LM3500
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SNVS231G – AUGUST 2003 – REVISED MAY 2013
LM3500 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
Internal Output Over-Voltage Protection (OVP)
Circuitry, with no External Zener Diode
Required
– LM3500-16: 15.5V OVP; LM3500-21: 20.5V
OVP.
True Shutdown Isolation
Input Undervoltage Lockout
Requires Only Small Ceramic Capacitors at the
Input and Output
Thermal Shutdown
0.1µA Shutdown Current
Small 8-Bump Thin DSBGA Package
LCD Bias Supplies
White LED Backlighting
Handheld Devices
Digital Cameras
Portable Applications
DESCRIPTION
The LM3500 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
(500mV), power efficiency of the LM3500 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 LM3500
operates with a fixed 1MHz switching frequency.
When used with ceramic input and output capacitors,
the LM3500 provides a small, low-noise, low-cost
solution.
Typical Application Circuit
L
22 PH
VIN
2.7V - 5.5V
B1
VIN
A3
CIN
1PF
Ceramic
>1.1V
VOUT, a voltage greater than VIN + 0.3V should not be applied to the VOUT or VSW pins.
For more detailed soldering information and specifications, please refer to Texas Instruments Application Note 1112: DSBGA Wafer
Level Chip Scale Package
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
Ambient Temperature (1)
−40°C to +85°C
−40°C to +125°C
Junction Temperature
Supply Voltage
(1)
2.7V to 7V
In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =
125ºC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the
part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).
Thermal Properties
Junction to Ambient Thermal Resistance (θJA) (1)
(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 specification apply to both LM3500-16 and
LM3500-21.
Symbol
IQ
Parameter
Conditions
Min (1)
Typ (2)
Max (1)
Quiescent Current, Device Not
Switching
FB > 0.54V
0.95
1.2
Quiescent Current, Device
Switching
FB = 0V
1.8
2.5
mA
Shutdown
SHDN = 0V
VFB
Feedback Voltage
VIN = 2.7V to 7V
ΔVFB
Feedback Voltage Line
Regulation
VIN = 2.7V to 7V
ICL
Switch Current Limit
(LM3500-16)
VIN = 2.7V,
Duty Cycle = 80%
Switch Current Limit
(LM3500-21)
Units
0.1
2
µA
0.5
0.53
V
0.1
0.4
%/V
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
0.47
mA
FB = 0.5V (3)
IB
FB Pin Bias Current
VIN
Input Voltage Range
RDSON
NMOS Switch RDSON
VIN = 2.7V, ISW = 300mA
PMOS Switch RDSON
VOUT = 6V, ISW = 300mA
Duty Cycle Limit (LM3500-16)
FB = 0V
80
87
Duty Cycle Limit (LM3500-21)
FB = 0V
85
94
0.85
1.0
1.15
SHDN = 5.5V
18
30
SHDN = 2.7V
9
16
µA
SHDN = GND
0.1
Switch Leakage Current
(LM3500-16)
VSW = 15V
0.01
0.5
µA
Switch Leakage Current
(LM3500-21)
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
(LM3500-16)
ON Threshold
15
15.5
16
OFF Threshold
14
14.6
15
Output Overvoltage Protection
(LM3500-21)
ON Threshold
20
20.5
21
OFF Threshold
19
19.5
20
VOUT Bias Current
(LM3500-16)
VOUT = 15V, SHDN = VIN
260
400
VOUT Bias Current
(LM3500-21)
VOUT = 20V, SHDN = VIN
300
460
PMOS Switch Leakage
Current (LM3500-16)
VOUT = 15V, VSW = 0V
0.01
3
PMOS Switch Leakage
Current (LM3500-21)
VOUT = 20V, VSW = 0V
0.01
3
DLimit
FSW
Switching Frequency
ISD
SHDN Pin Current
IL
UVP
OVP
IVout
IVL
(1)
(2)
(3)
(4)
4
2.7
(4)
0.43
1.1
2.3
Ω
%
MHz
V
V
µA
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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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 specification apply to both LM3500-16 and
LM3500-21.
Symbol
SHDN
Threshold
Parameter
Conditions
Min (1)
SHDN Low
SHDN High
1.1
Typ (2)
Max (1)
0.65
0.3
Units
V
0.65
Electrical Characteristics
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 specification apply to both LM3500-16 and
LM3500-21.
