
LM3502
OUT/PIN
= [(V
OUT–VFb
6 LED Efficiency vs LED Current 4 LED Efficiency vs LED Current
)*I
]/[VIN*IIN]. TA= 25˚C, unless otherwise stated.) (Continued)
OUT
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Cntrl Pin Current vs Cntrl Pin Voltage Maximum Duty Cycle vs Temperature
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En1 Pin Current vs En1 Pin Voltage En2 Pin Current vs En2 Pin Voltage
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20131718
Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3.
Efficiency: η =P
V
OUT1
OUT/PIN
= [(V
OUT–VFb
Pin Current vs V
)*I
]/[VIN*IIN]. TA= 25˚C, unless otherwise stated.) (Continued)
OUT
Pin Voltage Power NMOS R
OUT1
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(Figure 2: N1) vs V
DS(ON)
IN
LM3502
NMOS R
(Figure 2: N2) vs V
DS(ON)
Feedback Voltage vs Temperature
IN
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PMOS R
(Figure 2: P1) vs V
DS(ON)
IN
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Typical Performance Characteristics ( Circuit in Figure 1: L = DO1608C-223 and D = MBRM140T3.
Efficiency: η =P
LM3502
OUT/PIN
= [(V
OUT–VFb
Current Limit (LM3502-16) vs V
)*I
]/[VIN*IIN]. TA= 25˚C, unless otherwise stated.) (Continued)
OUT
IN
Current Limit (LM3502-16) vs Temperature
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Current Limit (LM3502-25) vs V
IN
20131756
Current Limit (LM3502-25) vs Temperature
Current Limit (LM3502-35/44) vs Temperature Current Limit (LM3502-35/44) vs V
20131755
20131757
IN
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20131729
Application Information
WHITE LED CURRENT SETTING
The LED current is set using the following equation:
20131730
I
: White LED Current.
LED
: Feedback Pin Voltage. VFb= 0.25V, Typical.
V
Fb
R1: Currrent Setting Resistor.
WHITE LED DIMMING
For dimming the white LED string with a pulse-widthmodulated (PWM) signal on the Cntrl pin, care must taken to
balance the tradeoffs between audible noise and white LED
LM3502
brightness control. For best PWM duty cycle vs. white LED
current linearity, the PWM frequency should be between
200Hz and 500Hz. Other PWM frequencies can be used, but
the linearity over input voltage and duty cycle variation will
not be as good as what the 200Hz to 500Hz PWM frequency
spectrum provides. To minimize audible noise interference, it
is recommended that a output capacitor with minimal susceptibility to piezoelectric induced stresses be used for the
particular applications that require minimal or no audible
noise interference.
If V
is not used , En2 must be grounded
OUT2
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FIGURE 5.
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Application Information (Continued)
LM3502
FIGURE 6. Inductor Current Waveform
CONTINUOUS AND DISCONTINUOUS MODES OF OPERATION
Since the LM3502 is a constant frequency pulse-widthmodulated step-up regulator, care must be taken to make
sure the maximum duty cycle specification is not violated.
The duty cycle equation depends on which mode of operation the LM3502 is in. The two operational modes of the
LM3502 are continuous conduction mode (CCM) and discontinuous conduction mode (DCM). Continuous conduction
mode refers to the mode of operation where during the
switching cycle, the inductor current never goes to and stays
at zero for any significant amount of time during the switching cycle. Discontinuous conduction mode refers to the
mode of operation where during the switching cycle, the
inductor current goes to and stays at zero for a significant
amount of time during the switching cycle. Figure 6 illustrates the threshold between CCM and DCM operation. In
Figure 6, the inductor current is right on the CCM/DCM
operational threshold. Using this as a reference, a factor can
be introduced to calculate when a particular application is in
CCM or DCM operation. R is a CCM/DCM factor we can use
to compute which mode of operation a particular application
is in. If R is ≥ 1, then the application is operating in CCM.
Conversely, if R is
The R factor inequalities are a result of the components that
make up the R factor. From Figure 6, the R factor is equal to
the average inductor current, I
inductor ripple current, ∆i
equation can be used to compute R factor:
<
1, the application is operating in DCM.
(avg), divided by half the
L
. Using Figure 6 the following
L
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VIN: Input Voltage.
: Output Voltage.
V
OUT
Eff: Efficiency of the LM3502.
Fs: Switching Frequency.
: White LED Current/Load Current.
I
OUT
L: Inductance Magnitude/Inductor Value.
D: Duty Cycle for CCM Operation.
: Inductor Ripple Current
∆i
L
(avg): Average Inductor Current
I
L
For CCM operation, the duty cycle can be computed with:
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D: Duty Cycle for CCM Operation.
: Output Voltage.
V
OUT
: Input Voltage.
