查询LM3501TL-16供应商
LM3501
Synchronous Step-up DC/DC Converter for White LED
Applications
Synchronous Step-up DC/DC Converter for White LED Applications
May 2005
General Description
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 synchronous 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 operates 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.
Two LM3501 options are available with different output voltage 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 protection circuitry on the output to prevent a destructive overvoltage event.
A single external resistor is used to set the maximum LED
current in LED-drive applications. The LED current can easily 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 completely disconnects the input from output, creating total isolation and preventing any leakage currents from trickling into
the LEDs.
Features
n Synchronous rectification, high efficiency and no
external schottky diode required
n Uses small surface mount components
n Can drive 2-5 white LEDs in series (may function with
more low V
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
n 0.1µA shutdown current
n Small 8-bump thin micro SMD package
LEDs)
F
Applications
n LCD Bias Supplies
n White LED Back-Lighting
n Handheld Devices
n Digital Cameras
n Portable Applications
Typical Application Circuit
20065301
FIGURE 1. Typical 3 LED Application
© 2005 National Semiconductor Corporation DS200653 www.national.com
Connection Diagram
LM3501
Top View
8-bump micro SMD
20065302
Ordering Information
Order Number Package Type NSC Package Drawing Top Mark Supplied As
LM3501TL-16 micro SMD TL08SSA 19 250 Units, Tape and Reel
LM3501TLX-16 micro SMD TL08SSA 19 3000 Units, Tape and Reel
LM3501TL-21 micro SMD TL08SSA 30 250 Units, Tape and Reel
LM3501TLX-21 micro SMD TL08SSA 30 3000 Units, Tape and Reel
Pin Description/Functions
Pin Name Function
A1 AGND Analog ground.
B1 V
C1 V
C2 V
IN
OUT
SW
C3 GND Power Ground.
B3 FB Output voltage feedback connection.
A3 CNTRL Analog LED current control.
A2 SHDN
AGND (pin A1): Analog ground pin. The analog ground pin
should tie directly to the GND pin.
V
(pin B1):Analog and Power supply pin. Bypass this pin
IN
with a capacitor, as close to the device as possible, connected between the V
(pin C1):Source connection of internal PMOS power
V
OUT
and GND pins.
IN
device. Connect the output capacitor between the V
GND pins as close as possible to the device.
(pin C2):Drain connection of internal NMOS and PMOS
V
SW
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.
Analog and Power supply input.
PMOS source connection for synchronous rectification.
Switch pin. Drain connections of both NMOS and PMOS power devices.
Shutdown control 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.
CNTRL (pin A3): Analog control of LED current. A voltage
above 125 mV will begin to regulate the LED current. De-
OUT
and
creasing the voltage below 75 mV will turn off the LEDs.
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.
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LM3501
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
V
IN
V
(LM3501-16)(Note 2) −0.3V to 16V
OUT
V
(LM3501-21)(Note 2) −0.3V to 21V
OUT
V
(Note 2) −0.3V to V
SW
FB Voltage −0.3V to 7.5V
SHDN Voltage
−0.3V to 7.5V
+0.3V
OUT
−0.3V to VIN+0.3V
ESD Ratings (Note 3)
Human Body Model 2kV
Machine Model 200V
Operating Conditions
Junction Temperature
(Note 4) −40˚C to +125˚C
Supply Voltage 2.7V to 7V
CNTRL Max. 2.7V
CNTRL −0.3V to 7.5V
Maximum Junction Temperature 150˚C
Lead Temperature
(Soldering 10 sec.) 300˚C
Thermal Properties
Junction to Ambient Thermal
Resistance (θ
) (Note 5)
JA
Vapor Phase
(60 sec.) 215˚C
Infrared
(15 sec.) 220˚C
Electrical Characteristics
Specifications in standard type face are for TA= 25˚C and those in boldface type apply over the Operating Temperature
Range of T
LM3501-21.
