LM3501
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SNVS230C – DECEMBER 2003 – REVISED MAY 2013
LM3501 Synchronous Step-up DC/DC Converter for White LED Applications
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FEATURES
APPLICATIONS
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Synchronous Rectification, High Efficiency
and no External Schottky Diode required
Uses Small Surface Mount Components
Can Drive 2-5 White LEDs in Series (May
Function with More Low VF LEDs)
2.7V to 7V Input Range
True Shutdown Isolation, no LED Leakage
Current
DC Voltage LED Current Control
Input Undervoltage Lockout
Internal Output Over-Voltage Protection (OVP)
Circuitry, with no External Zener Diode
Required LM3501-16: 15.5V OVP; LM3501-21:
20.5V OVP.
Requires Only a Small 16V (LM3501-16) or 25V
(LM3501-21) Ceramic Capacitor at the Input
and Output
Thermal Shutdown
0.1µA shutdown Current
Small 8-Bump Thin DSBGA Package
LCD Bias Supplies
White LED Back-Lighting
Handheld Devices
Digital Cameras
Portable Applications
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 integrated 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.
Typical Application Circuit
L
22 PH
VIN
2.7V - 5.5V
Voltage
Control
B1
VIN
A3
CIN
1PF
Ceramic
CNTRL
LM3501-16
>1.1V
VOUT, a voltage greater than VIN + 0.3V should not be applied to the VOUT or VSW pins.
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.
Operating Conditions
Junction Temperature
(1)
−40°C to +125°C
Supply Voltage
2.7V to 7V
CNTRL Max.
(1)
2.7V
The maximum allowable power dissipation is a function of the maximum operating junction temperature, TJ(MAX), the junction-to-ambient
thermal resistance, θJA, and the ambient temperature, TA. See the Thermal Properties section for the thermal resistance. The maximum
allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum
allowable power dissipation will cause excessive die temperature.
Thermal Properties
Junction to Ambient Thermal Resistance (θJA)
(1)
4
(1)
75°C/W
Junction-to-ambient thermal resistance (θJA) is highly application and board-layout dependent. The 75ºC/W figure provided was
measured on a 4-layer test 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.
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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.
Symbol
IQ
VFB
Parameter
Conditions
Min
(1)
Typ
Max
0.95
1.2
2
2.5
0.1
2
(2)
(1)
Quiescent Current, Device Not
Switching
FB > 0.54V
Quiescent Current, Device
Switching
FB = 0V
Shutdown
SHDN = 0V
Feedback Voltage
CNTRL = 2.7V,
VIN = 2.7V to 7V
0.485
0.515
0.545
CNTRL = 1V,
VIN = 2.7V to 7V
0.14
0.19
0.24
0.1
0.5
Units
mA
µA
V
ΔVFB
Feedback Voltage Line Regulation
VIN = 2.7V to 7V
ICL
Switch Current Limit
(LM3501-16)
VIN = 2.7V,
Duty Cycle = 80%
275
400
480
VIN = 3.0V,
Duty Cycle = 70%
255
400
530
VIN = 2.7V,
Duty Cycle = 70%
420
640
770
VIN = 3.0V,
Duty Cycle = 63%
450
670
800
45
200
nA
7.0
V
Switch Current Limit
(LM3501-21)
FB Pin Bias Current
VIN
Input Voltage Range
RDSON
NMOS Switch RDSON
VIN = 2.7V, ISW = 300 mA
PMOS Switch RDSON
VOUT = 6V, ISW = 300 mA
Duty Cycle Limit
(LM3501-16)
FB = 0V
Duty Cycle Limit
(LM3501-21)
FB = 0V
DLimit
FB = 0.5V
FSW
Switching Frequency
ISD
SHDN Pin Current
ICNTRL
IL
UVP
OVP
(1)
(2)
(3)
(4)
(4)
CNTRL Pin Current
(4)
mA
(3)
IB
%/V
2.7
0.43
1.3
Ω
2.3
80
87
85
94
0.85
1.0
1.15
1.8
4
SHDN = 2.7V
1
2.5
SHDN = GND
0.1
VCNTRL = 2.7V
10
20
VCNTRL = 1V
4
15
%
SHDN = 5.5V
Switch Leakage Current
(LM3501-16)
VSW = 15V
0.01
0.5
Switch Leakage Current
(LM3501-21)
VSW = 20V
0.01
2.0
Input Undervoltage Lockout
ON Threshold
2.4
2.5
2.6
OFF Threshold
2.3
2.4
2.5
Output Overvoltage Protection
(LM3501-16)
ON Threshold
15
15.5
16
OFF Threshold
14
14.6
15
Output Overvoltage Protection
(LM3501-21)
ON Threshold
20
20.5
21
OFF Threshold
19
19.5
20
MHz
µA
µA
µA
V
V
V
All limits specified at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are
production tested, specified through statistical analysis or specified by design. All limits at temperature extremes are specified via
correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level
(AOQL).