V
IN
For DCM operation, the duty cycle can be computed with:
20131743
Application Information (Continued)
20131744
D: Duty Cycle for DCM Operation.
: Output Voltage.
V
OUT
: Input Voltage.
V
IN
I
: White LED Current/Load Current.
OUT
Fs: Switching Frequency.
L: Inductor Value/Inductance Magnitude.
INDUCTOR SELECTION
In order to maintain inductance, an inductor used with the
LM3502 should have a saturation current rating larger than
the peak inductor current of the particular application. Inductors with low DCR values contribute decreased power losses
and increased efficiency. The peak inductor current can be
computed for both modes of operation: CCM and DCM.
The cycle-by-cycle peak inductor current for CCM operation
can be computed with:
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20131746
VIN: Input Voltage.
Eff: Efficiency of the LM3502.
Fs: Switching Frequency.
: White LED Current/Load Current.
I
OUT
L: Inductance Magnitude/Inductor Value.
D: Duty Cycle for CCM Operation.
: Peak Inductor Current.
I
PEAK
: Inductor Ripple Current.
∆i
L
(avg): Average Inductor Current.
I
L
The cycle-by-cycle peak inductor current for DCM operation
can be computed with:
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VIN: Input Voltage.
Fs: Switching Frequency.
L: Inductance Magnitude/Inductor Value.
D: Duty Cycle for DCM Operation.
: Peak Inductor Current.
I
PEAK
The minimum inductance magnitude/inductor value for the
LM3502 can be calculated using the following, which is only
valid when the duty cycle is
>
0.5:
20131748
D: Duty Cycle.
D: 1– D.
: NMOS Power Switch ON Resistance.
R
DS(ON)
: Input Voltage.
V
IN
L: Inductance Magnitude/Inductor Value.
This equation gives the value required to prevent subharmonic oscillations. The result of this equation and the inductor average and ripple current should be accounted for when
choosing an inductor value.
Some recommended inductor manufacturers included but
are not limited to:
CoilCraft
DO1608C-223
DT1608C-223
www.coilcraft.com
CAPACITOR SELECTION
Multilayer ceramic capacitors are the best choice for use
with the LM3502. Multilayer ceramic capacitors have the
lowest equivalent series resistance (ESR). Applied voltage
or DC bias, temperature, dielectric material type (X7R, X5R,
Y5V, etc), and manufacturer component tolerance have an
affect on the true or effective capacitance of a ceramic
capacitor. Be aware of how your application will affect a
particular ceramic capacitor by analyzing the aforementioned factors of your application. Before selecting a capacitor always consult the capacitor manufacturer’s data curves
to verify the effective or true capacitance in your application.
INPUT CAPACITOR SELECTION
The input capacitor serves as an energy reservoir for the
inductor. In addition to acting as an energy reservoir for the
inductor the input capacitor is necessary for the reduction in
input voltage ripple and noise experienced by the LM3502.
The reduction in input voltage ripple and noise helps ensure
the LM3502’s proper operation, and reduces the effect of the
LM3502 on other devices sharing the same supply voltage.
To ensure low input voltage ripple, the input capacitor must
have an extremely low ESR. As a result of the low input
voltage ripple requirement multilayer ceramic capacitors are
the best choice.A minimum capacitance of 2.0 µF is required
for normal operation, so consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance
requirement is going to be achieved for a particular application.
OUTPUT CAPACITOR SELECTION
The output capacitor serves as an energy reservoir for the
white LED load when the internal power FET switch (Figure
2: N1) is on or conducting current. The requirements for the
output capacitor must include worst case operation such as
when the load opens up and the LM3502 operates in overvoltage protection (OVP) mode operation. A minimum capacitance of 0.5µF is required to ensure normal operation.
Consult the capacitor manufacturer’s data curves to verify
whether the minimum capacitance requirement is going to
be achieved for a particular application.
