LM3501 [NSC]
Synchronous Step-up DC/DC Converter for White LED Applications; 同步升压型DC / DC转换器，用于白光LED应用
| 型号： | LM3501 |
| 厂家： | 
National Semiconductor |
| 描述： | Synchronous Step-up DC/DC Converter for White LED Applications |
| 文件： | 总18页 (文件大小：1035K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
May 2005
LM3501
Synchronous Step-up DC/DC Converter for White LED
Applications
General Description
Features
n Synchronous rectification, high efficiency and no
external schottky diode required
The LM3501 is a fixed-frequency step-up DC/DC converter
that is ideal for driving white LEDs for display backlighting
and other lighting functions. With fully intergrated synchro-
nous switching (no external schottky diode required) and a
low feedback voltage (515 mV), power efficiency of the
LM3501 circuit has been optimized for lighting applications
in wireless phones and other portable products (single cell
Li-Ion or 3-cell NiMH battery supplies). The LM3501 oper-
ates with a fixed 1 MHz switching frequency. When used with
ceramic input and output capacitors, the LM3501 provides a
small, low-noise, low-cost solution.
n Uses small surface mount components
n Can drive 2-5 white LEDs in series (may function with
more low VF LEDs)
n 2.7V to 7V input range
n True shutdown isolation, no LED leakage current
n DC voltage LED current control
n Input undervoltage lockout
n Internal output over-voltage protection (OVP) circuitry,
with no external zener diode required LM3501-16: 15.5V
OVP; LM3501-21: 20.5V OVP.
n Requires only a small 16V (LM3501-16) or 25V
(LM3501-21) ceramic capacitor at the input and output
n Thermal Shutdown
Two LM3501 options are available with different output volt-
age capabilities. The LM3501-21 has a maximum output
voltage of 21V and is typically suited for driving 4 or 5 white
LEDs in series. The LM3501-16 has a maximum output
voltage of 16V and is typically suited for driving 3 or 4 white
LEDs in series (maximum number of series LEDs dependent
on LED forward voltage). If the primary white LED network
should be disconnected, the LM3501 uses internal protec-
tion circuitry on the output to prevent a destructive overvolt-
age event.
n 0.1µA shutdown current
n Small 8-bump thin micro SMD package
Applications
n LCD Bias Supplies
n White LED Back-Lighting
n Handheld Devices
n Digital Cameras
A single external resistor is used to set the maximum LED
current in LED-drive applications. The LED current can eas-
ily be adjusted by varying the analog control voltage on the
control pin or by using a pulse width modulated (PWM)
signal on the shutdown pin. In shutdown, the LM3501 com-
pletely disconnects the input from output, creating total iso-
lation and preventing any leakage currents from trickling into
the LEDs.
n Portable Applications
Typical Application Circuit
20065301
FIGURE 1. Typical 3 LED Application
© 2005 National Semiconductor Corporation
DS200653
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Connection Diagram
Top View
20065302
8-bump micro SMD
Ordering Information
Order Number
LM3501TL-16
LM3501TLX-16
LM3501TL-21
LM3501TLX-21
Package Type
NSC Package Drawing
TL08SSA
Top Mark
Supplied As
micro SMD
micro SMD
micro SMD
micro SMD
19
19
30
30
250 Units, Tape and Reel
3000 Units, Tape and Reel
250 Units, Tape and Reel
3000 Units, Tape and Reel
TL08SSA
TL08SSA
TL08SSA
Pin Description/Functions
Pin
A1
B1
C1
C2
C3
B3
A3
A2
Name
AGND
VIN
Function
Analog ground.
Analog and Power supply input.
PMOS source connection for synchronous rectification.
VOUT
VSW
Switch pin. Drain connections of both NMOS and PMOS power devices.
Power Ground.
GND
FB
Output voltage feedback connection.
Analog LED current control.
CNTRL
SHDN
Shutdown control pin.