Typical numbers are at 25°C and represent the most likely norm.
Feedback current flows out of the pin.
Current flows into the pin.
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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.
Symbol
IVout
IVL
Parameter
Min
(1)
Typ
Max
(2)
(1)
VOUT Bias Current
(LM3501-16)
VOUT = 15V, SHDN = 1.5V
260
400
VOUT Bias Current
(LM3501-21)
VOUT = 20V, SHDN = 1.5V
300
460
PMOS Switch Leakage Current
(LM3501-16)
VOUT = 15V, VSW = 0V
0.01
3
PMOS Switch Leakage Current
(LM3501-21)
VOUT = 20V, VSW = 0V
0.01
3
CNTRL
Threshold
SHDN
Threshold
Conditions
µA
µA
LED power off
75
LED power on
125
SHDN low
0.65
SHDN High
Units
1.1
mV
0.3
0.65
V
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.
Symbol
IQ
VFB
Parameter
Conditions
Min
(1)
Typ
Max
0.95
1.2
2
2.5
0.1
2
(2)
Quiescent Current, Device Not
Switching
FB > 0.54V
Quiescent Current, Device
Switching
FB = 0V
Shutdown
SHDN = 0V
Feedback Voltage
CNTRL = 2.7V, VIN = 2.7V to 7V
0.485
0.515
0.545
CNTRL = 1V, VIN = 2.7V to 7V
0.14
0.19
0.24
0.5
Feedback Voltage Line Regulation
VIN = 2.7V to 7V
0.1
ICL
Switch Current Limit
(LM3501-16)
VIN = 3.0V,
Duty Cycle = 70%
400
Switch Current Limit
(LM3501-21)
VIN = 3.0V,
Duty Cycle = 63%
670
IB
FB Pin Bias Current
FB = 0.5V
VIN
Input Voltage Range
RDSON
NMOS Switch RDSON
VIN = 2.7V, ISW = 300 mA
PMOS Switch RDSON
VOUT = 6V, ISW = 300 mA
Duty Cycle Limit
(LM3501-16)
FB = 0V
Duty Cycle Limit
(LM3501-21)
FB = 0V
FSW
Switching Frequency
ISD
SHDN Pin Current
(1)
(2)
(3)
(4)
6
(4)
Units
mA
ΔVFB
DLimit
(1)
V
%/V
mA
(3)
45
2.7
200
nA
7.0
V
0.43
1.3
2.3
Ω
87
%
94
0.8
1.0
1.2
1.8
4
SHDN = 2.7V
1
2.5
SHDN = GND
0.1
SHDN = 5.5V
µA
MHz
µA
All limits specified at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are
production tested, specified through statistical analysis or specified by design. All limits at temperature extremes are specified via
correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level
(AOQL).
Typical numbers are at 25°C and represent the most likely norm.
Feedback current flows out of the pin.
Current flows into the pin.
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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.
Symbol
ICNTRL
IL
UVP
OVP
IVout
IVL
CNTRL Pin Current
(4)
Max
VCNTRL = 2.7V
10
20
VCNTRL = 1V
4
15
Conditions
Min
(1)
(2)
(1)
Switch Leakage Current
(LM3501-16)
VSW = 15V
0.01
0.5
Switch Leakage Current
(LM3501-21)
VSW = 20V
0.01
2.0
Input Undervoltage Lockout
ON Threshold
2.4
2.5
2.6
OFF Threshold
2.3
2.4
2.5
Output Overvoltage Protection
(LM3501-16)
ON Threshold
15
15.5
16
OFF Threshold
14
14.6
15
Output Overvoltage Protection
(LM3501-21)
ON Threshold
20
20.5
21
OFF Threshold
19
19.5
20
VOUT Leakage Current
(LM3501-16)
VOUT = 15V, SHDN = 1.5V
260
400
VOUT Leakage Current
(LM3501-21)
VOUT = 20V, SHDN = 1.5V
300
460
PMOS Switch Leakage Current
(LM3501-16)
VOUT = 15V, VSW = 0V
0.01
3
PMOS Switch Leakage Current
(LM3501-21)
VOUT = 20V, VSW = 0V
0.01
3
CNTRL
Threshold
SHDN
Threshold
Typ
Parameter
LED power on
125
SHDN low
0.65
1.1
V
V
mV
0.3
0.65
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µA
µA
75
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µA
µA
LED power off
SHDN High
Units
V
7
LM3501
SNVS230C – DECEMBER 2003 – REVISED MAY 2013
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Typical Performance Characteristics
8
Switching Quiescent Current
vs.
