Datasheet LM3500TL-16, LM3500TL-21 Datasheet (National) [ru]

February 2005

LM3500
Synchronous Step-up DC/DC Converter for White LED
Applications

LM3500 Synchronous Step-up DC/DC Converter for White LED Applications

General 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 3­cell 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, low­noise, low-cost solution.

Two LM3500 options are available with different output volt­age capabilities. The LM3500-21 has a maximum output
voltage of 21V and is typically suited for driving 4 or 5 white
LEDs in series. The LM3500-16 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 LM3500 uses internal protection
circuitry on the output to prevent a destructive over-voltage
event.

A single external resistor is used to set the maximum LED
current in LED-drive applications. The LED current can easily
be adjusted using a pulse width modulated (PWM) signal on
the shutdown pin. In shutdown, the LM3500 completely dis­connects the input from output, creating total isolation and
preventing any leakage currents from trickling into the LEDs.

Features

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 micro SMD package

■

Applications

LCD Bias Supplies

■

White LED Backlighting

■

Handheld Devices

■

Digital Cameras

■

Portable Applications

■

Typical Application Circuit

20065701

© 2007 National Semiconductor Corporation 200657 www.national.com

Connection Diagram

LM3500

Top View

8-bump micro SMD

20065702

Ordering Information

Maximum

Output Voltage

16V LM3500TL-16 micro SMD TL08SSA S18 250 Units, Tape and Reel

16V LM3500TLX-16 micro SMD TL08SSA S18 3000 Units, Tape and Reel

21V LM3500TL-21 micro SMD TL08SSA S23 250 Units, Tape and Reel

21V LM3500TLX-21 micro SMD TL08SSA S23 3000 Units, Tape and Reel

Order Number Package Type NSC Package

Drawing

Top Mark Supplied As

Pin Description/Functions

Pin Name Function

A1 AGND Analog ground.

B1 V

C1 V

C2 V

C3 GND Power Ground.

B3 FB Output voltage feedback connection.

A3 NC No internal connection made to this pin.

A2 SHDN Shutdown control pin.

AGND(pin A1): Analog ground pin. The analog ground pin
should tie directly to the GND pin.

VIN(pin B1): Analog and Power supply pin. Bypass this pin
with a capacitor, as close to the device as possible, connected
between the VIN and GND pins.

V

(pin C1): Source connection of internal PMOS power

OUT

device. Connect the output capacitor between the V
GND pins as close as possible to the device.

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.

IN

OUT

SW

Analog and Power supply input.

PMOS source connection for synchronous rectification.

Switch pin. Drain connections of both NMOS and PMOS power devices.

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.

NC(pin A3): No internal connection is made to this pin. The
maximum allowable voltage that can be applied to this pin is

OUT

and

7.5V.
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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LM3500

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

(LM3500-16)(Note 2)

OUT

V

(LM3500-21)(Note 2)

OUT

VSW(Note 2) −0.3V to V

FB, SHDN, and NC Voltages −0.3V to 7.5V

−0.3V to 7.5V

−0.3V to 16V

−0.3V to 21V
+0.3V

OUT

Operating Conditions

Ambient Temperature
(Note 5) −40°C to +85°C

Junction Temperature −40°C to +125°C
Supply Voltage 2.7V to 7V

Thermal Properties

Junction to Ambient Thermal
Resistance (θJA)(Note 6)

75°C/W

Maximum Junction Temperature 150°C
Lead Temperature

(Note 3) 300°C
ESD Ratings (Note 4)
Human Body Model 2kV
Machine Model 200V

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 Parameter Conditions

I

Q

Quiescent Current, Device Not
Switching

Quiescent Current, Device
Switching

FB > 0.54V

FB = 0V

Min

(Note 7)

Shutdown SHDN = 0V 0.1 2 µA

V

ΔV

FB

FB

Feedback Voltage VIN = 2.7V to 7V 0.47 0.5 0.53

Feedback Voltage Line

VIN = 2.7V to 7V

Regulation

I

CL

I

B

V

R

D

IN

DSON

Limit

Switch Current Limit
(LM3500-16)

Switch Current Limit
(LM3500-21)

VIN = 2.7V,
Duty Cycle = 80%

VIN = 3.0V,
Duty Cycle = 70%

VIN = 2.7V,
Duty Cycle = 70%

VIN = 3.0V,
Duty Cycle = 63%

275 400 480

255 400 530

420 640 770

450 670 800

FB Pin Bias Current FB = 0.5V (Note 9)

Input Voltage Range 2.7 7.0 V

NMOS Switch R

PMOS Switch R

DSON

DSON

VIN = 2.7V, ISW = 300mA

V

= 6V, ISW = 300mA

OUT

Duty Cycle Limit (LM3500-16) FB = 0V 80 87

Duty Cycle Limit (LM3500-21) FB = 0V 85 94

F

SW

I

SD

Switching Frequency

0.85 1.0 1.15 MHz

SHDN Pin Current (Note 10) SHDN = 5.5V 18 30

SHDN = GND 0.1

I

L

Switch Leakage Current

VSW = 15V 0.01 0.5 µA

(LM3500-16)

Switch Leakage Current

VSW = 20V 0.01 2.0

(LM3500-21)

UVP Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6

OFF Threshold 2.3 2.4 2.5

Typ

(Note 8)