AGND (pin A1): Analog ground pin. The analog ground pin
should tie directly to the GND pin.
FB (pin B3):Output voltage feedback connection. Set the
primary White LED network current with a resistor from the
FB pin to GND. Keep the current setting resistor close to the
device and connected between the FB and GND pins.
VIN (pin B1):Analog and Power supply pin. Bypass this pin
with a capacitor, as close to the device as possible, con-
nected between the VIN and GND pins.
CNTRL (pin A3): Analog control of LED current. A voltage
above 125 mV will begin to regulate the LED current. De-
creasing the voltage below 75 mV will turn off the LEDs.
VOUT (pin C1):Source connection of internal PMOS power
device. Connect the output capacitor between the VOUT and
GND pins as close as possible to the device.
SHDN (pin A2):Shutdown control pin. Disable the device
with a voltage less than 0.3V and enable the device with a
voltage greater than 1.1V. The white LED current can be
controlled using a PWM signal at this pin. There is an
internal pull down on the SHDN pin, the device is in a
normally off state.
VSW (pin C2):Drain connection of internal NMOS and PMOS
switch devices. Keep the inductor connection close to this
pin to minimize EMI radiation.
GND (pin C3):Power ground pin. Tie directly to ground
plane.
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2
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Ratings (Note 3)
Human Body Model
Machine Model
2kV
200V
VIN
−0.3V to 7.5V
−0.3V to 16V
Operating Conditions
Junction Temperature
(Note 4)
V
V
V
OUT (LM3501-16)(Note 2)
OUT (LM3501-21)(Note 2)
SW (Note 2)
−0.3V to 21V
−40˚C to +125˚C
2.7V to 7V
2.7V
−0.3V to VOUT+0.3V
−0.3V to 7.5V
−0.3V to VIN+0.3V
−0.3V to 7.5V
150˚C
Supply Voltage
CNTRL Max.
FB Voltage
SHDN Voltage
CNTRL
Thermal Properties
Junction to Ambient Thermal
Resistance (θJA) (Note 5)
Maximum Junction Temperature
Lead Temperature
(Soldering 10 sec.)
Vapor Phase
(60 sec.)
75˚C/W
300˚C
215˚C
220˚C
Infrared
(15 sec.)
Electrical Characteristics
Specifications in standard type face are for TA = 25˚C and those in boldface type apply over the Operating Temperature
Range of TA = −10˚C to +85˚C. Unless otherwise specified VIN = 2.7V and specifications apply to both LM3501-16 and
LM3501-21.
Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
Symbol
IQ
Parameter
Conditions
Units
>
Quiescent Current, Device Not
Switching
FB 0.54V
0.95
1.2
mA
µA
V
Quiescent Current, Device
Switching
FB = 0V
2
2.5
2
Shutdown
SHDN = 0V
0.1
VFB
Feedback Voltage
CNTRL = 2.7V,
VIN = 2.7V to 7V
CNTRL = 1V,
0.485
0.14
0.515
0.545
0.19
0.1
0.24
0.5
VIN = 2.7V to 7V
VIN = 2.7V to 7V
∆VFB
Feedback Voltage Line
Regulation
%/V
mA
ICL
Switch Current Limit
(LM3501-16)
VIN = 2.7V,
275
255
420
450
400
400
640
480
530
770
800
Duty Cycle = 80%
VIN = 3.0V,
Duty Cycle = 70%
VIN = 2.7V,
Switch Current Limit
(LM3501-21)
Duty Cycle = 70%
VIN = 3.0V,
670
45
Duty Cycle = 63%
FB = 0.5V (Note 8)
IB
FB Pin Bias Current
Input Voltage Range
NMOS Switch RDSON
PMOS Switch RDSON
Duty Cycle Limit
200
7.0
nA
V
VIN
2.7
80
RDSON
VIN = 2.7V, ISW = 300 mA
VOUT = 6V, ISW = 300 mA
FB = 0V
0.43
2.3
Ω
1.3
87
DLimit
(LM3501-16)
%
Duty Cycle Limit
FB = 0V
85
94
(LM3501-21)
FSW
Switching Frequency
0.85
1.0
1.15
MHz
3
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Electrical Characteristics (Continued)
Specifications in standard type face are for TA = 25˚C and those in boldface type apply over the Operating Temperature
Range of TA = −10˚C to +85˚C. Unless otherwise specified VIN = 2.7V and specifications apply to both LM3501-16 and
LM3501-21.
Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
4
Symbol
ISD
Parameter
Conditions
SHDN = 5.5V
Units
SHDN Pin Current (Note 9)
1.8
1
SHDN = 2.7V
SHDN = GND
VCNTRL = 2.7V
VCNTRL = 1V
VSW = 15V
2.5
µA
0.1
10
4
ICNTRL
IL
CNTRL Pin Current (Note 9)
20
15
µA
Switch Leakage Current
(LM3501-16)
0.01
0.01
0.5
2.0
µA
Switch Leakage Current
(LM3501-21)
VSW = 20V
UVP
OVP
Input Undervoltage Lockout
ON Threshold
2.4
2.3
15
14
20
19
2.5
2.4
2.6
2.5
16
15
21
20
V
V
V
OFF Threshold
ON Threshold
Output Overvoltage Protection
(LM3501-16)
15.5
14.6
20.5
19.5
OFF Threshold
ON Threshold
Output Overvoltage Protection
(LM3501-21)
OFF Threshold
VOUT = 15V, SHDN = 1.5V
IVout
VOUT Bias Current
(LM3501-16)
260
300
400
460
3
µA
µA
VOUT Bias Current
(LM3501-21)
VOUT = 20V, SHDN = 1.5V
VOUT = 15V, VSW = 0V
VOUT = 20V, VSW = 0V
IVL
PMOS Switch Leakage Current
(LM3501-16)
0.01
0.01
PMOS Switch Leakage Current
(LM3501-21)
3
CNTRL
LED power off
LED power on
75
mV
V
Threshold
125
0.65
0.65
SHDN
SHDN low
0.3
Threshold
SHDN High
1.1
Specifications in standard type face are for TJ = 25˚C and those in boldface type apply over the full Operating Temperature
Range (TJ = −40˚C to +125˚C). Unless otherwise specified VIN =2.7V and specifications apply to both LM3501-16 and
LM3501-21.
Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
Symbol
IQ
Parameter
Conditions
Units
>
Quiescent Current, Device Not
Switching
FB 0.54V
0.95
2
1.2
2.5
mA
Quiescent Current, Device
Switching
FB = 0V
Shutdown
SHDN = 0V
0.1
0.515
0.19
2
µA
V
VFB
∆VFB
ICL
Feedback Voltage
CNTRL = 2.7V, VIN = 2.7V to 7V
CNTRL = 1V, VIN = 2.7V to 7V
VIN = 2.7V to 7V
0.485
0.14
0.545
0.24
Feedback Voltage Line
Regulation
0.1
0.5
%/V
mA
Switch Current Limit
(LM3501-16)
VIN = 3.0V,
400
Duty Cycle = 70%
VIN = 3.0V,
Switch Current Limit
(LM3501-21)
670
45
Duty Cycle = 63%
FB = 0.5V (Note 8)
IB
FB Pin Bias Current
Input Voltage Range
200
7.0
nA
V
VIN
2.7
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4
Electrical Characteristics (Continued)
Specifications in standard type face are for TJ = 25˚C and those in boldface type apply over the full Operating Temperature
Range (TJ = −40˚C to +125˚C). Unless otherwise specified VIN =2.7V and specifications apply to both LM3501-16 and
LM3501-21.
Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
0.43
Symbol
Parameter
Conditions
Units
RDSON
NMOS Switch RDSON
PMOS Switch RDSON
Duty Cycle Limit
VIN = 2.7V, ISW = 300 mA
VOUT = 6V, ISW = 300 mA
FB = 0V
Ω
1.3
87
2.3
DLimit
(LM3501-16)
%
Duty Cycle Limit
FB = 0V
94
(LM3501-21)
FSW
ISD
Switching Frequency
SHDN Pin Current (Note 9)
0.8
1.0
1.8
1
1.2
4
MHz
µA
SHDN = 5.5V
SHDN = 2.7V
SHDN = GND
VCNTRL = 2.7V
VCNTRL = 1V
VSW = 15V
2.5
0.1
10
ICNTRL
IL
CNTRL Pin Current (Note 9)
20
15
µA
µA
V
4
Switch Leakage Current
(LM3501-16)
0.01
0.5
Switch Leakage Current
(LM3501-21)
VSW = 20V
0.01
2.0
UVP
OVP
Input Undervoltage Lockout
ON Threshold
2.4
2.3
15
14
20
19
2.5
2.4
2.6
2.5
16
15
21
20
OFF Threshold
ON Threshold
Output Overvoltage Protection
(LM3501-16)
15.5
14.6
20.5
19.5
OFF Threshold
ON Threshold
V
Output Overvoltage Protection
(LM3501-21)
OFF Threshold
VOUT = 15V, SHDN = 1.5V
IVout
VOUT Leakage Current
(LM3501-16)
260
300
400
460
3
µA
µA
VOUT Leakage Current
(LM3501-21)
VOUT = 20V, SHDN = 1.5V
VOUT = 15V, VSW = 0V
VOUT = 20V, VSW = 0V
IVL
PMOS Switch Leakage Current
(LM3501-16)
0.01
0.01
PMOS Switch Leakage Current
(LM3501-21)
3
CNTRL
LED power off
LED power on
75
mV
V
Threshold
125
0.65
0.65
SHDN
SHDN low
0.3
Threshold
SHDN High
1.1
Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to
be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.
<
>
V
Note 2: This condition applies if V
V
. If V
, a voltage greater than V + 0.3V should not be applied to the V
or V
pins.
SW
IN
OUT
IN
OUT
IN
OUT
Note 3: 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 4: The maximum allowable power dissipation is a function of the maximum operating junction temperature, T
, the junction-to-ambient thermal
J(MAX)
resistance, θ , and the ambient temperature, T . See the Thermal Properties section for the thermal resistance. The maximum allowable power dissipation at any
JA
A
ambient temperature is calculated using: P (MAX) = (T
− T )/θ . Exceeding the maximum allowable power dissipation will cause excessive die temperature.
A JA
D
J(MAX)
Note 5: Junction-to-ambient thermal resistance (θ ) is highly application and board-layout dependent. The 75oC/W figure provided was measured on a 4-layer test
JA
board conforming to JEDEC standards. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues when
designing the board layout.
Note 6: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are production
tested, guaranteed through statistical analysis or guaranteed by design. All limits at temperature extremes are guaranteed via correlation using standard Statistical
Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 7: Typical numbers are at 25˚C and represent the most likely norm.
Note 8: Feedback current flows out of the pin.
Note 9: Current flows into the pin.