Symbol
IQ
Parameter
Conditions
Min
(1)
Typ
Max
(2)
(1)
Quiescent Current, Device Not
Switching
FB > 0.54V
0.95
1.2
Quiescent Current, Device
Switching
FB = 0V
1.8
2.5
Units
mA
Shutdown
SHDN = 0V
VFB
Feedback Voltage
VIN = 2.7V to 7V
0.1
2
µA
0.5
0.53
V
ΔVFB
Feedback Voltage Line
Regulation
VIN = 2.7V to 7V
0.1
0.4
%/V
ICL
Switch Current Limit
(LM3500-16)
VIN = 3.0V, Duty Cycle = 70%
400
Switch Current Limit
(LM3500-21)
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 = 300mA
PMOS Switch RDSON
VOUT = 6V, ISW = 300mA
1.1
Duty Cycle Limit (LM3500-16)
FB = 0V
87
Duty Cycle Limit (LM3500-21)
FB = 0V
94
DLimit
FSW
Switching Frequency
ISD
SHDN Pin Current (4)
IL
UVP
OVP
(1)
(2)
(3)
(4)
0.47
mA
(3)
45
2.7
200
nA
7.0
V
0.43
0.8
2.3
Ω
%
1.0
1.2
18
30
SHDN = 2.7V
9
16
µA
SHDN = GND
0.1
Switch Leakage Current
(LM3500-16)
VSW = 15V
0.01
0.5
µA
Switch Leakage Current
(LM3500-21)
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
(LM3500-16)
ON Threshold
15
15.5
16
OFF Threshold
14
14.6
15
Output Overvoltage Protection
(LM3500-21)
ON Threshold
20
20.5
21
OFF Threshold
19
19.5
20
SHDN = 5.5V
MHz
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 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 specification apply to both LM3500-16 and
LM3500-21.
Symbol
IVout
IVL
SHDN
Threshold
6
Parameter
Conditions
Min
(1)
Typ
Max
(2)
(1)
VOUT Bias Current
(LM3500-16)
VOUT = 15V, SHDN = VIN
260
400
VOUT Bias Current
(LM3500-21)
VOUT = 20V, SHDN = VIN
300
460
PMOS Switch Leakage
Current (LM3500-16)
VOUT = 15V, VSW = 0V
0.01
3
PMOS Switch Leakage
Current (LM3500-21)
VOUT = 20V, VSW = 0V
0.01
3
0.65
0.3
µA
µA
SHDN Low
SHDN High
1.1
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Units
0.65
V
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Typical Performance Characteristics
Switching Quiescent Current
vs
VIN
Non-Switching Quiescent Current
vs
VIN
Figure 2.
Figure 3.
2 LED Efficiency
vs
LED Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(2VLED*ILED))
2 LED Efficiency
vs
LED Current
L = TDK VLP4612T-220MR34,
Efficiency = 100*(PIN/(2VLED*ILED))
Figure 4.
Figure 5.
3 LED Efficiency
vs
LED Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(3VLED*ILED))
3 LED Efficiency
vs
LED Current
L = TDK VLP4612T-220MR34,
Efficiency = 100*(PIN/(3VLED*ILED))
Figure 6.
Figure 7.
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Typical Performance Characteristics (continued)
8
4 LED Efficiency
vs
LED Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(4VLED*ILED))
4 LED Efficiency
vs
LED Current
L = TDK VLP4612T-220MR34,
Efficiency = 100*(PIN/(4VLED*ILED))
Figure 8.
Figure 9.
2 LED Efficiency
vs
VIN
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(2VLED*ILED))
3 LED Efficiency
vs
VIN
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(3VLED*ILED))
Figure 10.
Figure 11.
4 LED Efficiency
vs
VIN
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(4VLED*ILED))
SHDN Pin Current
vs
SHDN Pin Voltage
Figure 12.
Figure 13.
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Typical Performance Characteristics (continued)
Output Power
vs
VIN: LM3500-16
(L = Coilcraft DT1608C-223)
Output Power
vs
Temperature: LM3500-16
(L = Coilcraft DT1608C-223)
Figure 14.
Figure 15.
Switch Current Limit
vs
VIN: LM3500-16
Switch Current Limit
vs
Temperature
LM3500-16, VOUT=8V
Figure 16.
Figure 17.
Switch Current Limit
vs
Temperature
LM3500-16, VOUT=12V
Switch Current Limit
vs
VIN: LM3500-21
1100
CURRENT LIMIT (mA)
1000
VOUT = 8V
900
UT
VO
800
=1
2V
700
600
UT
VO
500
=1
5V
400
VOUT = 18V
300
200
2.5
3.0 3.5
4.0
4.5 5.0
5.5
6.0
6.5
INPUT VOLTAGE (V)
Figure 18.