Symbol Parameter Conditions
I
Q
V
FB
∆V
FB
I
CL
I
B
V
IN
R
DSON
D
Limit
F
SW
= −10˚C to +85˚C. Unless otherwise specified VIN= 2.7V and specifications apply to both LM3501-16 and
A
Quiescent Current, Device Not
FB>0.54V
Switching
Quiescent Current, Device
FB=0V
Switching
Shutdown SHDN = 0V
Feedback Voltage CNTRL = 2.7V,
= 2.7V to 7V
V
IN
CNTRL = 1V,
= 2.7V to 7V
V
IN
Feedback Voltage Line
VIN= 2.7V to 7V
Regulation
Switch Current Limit
(LM3501-16)
VIN= 2.7V,
Duty Cycle = 80%
= 3.0V,
V
IN
Duty Cycle = 70%
Switch Current Limit
(LM3501-21)
VIN= 2.7V,
Duty Cycle = 70%
= 3.0V,
V
IN
Duty Cycle = 63%
Min
(Note 6)
0.485 0.515 0.545
0.14 0.19 0.24
275 400 480
255 400 530
420 640 770
450 670 800
Typ
(Note 7)
Max
(Note 6)
0.95 1.2
2 2.5
0.1 2 µA
0.1 0.5 %/V
FB Pin Bias Current FB = 0.5V (Note 8) 45 200 nA
Input Voltage Range 2.7 7.0 V
NMOS Switch R
PMOS Switch R
Duty Cycle Limit
(LM3501-16)
Duty Cycle Limit
(LM3501-21)
DSON
DSON
VIN= 2.7V, ISW= 300 mA 0.43
V
= 6V, ISW= 300 mA 1.3 2.3
OUT
FB=0V
FB=0V
80 87
85 94
Switching Frequency 0.85 1.0 1.15 MHz
75˚C/W
Units
mA
V
mA
Ω
%
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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
LM3501
Range of T
LM3501-21.
Symbol Parameter Conditions
I
SD
I
CNTRL
I
L
UVP Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6
OVP Output Overvoltage Protection
I
Vout
I
VL
CNTRL
Threshold
SHDN
Threshold
Specifications in standard type face are for TJ= 25˚C and those in boldface type apply over the full Operating Temperature
Range (T
LM3501-21.
Symbol Parameter Conditions
I
Q
V
FB
∆V
FB
I
CL
I
B
V
IN
= −10˚C to +85˚C. Unless otherwise specified VIN= 2.7V and specifications apply to both LM3501-16 and
A
Min
(Note 6)
Typ
(Note 7)
Max
(Note 6)
SHDN Pin Current (Note 9) SHDN = 5.5V 1.8 4
1 2.5
CNTRL Pin Current (Note 9) V
Switch Leakage Current
(LM3501-16)
Switch Leakage Current
(LM3501-21)
SHDN = GND
= 2.7V 10 20
CNTRL
V
=1V 4 15
CNTRL
VSW= 15V
VSW= 20V
0.1
0.01 0.5
0.01 2.0
OFF Threshold 2.3 2.4 2.5
ON Threshold 15 15.5 16
(LM3501-16)
Output Overvoltage Protection
(LM3501-21)
V
Bias Current
OUT
(LM3501-16)
V
Bias Current
OUT
(LM3501-21)
PMOS Switch Leakage Current
(LM3501-16)
PMOS Switch Leakage Current
(LM3501-21)
OFF Threshold 14 14.6 15
ON Threshold 20 20.5 21
OFF Threshold 19 19.5 20
V
= 15V, SHDN = 1.5V
OUT
V
= 20V, SHDN = 1.5V
OUT
V
= 15V, VSW=0V
OUT
= 20V, VSW=0V
V
OUT
260 400
300 460
0.01 3
0.01 3
LED power off 75
LED power on 125
SHDN low
SHDN High
= −40˚C to +125˚C). Unless otherwise specified VIN=2.7V and specifications apply to both LM3501-16 and
J
1.1 0.65
Min
(Note 6)
Quiescent Current, Device Not
FB>0.54V
Switching
Quiescent Current, Device
FB=0V
Switching
Shutdown SHDN = 0V
0.65 0.3
Typ
(Note 7)
Max
(Note 6)
0.95 1.2
2 2.5
0.1 2 µA
Feedback Voltage CNTRL = 2.7V, VIN= 2.7V to 7V 0.485 0.515 0.545
Feedback Voltage Line
Regulation
Switch Current Limit
(LM3501-16)
Switch Current Limit
(LM3501-21)
CNTRL = 1V, V
VIN= 2.7V to 7V
VIN= 3.0V,
Duty Cycle = 70%
VIN= 3.0V,
Duty Cycle = 63%
= 2.7V to 7V 0.14 0.19 0.24
IN
0.1 0.5 %/V
400
670
FB Pin Bias Current FB = 0.5V (Note 8) 45 200 nA
Input Voltage Range 2.7 7.0 V
Units
µASHDN = 2.7V
µA
µA
V
V
V
µA
µA
mV
V
Units
mA
V
mA
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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 (T
LM3501-21.