5
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Typical Performance Characteristics
Switching Quiescent Current vs. VIN
Non-Switching Quiescent Current vs. VIN
20065355
20065356
2 LED Efficiency vs. Load Current
L = Coilcraft DT1608C-223,
3 LED Efficiency vs. Load Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(2VLED*ILED))
Efficiency = 100*(PIN/(3VLED*ILED))
20065357
20065358
4 LED Efficiency vs. Load Current
L = Coilcraft DT1608C-223,
Output Power vs. VIN
Efficiency = 100*(PIN/(4VLED*ILED))
(LM3501-16, L = Coilcraft DT1608C-223)
20065359
20065386
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Typical Performance Characteristics (Continued)
Output Power vs. Temperature
(LM3501-16, L = Coilcraft DT1608C-223)
FB Pin Current vs. Temperature
20065387
20065360
SHDN Pin Current vs. SHDN Pin Voltage
CNTRL Pin Current vs. CNTRL Pin Voltage
20065378
20065377
Switch Current Limit vs. VIN
(LM3501-16)
FB Voltage vs. CNTRL Voltage
20065379
20065362
7
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Typical Performance Characteristics (Continued)
Switch Current Limit vs. Temperature
(LM3501-16, VOUT = 8V)
Switch Current Limit vs. Temperature
(LM3501-16, VOUT = 12V)
20065376
20065363
Switch Current Limit vs. VIN
(LM3501-21)
Switch Current Limit vs. Temperature
(LM3501-21, VOUT = 8V)
20065332
20065331
Switch Current Limit vs. Temperature
(LM3501-21, VOUT = 12V)
Switch Current Limit vs. Temperature
(LM3501-21, VOUT = 18V)
20065345
20065333
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Typical Performance Characteristics (Continued)
VOUT DC Bias vs. VOUT Voltage
(LM3501-16)
Oscillator Frequency vs. VIN
20065364
20065365
FB Voltage vs. Temperature
FB Voltage vs. Temperature
20065380
20065382
NMOS RDSON vs. VIN
(ISW = 300 mA)
FB Voltage vs. VIN
20065381
20065374
9
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Typical Performance Characteristics (Continued)
PMOS RDSON vs. Temperature
Typical VIN Ripple
20065368
3 LEDs, R
= 22Ω, V = 3.0V, CNTRL = 2.7V
LED
IN
1) SW, 10 V/div, DC
3) I , 100 mA/div, DC
20065375
L
4) V , 100 mV/div, AC
IN
T = 250 ns/div
Start-Up (LM3501-16)
SHDN Pin Duty Cycle Control Waveforms
20065371
20065346
3 LEDs, R
= 22Ω, V = 3.0V, CNTRL = 2.7V
LM3501-16, 3 LEDs, R
= 22Ω, V = 3.0V, SHDN frequency = 200 Hz
LED IN
LED
IN
1) SHDN, 1 V/div, DC
2) I , 100 mA/div, DC
1) SHDN, 1 V/div, DC
2) I , 100 mA/div, DC
L
L
3) I
, 20 mA/div, DC
3) I
, 20 mA/div, DC
LED
LED
T = 100 µs/div
4) V
, 10 V/div, DC
OUT
T = 1 ms/div
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Typical Performance Characteristics (Continued)
Typical VOUT Ripple, OVP Functioning (LM3501-16)
Typical VOUT Ripple, OVP Functioning (LM3501-21)
20065383
20065347
V
open circuit and equals approximately 15V DC, V = 3.0V
V
open circuit and equals approximately 20V DC, V = 3.0V
IN
OUT
IN
OUT
3) V
, 200 mV/div, AC
1) V
, 200 mV/div, AC
OUT
OUT
T = 1 ms/div
T = 400 µs/div
11
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Operation
20065304
FIGURE 2. LM3501 Block Diagram
The LM3501 utilizes a synchronous Current Mode PWM
control scheme to regulate the feedback voltage over almost
all load conditions. The DC/DC controller acts as a controlled
current source ideal for white LED applications. The LM3501
is internally compensated preventing the use of any external
compensation components providing a compact overall so-
lution. The operation can best be understood referring to the
block diagram in Figure 2. At the start of each cycle, the
oscillator sets the driver logic and turns on the NMOS power
device conducting current through the inductor and turns off
the PMOS power device isolating the output from the VSW
pin. The LED current is supplied by the output capacitor
when the NMOS power device is active. During this cycle,
the output voltage of the EAMP 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 minimizing EMI radiation. The EAMP voltage is
compared with a voltage ramp and the sensed switch volt-
age. Once this voltage reaches the EAMP output voltage,
the PWM COMP will then reset the logic turning off the
NMOS power device and turning on the PMOS power de-
vice. The inductor current then flows through the PMOS
power device to the white LED load and output capacitor.