Figure 19.
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Typical Performance Characteristics (continued)
Switch Current Limit
vs
Temperature
LM3500-21, VOUT=12V
1300
850
1200
800
VIN = 5.5V
1100
CURRENT LIMIT (mA)
CURRENT LIMIT (mA)
Switch Current Limit
vs
Temperature
LM3500-21, VOUT=12V
1000
VIN = 4.2V
900
800
700
600
500
-40
VIN = 5.5V
750
700
650
VIN = 4.2V
600
550
500
VIN = 3.0V
VIN = 3.0V
450
-40
-15
10
35
60
-15
85
10
35
60
85
TEMPERATURE (ºC)
TEMPERATURE (ºC)
Figure 20.
Figure 21.
Switch Current Limit
vs
Temperature
LM3500-21, VOUT=18V
Oscillator Frequency
vs
VIN
440
420
CURRENT LIMIT (mA)
400
V IN = 5.5V
V IN = 3.0V
380
360
340
V IN = 4.2V
320
300
280
260
240
-40
-15
10
35
60
85
INPUT VOLTAGE (V)
Figure 22.
Figure 23.
VOUT DC Bias
vs
VOUT Voltage: LM3500-16
VOUT DC Bias
vs
VOUT Voltage: LM3500-21
VOUT DC BIAS CURRENT (PA)
400
350
T = -40qC
300
250
T = 125qC
200
150
T = 25qC
100
50
0
0
2
4
6
8 10 12 14 16 18 20 22
VOUT (V)
Figure 24.
10
Figure 25.
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Typical Performance Characteristics (continued)
FB Voltage
vs
Temperature
FB Voltage
vs
VIN
Figure 26.
Figure 27.
NMOS RDSON
vs
VIN
(ISW = 300mA)
PMOS RDSON
vs
Temperature
Figure 28.
Figure 29.
Typical VIN Ripple
Start-Up: LM3500-16
LM3500-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V
1) SW, 10V/div, DC
3) IL, 100mA/div, DC
4) VIN, 100mV/div, AC
T = 250ns/div
Figure 30.
3 LEDs, RLED = 22Ω, VIN = 3.0V
1) SHDN, 1V/div, DC
2) IL, 100mA/div, DC
3) ILED, 20mA/div, DC
T = 100µs/div
Figure 31.
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Typical Performance Characteristics (continued)
Start-Up: LM3500-21
SHDN Pin Duty Cycle Control Waveforms
T
1
4
2
LM3500-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V, SHDN frequency =
200Hz
1) SHDN, 1V/div, DC
2) IL, 100mA/div, DC
3) ILED, 20mA/div, DC
4) VOUT, 10V/div, DC
T = 1ms/div
Figure 33.
3 LEDs, RLED = 22Ω, VIN = 3.0V
1) SHDN, 1V/div, DC
4) IL, 100mA/div, DC
2) VOUT, 10/div, DC
T = 200µs/div
VCONT = 2.7V
Figure 32.
Typical VOUT Ripple, OVP Functioning: LM3500-16
Typical VOUT Ripple, OVP Functioning: LM3500-21
T
1
VOUT open circuit and equals approximately 15V DC, VIN = 3.0V
3) VOUT, 200mV/div, AC
T = 1ms/div
Figure 34.
12
VOUT open circuit and equals approximately 20V DC, VIN = 3.0V
1) VOUT, 200mV/div, AC
T = 400µs/div
Figure 35.
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Operation
L
VIN
B1 VIN
C2
UVP
REF
UVP
COMP
REF
+
OVP
COMP
THERMAL
SHUTDOWN
+
LIGHT LOAD
COMP
VSW
VOUT
C1
+
-
OVP
REF
x
CIN
x
FB
-
B3
COUT
Reset Reset Reset
Reset
DriveP
LOGIC
PWM
COMP
+
EAMP
+
Reset
SET Reset
x
DriveN
Reset
Body Diode
Control
Current
Sense
0.5V
osc
NC
A3
+
Dlimit
A1
Duty Limit
Comp
FB
R
LED
SHUTDOWN
COMP
-
A2
AGND
SHDN
C3
GND
Figure 36. LM3500 Block Diagram
The LM3500 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 LM3500 is internally compensated thus eliminating the need for any external compensation
components providing a compact overall solution. The operation can best be understood referring to the block
diagram in Figure 36. 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 LM3500 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 LM3500 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(LM3500-16) and 20.5V(LM3500-21) when the primary white LED network is removed or if there is an LED
failure, allowing the use of small (16V for LM3500-16 and 25V for LM3500-21) ceramic capacitors at the output.