Symbol Parameter Conditions
R
DSON
D
Limit
F
SW
I
SD
I
CNTRL
I
L
UVP Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6
OVP Output Overvoltage Protection
I
Vout
I
VL
CNTRL
Threshold
SHDN
Threshold
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.
Note 2: This condition applies if V
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
resistance, θ
ambient temperature is calculated using: P
Note 5: Junction-to-ambient thermal resistance (θ
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.
= −40˚C to +125˚C). Unless otherwise specified VIN=2.7V and specifications apply to both LM3501-16 and
J
NMOS Switch R
PMOS Switch R
Duty Cycle Limit
(LM3501-16)
Duty Cycle Limit
(LM3501-21)
DSON
DSON
Min
(Note 6)
VIN= 2.7V, ISW= 300 mA 0.43
V
= 6V, ISW= 300 mA 1.3 2.3
OUT
FB=0V
FB=0V
Typ
(Note 7)
87
94
Max
(Note 6)
Switching Frequency 0.8 1.0 1.2 MHz
SHDN Pin Current (Note 9) SHDN = 5.5V 1.8 4
1 2.5
CNTRL Pin Current (Note 9) V
Switch Leakage Current
SHDN = GND
= 2.7V 10 20
CNTRL
V
=1V 4 15
CNTRL
VSW= 15V 0.01 0.5
0.1
(LM3501-16)
Switch Leakage Current
VSW= 20V 0.01 2.0
(LM3501-21)
OFF Threshold 2.3 2.4 2.5
ON Threshold 15 15.5 16
(LM3501-16)
Output Overvoltage Protection
(LM3501-21)
V
Leakage Current
OUT
(LM3501-16)
V
Leakage Current
OUT
(LM3501-21)
PMOS Switch Leakage Current
(LM3501-16)
PMOS Switch Leakage Current
(LM3501-21)
OFF Threshold 14 14.6 15
ON Threshold 20 20.5 21
OFF Threshold 19 19.5 20
V
= 15V, SHDN = 1.5V
OUT
V
= 20V, SHDN = 1.5V
OUT
V
= 15V, VSW=0V
OUT
= 20V, VSW=0V
V
OUT
260 400
300 460
0.01 3
0.01 3
LED power off 75
LED power on 125
SHDN low
SHDN High
<
V
IN
OUT
, and the ambient temperature, TA. See the Thermal Properties section for the thermal resistance. The maximum allowable power dissipation at any
JA
D
>
.IfV
V
IN
(MAX) = (T
JA
, a voltage greater than VIN+ 0.3V should not be applied to the V
OUT
J(MAX)−TA
) is highly application and board-layout dependent. The 75oC/W figure provided was measured on a 4-layer test
)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature.
1.1 0.65
0.65 0.3
or VSWpins.