The inductor current recharges the output capacitor and
supplies the current for the white LED branches. The oscil-
lator then sets the driver logic again repeating the process.
The Duty Limit Comp is always operational preventing the
NMOS power switch from being on more than one cycle and
conducting large amounts of current.
The LM3501 has dedicated protection circuitry active during
normal operation to protect the IC and the external compo-
nents. The Thermal Shutdown circuitry turns off both the
NMOS and PMOS power devices when the die temperature
reaches excessive levels. The LM3501 has a UVP Comp
that disables both the NMOS and PMOS power devices
when battery voltages are too low preventing an on state of
the power devices which could conduct large amounts of
current. The OVP Comp prevents the output voltage from
increasing beyond 15.5V (LM3501-16) and 20.5V (LM3501-
21) when the primary white LED network is removed or if
there is an LED failure, allowing the use of small (16V for
LM3501-16 and 25V for LM3501-21) ceramic capacitors at
the output. This comparator has hysteresis that will regulate
the output voltage between 15.5V and 14.6V typically for the
LM3501-16, and between 20.5V and 19.5V for the LM3501-
21. The LM3501 features a shutdown mode that reduces the
supply current to 0.1 uA and isolates the input and output of
the converter. The CNTRL pin can be used to change the
white LED current. A CNTRL voltage above 125 mV will
enable power to the LEDs and a voltage lower than 75 mV
will turn off the power to the LEDs.
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Application Information
Maximum LED VF
# of LEDs
(in series)
LM3501-16
LM3501-21
6.49V
ADJUSTING LED CURRENT
3
4
5
6
7
4.82V
3.61V
2.89V
X
The maximum White LED current is set using the following
equation:
4.86V
3.89V
3.24V
X
2.78V
The LED current can be controlled using an external DC
voltage. The recommended operating range for the voltage
on the CNTRL pin is 0V to 2.7V. When CNTRL is 2.7V, FB =
0.515V (typ.) The FB voltage will continue to increase if
CNTRL is brought above 2.7V (not recommended). The
CNTRL to FB voltage relationship is:
For the LM3501 to operate properly, the output voltage must
be kept above the input voltage during operation. For most
applications, this requires a minimum of 2 LEDs (total of 6V
or more) between the FB and VOUT pins.
OUTPUT OVERVOLTAGE PROTECTION
The LM3501 contains dedicated circuitry for monitoring the
output voltage. In the event that the primary LED network is
disconnected from the LM3501-16, the output voltage will
increase and be limited to 15.5V (typ.). There is a 900 mV
hysteresis associated with this circuitry which will cause the
output to fluctuate between 15.5V and 14.6V (typ.) if the
primary network is disconnected. In the event that the net-
work is reconnected regulation will begin at the appropriate
output voltage. The 15.5V limit allows the use of 16V 1 µF
ceramic output capacitors creating an overall small solution
for white LED applications.
The LED current can be controlled using a PWM signal on
the SHDN pin with frequencies in the range of 100 Hz
(greater than visible frequency spectrum) to 1 kHz. For
controlling LED currents down to the µA levels, it is best to
use a PWM signal frequency between 200-500 Hz. The
LM3501 LED current can be controlled with PWM signal
frequencies above 1 kHz but the controllable current de-
creases with higher frequency. The maximum LED current
would be achieved using the equation above with 100% duty
cycle, ie. the SHDN pin always high.
In the event that the primary LED network is disconnected
from the LM3501-21, the output voltage will increase and be
limited to 20.5V (typ.). There is a 1V hysteresis associated
with this circuitry which will cause the output to fluctuate
between 20.5V and 19.5V (typ.) if the primary network is
disconnected. In the event that the network is reconnected
regulation will begin at the appropriate output voltage. The
20.5V limit allows the use of 25V 1 µF ceramic output
capacitors.