VIN
Non-Switching Quiescent Current
vs.
VIN
Figure 3.
Figure 4.
2 LED Efficiency
vs.
Load Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(2VLED*ILED))
3 LED Efficiency
vs.
Load Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(3VLED*ILED))
Figure 5.
Figure 6.
4 LED Efficiency
vs.
Load Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(4VLED*ILED))
Output Power
vs.
VIN
(LM3501-16, L = Coilcraft DT1608C-223)
Figure 7.
Figure 8.
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Typical Performance Characteristics (continued)
Output Power
vs.
Temperature
(LM3501-16, L = Coilcraft DT1608C-223)
FB Pin Current
vs.
Temperature
Figure 9.
Figure 10.
SHDN Pin Current
vs.
SHDN Pin Voltage
CNTRL Pin Current
vs.
CNTRL Pin Voltage
Figure 11.
Figure 12.
FB Voltage
vs.
CNTRL Voltage
Switch Current Limit
vs.
VIN
(LM3501-16)
Figure 13.
Figure 14.
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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)
Figure 15.
Figure 16.
Switch Current Limit
vs.
VIN
(LM3501-21)
Switch Current Limit
vs.
Temperature
(LM3501-21, VOUT = 8V)
1300
1100
1200
VOUT = 8V
900
UT
VO
800
=1
2V
CURRENT LIMIT (mA)
CURRENT LIMIT (mA)
1000
700
600
UT
=1
5V
VO
500
400
VOUT = 18V
3.0 3.5
4.0
4.5 5.0
5.5
6.0
1000
VIN = 4.2V
900
800
700
600
300
200
2.5
VIN = 5.5V
1100
VIN = 3.0V
500
-40
6.5
-15
INPUT VOLTAGE (V)
35
60
Figure 17.
Figure 18.
Switch Current Limit
vs.
Temperature
(LM3501-21, VOUT = 12V)
Switch Current Limit
vs.
Temperature
(LM3501-21, VOUT = 18V)
850
85
440
800
420
VIN = 5.5V
V IN = 5.5V
400
750
CURRENT LIMIT (mA)
CURRENT LIMIT (mA)
10
TEMPERATURE (ºC)
700
650
VIN = 4.2V
600
550
V IN = 3.0V
380
360
340
V IN = 4.2V
320
300
280
500
450
-40
VIN = 3.0V
-15
10
35
260
60
85
-15
10
35
60
85
TEMPERATURE °C
TEMPERATURE (ºC)
Figure 19.
10
240
-40
Figure 20.
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Typical Performance Characteristics (continued)
Oscillator Frequency
vs.
VIN
VOUT DC Bias
vs.
VOUT Voltage
(LM3501-16)
Figure 21.
Figure 22.
FB Voltage
vs.
Temperature
FB Voltage
vs.
Temperature
Figure 23.
Figure 24.
FB Voltage
vs.
VIN
NMOS RDSON
vs.
VIN
(ISW = 300 mA)
Figure 25.
Figure 26.
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Typical Performance Characteristics (continued)
PMOS RDSON
vs.
Temperature
Typical VIN Ripple
3 LEDs, RLED = 22Ω, VIN = 3.0V, CNTRL = 2.7V
1) SW, 10 V/div, DC
3) IL, 100 mA/div, DC
4) VIN, 100 mV/div, AC
T = 250 ns/div
Figure 28.
Figure 27.
Start-Up (LM3501-16)
SHDN Pin Duty Cycle Control Waveforms
3 LEDs, RLED = 22Ω, VIN = 3.0V, CNTRL = 2.7V
1) SHDN, 1 V/div, DC
2) IL, 100 mA/div, DC
3) ILED, 20 mA/div, DC
T = 100 µs/div
LM3501-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V, SHDN frequency = 200
Hz
1) SHDN, 1 V/div, DC
2) IL, 100 mA/div, DC
3) ILED, 20 mA/div, DC
4) VOUT, 10 V/div, DC
T = 1 ms/div
Figure 30.
Figure 29.