This comparator has hysteresis that will regulate the output voltage between 15.5V and 14.6V typically for the
LM3500-16, and between 20.5V and 19.5V for the LM3500-21. The LM3500 features a shutdown mode that
reduces the supply current to 0.1uA and isolates the input and output of the converter.
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APPLICATION INFORMATION
ADJUSTING LED CURRENT
The White LED current is set using the following equation:
ILED = VFB/RLED
(1)
The LED current can be controlled using a PWM signal on the SHDN pin with frequencies in the range of 100Hz
(greater than visible frequency spectrum) to 1kHz. For controlling LED currents down to the µA levels, it is best
to use a PWM signal frequency between 200-500Hz. The LM3500 LED current can be controlled with PWM
signal frequencies above 1kHz 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.
LED-DRIVE CAPABILITY
The maximum number of LEDs that can be driven by the LM3500 is limited by the output voltage capability of the
LM3500. When using the LM3500 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 LM3500 feedback pin (VFB-MAX = 0.53V), and the
minimum output over-voltage protection level of the chosen LM3500 option (LM3500-16: OVPMIN = 15V;
LM3500-21: OVPMIN = 20V). For the circuit to function properly, the following inequality must be met:
(NMAX × VF-MAX) + 0.53V ≤ OVPMIN
(2)
When inserting a value for maximim LED VF, LED forward voltage variation over the operating temperature range
should be considered. The table below provides maximum LED voltage numbers for the LM3500-16 and
LM3500-21 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)
LM3500-16
LM3500-21
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 LM3500 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 LM3500 contains dedicated circuitry for monitoring the output voltage. In the event that the primary LED
network is disconnected from the LM3500-16, the output voltage will increase and be limited to 15.5V (typ.).
There is a 900mV 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 1µF ceramic output capacitors
creating an overall small solution for white LED applications.
In the event that the primary LED network is disconnected from the LM3500-21, 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 1µF
ceramic output capacitors.
RELIABILITY AND THERMAL SHUTDOWN
The maximum continuous pin current for the 8 pin thin DSBGA package is 535mA. When driving the device near
its power output limits the VSW pin can see a higher DC current than 535mA (see INDUCTOR SELECTION
section for average switch current). To preserve the long term reliability of the device the average switch current
should not exceed 535mA.
14
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The LM3500 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 LM3500 will return to normal operation.
INDUCTOR SELECTION
The inductor used with the LM3500 must have a saturation current greater than the cycle by cycle peak inductor
current (see Table 1 table below). Choosing inductors with low DCR decreases power losses and increases
efficiency.
The minimum inductor value required for the LM3500-16 can be calculated using the following equation:
L>
VIN RDSON
D
-1
D'
0.29
(3)
The minimum inductor value required for the LM3500-21 can be calculated using the following equation:
L>
VIN RDSON
D
-1
D'
0.58
(4)
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 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 22µH 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
(5)
where VOUT is the voltage at pin C1.
Table 1. Typical Peak Inductor Currents (mA)
VIN
(V)
# LEDs
(in
series)
2.7
3.3
4.2
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
2
64
76
96
116
142
162
3
102
116
148
180
210
246
4
122
146
186
232
272
318
5
179
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
(6)
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:
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IOUT
KD'
(7)
The maximum output current capability of the LM3500 can be estimated with the following equation:
IOUT | KD' ICL -
VIND
2LFSW
(8)
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 1µF ceramic output capacitor is sufficient.
Local bypassing for the input is needed on the LM3500. Multilayer X7R or X5R ceramic capacitors with low ESR
are a good choice for this as well. A 1µF ceramic capacitor is sufficient for most applications. However, for some
applications at least a 4.7µF ceramic capacitor may be required for proper startup of the LM3500. Using
capacitors with low ESR decreases input voltage ripple. For additional bypassing, a 100nF 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 36, 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 100nF 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 LM3500 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
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 LM3500 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 37 and Figure 38 for an
example of a good layout as used for the LM3500 evaluation board.
16
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Figure 37. Evaluation Board Layout (2X Magnification)
Top Layer
Figure 38. Evaluation Board Layout (2X Magnification)
Bottom Layer (as viewed from the top)
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L
22 PH
VIN
2.7V - 5.5V
B1
VIN
A3
CIN
1PF
Ceramic
C2
VSW
NC
LM3500-16
>1.1V
1.1V
1.1V
1.1V