Applying a voltage greater than 125 mV to the CNTRL pin
will begin regulating current to the LEDs. A voltage below 75
mV will prevent application or regulation of the LED current.
LED-DRIVE CAPABILITY
The maximum number of LEDs that can be driven by the
LM3501 is limited by the output voltage capability of the
LM3501. When using the LM3501 in the typical application
configuration, with LEDs stacked in series between the VOUT
and FB pins, the maximum number of LEDs that can be
placed in series (NMAX) is dependent on the maximum LED
forward voltage (VF-MAX), the voltage of the LM3501 feed-
back pin (VFB-MAX = 0.545V), and the minimum output over-
voltage protection level of the chosen LM3501 option
(LM3501-16: OVPMIN = 15V; LM3501-21: OVPMIN = 20V).
For the circuit to function properly, the following inequality
must be met:
RELIABILITY AND THERMAL SHUTDOWN
The maximum continuous pin current for the 8 pin thin micro
SMD package is 535 mA. When driving the device near its
power output limits the VSW pin can see a higher DC current
than 535 mA (see INDUCTOR SELECTION section for av-
erage switch current). To preserve the long term reliability of
the device the average switch current should not exceed 535
mA.
The LM3501 has an internal thermal shutdown function to
protect the die from excessive temperatures. The thermal
shutdown trip point is typically 150˚C. There is a hysteresis
of typically 35˚C so the die temperature must decrease to
approximately 115˚C before the LM3501 will return to normal
operation.
(NMAX x VF-MAX) + 0.545V ≤ OVPMIN
When inserting a value for maximum LED VF, LED forward
voltage variation over the operating temperature range
should be considered. The table below provides maximum
LED voltage numbers for the LM3501-16 and LM3501-21 in
the typical application circuit configuration (with 3, 4, 5, 6, or
7 LEDs placed in series between the VOUT and FB pins).
INDUCTOR SELECTION
The inductor used with the LM3501 must have a saturation
current greater than the cycle by cycle peak inductor current
(see Typical Peak Inductor Currents table below). Choosing
inductors with low DCR decreases power losses and in-
creases efficiency.
13
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The typical cycle-by-cycle peak inductor current can be cal-
culated from the following equation:
Application Information (Continued)
The minimum inductor value required for the LM3501-16 can
be calculated using the following equation:
where IOUT is the total load current, FSW is the switching
frequency, L is the inductance and η is the converter effi-
ciency of the total driven load. A good typical number to use
for η is 0.8. The value of η can vary with load and duty cycle.
The average inductor current, which is also the average VSW
pin current, is given by the following equation:
The minimum inductor value required for the LM3501-21 can
be calculated using the following equation:
For both equations above, L is in µH, VIN is the input supply
of the chip in Volts, RDSON is the ON resistance of the NMOS
power switch found in the Typical Performance Characteris-
tics section in ohms and D is the duty cycle of the switching
regulator. The above equation is only valid for D greater than
or equal to 0.5. For applications where the minimum duty
cycle is less than 0.5, a 22 µH inductor is the typical recom-
mendation for use with most applications. Bench-level veri-
fication of circuit performance is required in these special
cases, however. The duty cycle, D, is given by the following
equation:
The maximum output current capability of the LM3501 can
be estimated with the following equation:
where ICL is the current limit. Some recommended inductors
include but are not limited to:
Coilcraft DT1608C series
Coilcraft DO1608C series
TDK VLP4612 series
TDK VLP5610 series
TDK VLF4012A series
where VOUT is the voltage at pin C1.
CAPACITOR SELECTION
Typical Peak Inductor Current (mA)(Note 10)
Choose low ESR ceramic capacitors for the output to mini-
mize output voltage ripple. Multilayer X7R or X5R type ce-
ramic capacitors are the best choice. For most applications,
a 1 µF ceramic output capacitor is sufficient.