Typical VOUT Ripple, OVP Functioning (LM3501-16)
Typical VOUT Ripple, OVP Functioning (LM3501-21)
T
1
VOUT open circuit and equals approximately 15V DC, VIN = 3.0V
3) VOUT, 200 mV/div, AC
T = 1 ms/div
Figure 31.
12
VOUT open circuit and equals approximately 20V DC, VIN = 3.0V
1) VOUT, 200 mV/div, AC
T = 400 µs/div
Figure 32.
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Operation
L
VIN
B1 VIN
C2
UVP
REF
UVP
COMP
THERMAL
SHUTDOWN
+
REF
+
OVP
COMP
LIGHT LOAD
COMP
VSW
VOUT
C1
+
-
OVP
REF
x
CIN
FB
B3
COUT
Reset Reset Reset
Reset
DriveP
-
LOGIC
PWM
COMP
+
EAMP
+
Reset
SET Reset
DriveN
Reset
Body Diode
Control
+
Current
Sense
osc
FB
R LED
CNTRL
A3
+
Dlimit
A1
Duty Limit
Comp
SHUTDOWN
COMP
-
A2
AGND
SHDN
C3
GND
Figure 33. 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 thus eliminating the requirement for any external
compensation components providing a compact overall solution. The operation can best be understood referring
to the block diagram in Figure 33. 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 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.
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APPLICATION INFORMATION
ADJUSTING LED CURRENT
The maximum White LED current is set using the following equation:
ILED = VFB(MAX)/RLED
(1)
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:
FB = 0.191*CNTRL
(2)
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 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 feedback pin (VFB-MAX = 0.545V), and the
minimum output overvoltage protection level of the chosen LM3501 option (LM3501-16: OVPMIN = 15V; LM350121: OVPMIN = 20V). For the circuit to function properly, the following inequality must be met:
(NMAX x VF-MAX) + 0.545V ≤ OVPMIN
(3)
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).
Maximum LED VF
# of LEDs
(in series)
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 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 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.
14
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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 DSBGA 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 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 Table 1 below). Choosing inductors with low DCR decreases power losses and increases efficiency.
The minimum inductor value required for the LM3501-16 can be calculated using the following equation:
L>
VIN RDSON
D
-1
D'
0.29
(4)
The minimum inductor value required for the LM3501-21 can be calculated using the following equation:
L>
VIN RDSON
D
-1
D'
0.58
(5)
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 Typical Performance Characteristics 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:
VIN
=1-D
D' = V
OUT
(6)
where VOUT is the voltage at pin C1.
Table 1. Typical Peak Inductor Current (mA) (1)
VIN
(V)
# LEDs
(in series)
2.7
3.3
(1)
LED Current
15
mA
20
mA
30
mA
40
mA
50
mA
60
mA
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
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
CIN = COUT = 1 μF, L = 22 μH, 160 mΩ DCR max. Coilcraft DT1608C-2232 and 3 LED applications: LM3501-16 or LM3501-21; LED VF
= 3.77V at 20mA; TA = 25°C4 LED applications: LM3501-16 or LM3501-21; LED VF = 3.41V at 20mA; TA = 25°C5 LED applications:
LM3501-21 only; LED VF = 3.28V at 20mA; TA = 25°C
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Table 1. Typical Peak Inductor Current (mA)(1) (continued)
LED Current
VIN
(V)
# LEDs
(in series)
4.2
2
64
3
102
4
122
5
179
15
mA
20
mA
30
mA
40
mA
50
mA
60
mA
76
96
116
142
162
116
148
180
210
246
146
186
232
272
318
206
263
324
388
456
The typical cycle-by-cycle peak inductor current can be calculated from the following equation:
IPK |
IOUT
KD'
+
VIND
2LFSW
(7)
where IOUT is the total load current, FSW is the switching 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 VSW pin current, is given by the following
equation:
IL(AVE) |
IOUT
KD'
(8)
The maximum output current capability of the LM3501 can be estimated with the following equation:
IOUT | KD' ICL -
VIND
2LFSW
(9)
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
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 CIN, as shown in Figure 33, 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 current setting
16
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resistor, RLED, 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 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 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 34 and Figure 35 for an
example of a good layout as used for the LM3501 evaluation board.
Figure 34. Evaluation Board Layout (2X Magnification)
Top Layer
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Figure 35. Evaluation Board Layout (2X Magnification)
Bottom Layer (as viewed from the top)
L
22 PH
VIN
2.7V - 5.5V
Voltage
Control
B1
VIN
A3
CIN
1PF
Ceramic
C2
VSW
CNTRL
LM3501-16
>1.1V
1.1V
1.1V
1.1V
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