OUT
, the junction-to-ambient thermal
J(MAX)
LM3501
Units
Ω
%
µASHDN = 2.7V
µA
µA
V
V
µA
µA
mV
V
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Typical Performance Characteristics
LM3501
Switching Quiescent Current vs. V
IN
Non-Switching Quiescent Current vs. V
IN
2 LED Efficiency vs. Load Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(P
IN
/(2V
LED*ILED
4 LED Efficiency vs. Load Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(P
IN
/(4V
LED*ILED
20065355
20065356
3 LED Efficiency vs. Load Current
L = Coilcraft DT1608C-223,
))
20065357 20065358
))
Efficiency = 100*(PIN/(3V
Output Power vs. V
LED*ILED
))
IN
(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
LM3501
FB Voltage vs. CNTRL Voltage
20065377
20065379
Switch Current Limit vs. V
(LM3501-16)
20065378
IN
20065362
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Typical Performance Characteristics (Continued)
LM3501
Switch Current Limit vs. Temperature
(LM3501-16, V
Switch Current Limit vs. V
OUT
= 8V)
20065363
IN
(LM3501-21)
Switch Current Limit vs. Temperature
(LM3501-16, V
OUT
= 12V)
20065376
Switch Current Limit vs. Temperature
(LM3501-21, V
OUT
= 8V)
20065331
Switch Current Limit vs. Temperature
(LM3501-21, V
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OUT
= 12V)
20065333
20065332
Switch Current Limit vs. Temperature
(LM3501-21, V
OUT
= 18V)
20065345
Typical Performance Characteristics (Continued)
Oscillator Frequency vs. V
IN
V
DC Bias vs. V
OUT
(LM3501-16)
OUT
LM3501
Voltage
20065364
FB Voltage vs. Temperature FB Voltage vs. Temperature
20065380 20065382
FB Voltage vs. V
NMOS R
IN
(ISW= 300 mA)
DSON
vs. V
IN
20065365
20065381
20065374
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Typical Performance Characteristics (Continued)
LM3501
PMOS R
vs. Temperature Typical VINRipple
DSON
20065375
3 LEDs, R
1) SW, 10 V/div, DC
, 100 mA/div, DC
3) I
L
4) V
IN
=22Ω,VIN= 3.0V, CNTRL = 2.7V
LED
, 100 mV/div, AC
T = 250 ns/div
Start-Up (LM3501-16) SHDN Pin Duty Cycle Control Waveforms
20065368
3 LEDs, R
=22Ω,VIN= 3.0V, CNTRL = 2.7V
LED
20065371
1) SHDN, 1 V/div, DC
2) IL, 100 mA/div, DC
, 20 mA/div, DC
3) I
LED
T = 100 µs/div
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LM3501-16, 3 LEDs, R
1) SHDN, 1 V/div, DC
2) IL, 100 mA/div, DC
, 20 mA/div, DC
3) I
LED
, 10 V/div, DC
4) V
OUT
T = 1 ms/div
=22Ω,VIN= 3.0V, SHDN frequency = 200 Hz
LED
20065346
Typical Performance Characteristics (Continued)
LM3501
Typical V
V
open circuit and equals approximately 15V DC, VIN= 3.0V
OUT
, 200 mV/div, AC
3) V
OUT
T = 1 ms/div
Ripple, OVP Functioning (LM3501-16) Typical V
OUT
20065383
Ripple, OVP Functioning (LM3501-21)
OUT
V
open circuit and equals approximately 20V DC, VIN= 3.0V
OUT
, 200 mV/div, AC
1) V
OUT
T = 400 µs/div
20065347
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Operation
LM3501
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 solution. 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 V
SW
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 voltage. 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 device. 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 oscillator 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 components. 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.
www.national.com 12
LM3501
Application Information
ADJUSTING LED CURRENT
The maximum White LED current is set using the following
equation:
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:
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 decreases with higher frequency. The maximum LED current
would be achieved using the equation above with 100% duty
cycle, ie. the SHDN pin always high.
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 V
and FB pins, the maximum number of LEDs that can be
placed in series (N
forward voltage (V
back pin (V
FB-MAX
) is dependent on the maximum LED
MAX
), the voltage of the LM3501 feed-
F-MAX
= 0.545V), and the minimum output overvoltage protection level of the chosen LM3501 option
(LM3501-16: OVP
= 15V; LM3501-21: OVP
MIN
MIN
For the circuit to function properly, the following inequality
must be met:
(N
MAXxVF-MAX
) + 0.545V ≤ OVP
MIN
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 V
and FB pins).
OUT
OUT
= 20V).