LED Current
30 40
mA mA mA mA mA mA
82 100 134 160 204 234
118 138 190 244 294 352
# LEDs
(in
series)
VIN
(V)
2.7
15
20
50
60
Local bypassing for the input is needed on the LM3501.
Multilayer X7R or X5R ceramic capacitors with low ESR are
a good choice for this as well. A 1 µF ceramic capacitor is
sufficient for most applications. However, for some applica-
tions at least a 4.7 µF ceramic capacitor may be required for
proper startup of the LM3501. Using capacitors with low
ESR decreases input voltage ripple. For additional bypass-
ing, a 100 nF ceramic capacitor can be used to shunt high
frequency ripple on the input. Some recommended capaci-
tors include but are not limited to:
2
3
4
5
2
3
4
5
2
3
4
5
142 174 244 322
191 232 319 413
X
X
X
X
3.3
4.2
76
90
116 136 172 198
110 126 168 210 250 290
132 158 212 270 320
183 216 288 365 446
X
X
TDK C2012X7R1C105K
64
76
96
116 142 162
Taiyo-Yuden EMK212BJ105 G
102 116 148 180 210 246
122 146 186 232 272 318
179 206 263 324 388 456
LAYOUT CONSIDERATIONS
The input bypass capacitor CIN, as shown in Figure 2, must
be placed close to the device and connect between the VIN
and GND pins. This will reduce copper trace resistance
which effects the input voltage ripple of the IC. For additional
input voltage filtering, a 100 nF bypass capacitor can be
placed in parallel with CIN to shunt any high frequency noise
to ground. The output capacitor, COUT, should also be placed
close to the LM3501 and connected directly between the
VOUT and GND pins. Any copper trace connections for the
COUT capacitor can increase the series resistance, which
directly effects output voltage ripple and efficiency. The cur-
rent setting resistor, RLED, should be kept close to the FB pin
Note 10: C = C
= 1 µF
OUT
IN
L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223
2 and 3 LED applications: LM3501-16 or LM3501-21; LED V = 3.77V at
F
20mA; T = 25˚C
A
4 LED applications: LM3501-16 or LM3501-21; LED V = 3.41V at 20mA; T
F
A
= 25˚C
5 LED applications: LM3501-21 only; LED V = 3.28V at 20mA; T = 25˚C
F
A
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14
limit its current driving capability. Trace connections made to
the inductor should be minimized to reduce power dissipa-
tion, EMI radiation and increase overall efficiency. It is good
practice to keep the VSW routing away from sensitive pins
such as the FB pin. Failure to do so may inject noise into the
FB pin and affect the regulation of the device. See Figure 3
and Figure 4 for an example of a good layout as used for the
LM3501 evaluation board.
Application Information (Continued)
to minimize copper trace connections that can inject noise
into the system. The ground connection for the current set-
ting resistor should connect directly to the GND pin. The
AGND pin should connect directly to the GND pin. Not
connecting the AGND pin directly, as close to the chip as
possible, may affect the performance of the LM3501 and
20065384
FIGURE 3. Evaluation Board Layout (2X Magnification)
Top Layer
20065385
FIGURE 4. Evaluation Board Layout (2X Magnification)
Bottom Layer (as viewed from the top)
15
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Application Information (Continued)
20065309
FIGURE 5. 2 White LED Application
20065366
FIGURE 6. Multiple 2 LED String Application
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16
Application Information (Continued)
20065367
FIGURE 7. Multiple 3 LED String Application
20065369
FIGURE 8. LM3501-21 5 LED Application
17
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Physical Dimensions inches (millimeters) unless otherwise noted
8-Bump micro SMD Package (TL)
For Ordering, Refer to Ordering Information Table
NS Package Number TLA08A
X1 = 1.92 mm ( 0.03 mm), X2 = 1.92 mm ( 0.03 mm), X3 = 0.6 mm ( 0.075 mm)
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:
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which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
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.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
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