Some recommended capacitor manufacturers included but
are not limited to:
LM3502
www.national.com15
Application Information (Continued)
LM3502
Taiyo
Yuden
muRata GRM40-035X7R105K
TDK C3216X7R1H105KT
AVX 08053D105MAT
DIODE SELECTION
To maintain high efficiency it is recommended that the average current rating (I
larger than the peak inductor current (I
mum, the average current rating of the diode should be
larger than the maximum LED current. To maintain diode
integrity the peak repetitive forward current (I
greater than or equal to the peak inductor current (I
Diodes with low forward voltage ratings (V
capacitance magnitudes (C
high efficiency. The chosen diode must have a reverse
breakdown voltage rating (V
than the output voltage (V
is chosen, Schottky or not, certain selection criteria must be
followed:
1. V
R
2. IFor IO≥ I
3. I
FRM
Some recommended diode manufacturers included but are
not limited to:
Vishay SS12(1A/20V) www.vishay.com
On
Semiconductor
Central
Semiconductor
SHUTDOWN AND START-UP
On startup, the LM3502 contains special circuitry that limits
the peak inductor current which prevents large current
spikes from loading the battery or power supply. When Cntrl
≥ 1.4V and both the En1 and En2 signals are less than 0.3V,
the LM3502 will enter a low I
During this low I
below the supply voltage and the soft-start will be reset to
GMK212BJ105MD
(0805/35V)
(0805/50V)
(1206/50V)
C3216X7R1C475K
(1206/16V)
(0805/25V)
08056D475KAT
(0805/6.3V)
1206ZD475MAT
(1206/10V)
and V
RRM
LOAD
≥ I
Lpeak
Q
www.t-yuden.com
www.murata.com
www.tdktca.com
www.avxcorp.com
or IO) of the selected diode should be
F
or CTor CD) are conducive to
J
and/or V
R
). No matter what type of diode
out
>
V
OUT
or I
OUT
). At the mini-
Lpeak
) and low junction
F
) that is larger
RRM
SS14(1A/40V)
SS16(1A/60V)
MBRM120E
www.onsemi.com
(1A/20V)
MBRS1540T3
(1.5A/40V)
MBR240LT
(2A/40V)
CMSH1- 40M
www.centralsemi.com
(1A/40V)
state and regulation will end.
Q
mode the output voltage is a diode drop
) must be
FRM
Lpeak
limit the peak inductor current at the next startup. When both
En1 and En2 are less than 0.3V, the P1 PMOS and N2
NMOS switches will turn off.
When Cntrl
<
0.3V for more than 12ms, typicaly, the LM3502
will shutdown and the output voltage will be a diode drop
below the supply voltage. If the Cntrl pin is low for more than
12ms, the soft-start will reset to limit the peak inductor current at the next startup.
<
When Cntrl is
0.3 but for less than 12ms, typically, the
device will not shutdown and reset the soft-start but shut off
the NMOS N1 Power Device to allow for PWM contrl of the
LED current.
THERMAL SHUTDOWN
The LM3502 stops regulating when the internal semiconductor junction temperature reaches approximately 140˚C. The
internal thermal shutdown has approximately 20˚C of hysteresis which results in the LM3502 turning back on when the
internal semiconductor junction temperature reaches 120˚C.
When the thermal shutdown temperature is reached, the
softstart is reset to prevent inrush current when the die
temperature cools.
UNDER VOLTAGE PROTECTION
The LM3503 contains protection circuitry to prevent operation for low input supply voltages. When Vin drops below
2.3V, typically the LM3502 will no longer regulate. In this
).
mode, the output volage will be one diode drop below Vin
and the softstart will be reset. When Vin increases above
2.4V, typically, the device will begin regulating again.
OVER VOLTAGE PROTECTION
The LM3502 contains dedicated circuitry for monitoring the
output voltage. In the event that the LED network is disconnected from the LM3502, the output voltage will increase
and be limited to 15.5V(typ.) for the 16V version , 24V(typ.)
for the 25V version, 34V(typ.) for the 35V version and
42V(typ.) for the 44V version (see eletrical table for more
details). In the event that the network is reconnected, regulation will resume at the appropriate output voltage.
LAYOUT CONSIDERATIONS
All components, except for the white LEDs, must be placed
as close as possible to the LM3502. The die attach pad
(DAP) must be soldered to the ground plane.
The input bypass capacitor C
be placed close to the IC and connect between the V
, as shown in Figure 1, must
IN
IN
and
PGND pins. This will reduce copper trace resistance which
effects input voltage ripple of the IC. For additional input
voltage filtering, a 100nF bypass capacitor can be placed in
parallel with C
ground. The output capacitor, C
the IC and be connected between the V
pins. Any copper trace connections for the C
to shunt any high frequency noise to
IN
, must be placed close to
OUT
OUT1
OUT
and PGND
capacitor
can increase the series resistance, which directly effects
output voltage ripple and efficiency. The current setting resistor, R1, should be kept close to the Fb pin to minimize
copper trace connections that can inject noise into the system. The ground connection for the current setting resistor
network should connect directly to the PGND pin. The AGND
pin should be tied directly to the PGND pin. Trace connections made to the inductor should be minimized to reduce
power dissipation and increase overall efficiency while reducing EMI radiation. For more details regarding layout
guidelines for switching regulators, refer to Applications Note
AN-1149.
www.national.com 16
Physical Dimensions inches (millimeters) unless otherwise noted
LM3502
16-Lead Thin Leadless Leadframe Package
NS Package Number SQA16A
TLP10: 10-Bump Thin Micro SMD
X1 = 1.958 mm
X2 = 2.135 mm
X3 = 0.6 mm
NS Package No. TLP10
www.national.com17
Notes
LM3502 Step-Up Converter for White LED Applications
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
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properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
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Support Center
Email: new.feedback@nsc.com
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