# of LEDs
(in series)
Maximum LED V
F
LM3501-16 LM3501-21
3 4.82V 6.49V
4 3.61V 4.86V
5 2.89V 3.89V
6 X 3.24V
7 X 2.78V
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 V
OUT
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 network 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.
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.
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 V
pin can see a higher DC current
SW
than 535 mA (see INDUCTOR SELECTION section for average 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.
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 increases efficiency.
www.national.com13
Application Information (Continued)
The minimum inductor value required for the LM3501-16 can
LM3501
be calculated using the following equation:
The typical cycle-by-cycle peak inductor current can be calculated from 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, VINis the input supply
of the chip in Volts, R
is the ON resistance of the NMOS
DSON
power switch found in the Typical Performance Characteristics 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 recommendation for use with most applications. Bench-level verification of circuit performance is required in these special
cases, however. The duty cycle, D, is given by the following
equation:
where V
is the voltage at pin C1.
OUT
Typical Peak Inductor Current (mA)(Note 10)
LED Current
mA
VIN
(V)
# LEDs
(in
series)
15mA20mA30mA40mA50mA60
2.7 2 82 100 134 160 204 234
3 118 138 190 244 294 352
4 142 174 244 322 X X
5 191 232 319 413 X X
3.3 2 76 90 116 136 172 198
3 110 126 168 210 250 290
4 132 158 212 270 320 X
5 183 216 288 365 446 X
2 64 76 96 116 142 162
4.2
3 102 116 148 180 210 246
4 122 146 186 232 272 318
5 179 206 263 324 388 456
Note 10: CIN=C
L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223
2 and 3 LED applications: LM3501-16 or LM3501-21; LED V
20mA; T
4 LED applications: LM3501-16 or LM3501-21; LED V
= 25˚C
5 LED applications: LM3501-21 only; LED V
= 25˚C
A
OUT
=1µF
= 3.77V at
F
= 3.41V at 20mA; T
F
= 3.28V at 20mA; TA= 25˚C
F
where I
is the total load current, FSWis the switching
OUT
frequency, L is the inductance and η is the converter efficiency 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 V
SW
pin current, is given by the following equation:
The maximum output current capability of the LM3501 can
be estimated with the following equation:
where ICLis 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
CAPACITOR SELECTION
Choose low ESR ceramic capacitors for the output to minimize output voltage ripple. Multilayer X7R or X5R type ceramic capacitors are the best choice. For most applications,
a 1 µF ceramic output capacitor is sufficient.
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 applications 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 bypassing, a 100 nF ceramic capacitor can be used to shunt high
frequency ripple on the input. Some recommended capacitors include but are not limited to:
TDK C2012X7R1C105K
Taiyo-Yuden EMK212BJ105 G
LAYOUT CONSIDERATIONS
The input bypass capacitor C
be placed close to the device and connect between the V
, as shown in Figure 2, must
IN
IN
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
A
placed in parallel with C
to ground. The output capacitor, C
to shunt any high frequency noise
IN
, should also be placed
OUT
close to the LM3501 and connected directly between the
and GND pins. Any copper trace connections for the
V
OUT
capacitor can increase the series resistance, which
C
OUT
directly effects output voltage ripple and efficiency. The current setting resistor, R
, should be kept close to the FB pin
LED
www.national.com 14
Application Information (Continued)
to minimize copper trace connections that can inject noise
into the system. The ground connection for the current setting 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
limit its current driving capability. Trace connections made to
the inductor should be minimized to reduce power dissipation, EMI radiation and increase overall efficiency. It is good
practice to keep the V
routing away from sensitive pins
SW
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.
LM3501
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)
www.national.com15
Application Information (Continued)
LM3501
20065309
FIGURE 5. 2 White LED Application
FIGURE 6. Multiple 2 LED String Application
www.national.com 16
20065366
Application Information (Continued)
FIGURE 7. Multiple 3 LED String Application
LM3501
20065367
FIGURE 8. LM3501-21 5 LED Application
20065369
www.national.com17
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.
Synchronous Step-up DC/DC Converter for White LED Applications
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