Max

(Note 7)

0.95 1.2

1.8 2.5

0.1 0.4 %/V

45 200 nA

0.43

1.1 2.3

Units

mA

V

mA

Ω

%

µASHDN = 2.7V 9 16

V

3 www.national.com

Symbol Parameter Conditions

LM3500

OVP Output Overvoltage Protection

(LM3500-16)

Output Overvoltage Protection
(LM3500-21)

I

Vout

V

Bias Current

OUT

ON Threshold 15 15.5 16

OFF Threshold 14 14.6 15

ON Threshold 20 20.5 21

OFF Threshold 19 19.5 20

V

= 15V, SHDN = V

OUT

(LM3500-16)

V

Bias Current

OUT

V

= 20V, SHDN = V

OUT

(LM3500-21)

I

VL

PMOS Switch Leakage

V

= 15V, VSW = 0V

OUT

Current (LM3500-16)

PMOS Switch Leakage

V

= 20V, VSW = 0V

OUT

Current (LM3500-21)

SHDN
Threshold

SHDN Low 0.65 0.3

SHDN High 1.1 0.65

Min

(Note 7)

IN

IN

Typ

(Note 8)

260 400

300 460

0.01 3

0.01 3

Max

(Note 7)

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 Parameter Conditions

I

Q

Quiescent Current, Device Not
Switching

Quiescent Current, Device
Switching

FB > 0.54V

FB = 0V

Min

(Note 7)

Typ

(Note 8)

0.95 1.2

1.8 2.5

Max

(Note 7)

Shutdown SHDN = 0V 0.1 2 µA

V

ΔV

I

CL

I

B

V

R

D

FB

FB

IN

DSON

Limit

Feedback Voltage VIN = 2.7V to 7V 0.47 0.5 0.53

Feedback Voltage Line
Regulation

Switch Current Limit
(LM3500-16)

Switch Current Limit
(LM3500-21)

FB Pin Bias Current FB = 0.5V (Note 9)

VIN = 2.7V to 7V

VIN = 3.0V, Duty Cycle = 70%

VIN = 3.0V, Duty Cycle = 63%

0.1 0.4 %/V

400

670

45 200 nA

Input Voltage Range 2.7 7.0 V

NMOS Switch R

PMOS Switch R

DSON

DSON

VIN = 2.7V, ISW = 300mA

V

= 6V, ISW = 300mA

OUT

0.43

1.1 2.3

Duty Cycle Limit (LM3500-16) FB = 0V 87

Duty Cycle Limit (LM3500-21) FB = 0V 94

F

SW

I

SD

Switching Frequency

0.8 1.0 1.2 MHz

SHDN Pin Current (Note 10) SHDN = 5.5V 18 30

SHDN = GND 0.1

I

L

Switch Leakage Current

VSW = 15V 0.01 0.5 µA

(LM3500-16)

Switch Leakage Current

VSW = 20V 0.01 2.0

(LM3500-21)

UVP Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6

OFF Threshold 2.3 2.4 2.5

OVP Output Overvoltage Protection

(LM3500-16)

Output Overvoltage Protection
(LM3500-21)

ON Threshold 15 15.5 16

OFF Threshold 14 14.6 15

ON Threshold 20 20.5 21

OFF Threshold 19 19.5 20

Units

V

µA

µA

V

Units

mA

V

mA

Ω

%

µASHDN = 2.7V 9 16

V

V

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LM3500

Symbol Parameter Conditions

I

Vout

V

Bias Current

OUT

V

= 15V, SHDN = V

OUT

IN

Min

(Note 7)

(LM3500-16)

V

Bias Current

OUT

V

= 20V, SHDN = V

OUT

IN

(LM3500-21)

I

VL

PMOS Switch Leakage

V

= 15V, VSW = 0V

OUT

Current (LM3500-16)

PMOS Switch Leakage

V

= 20V, VSW = 0V

OUT

Current (LM3500-21)

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 VIN < V

Note 3: For more detailed soldering information and specifications, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip

Scale Package (AN-1112), available at www.national.com.

Note 4: 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 5: 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 (T
dissipation of the device in the application (P
following equation: T

Note 6: 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.

Note 7: 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 8: Typical numbers are at 25°C and represent the most likely norm.

Note 9: Feedback current flows out of the pin.

Note 10: Current flows into the pin.

SHDN Low 0.65 0.3

SHDN High 1.1 0.65

A-MAX

= T

J-MAX-OP

. If VIN > V

OUT

A-MAX

D-MAX

– (θJA × P

, a voltage greater than VIN + 0.3V should not be applied to the V

OUT

) is dependent on the maximum operating junction temperature (T

), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the

).

D-MAX

Typ

(Note 8)

Max

(Note 7)

260 400

300 460

0.01 3

0.01 3

or VSW pins.

OUT

= 125ºC), the maximum power

J-MAX-OP

Units

µA

µA

V

Typical Performance Characteristics

Switching Quiescent Current vs V

IN

20065755

Non-Switching Quiescent Current vs V

20065756

IN

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LM3500

2 LED Efficiency vs LED Current

L = Coilcraft DT1608C-223,

Efficiency = 100*(PIN/(2V

LED*ILED

2 LED Efficiency vs LED Current

L = TDK VLP4612T-220MR34,

))

Efficiency = 100*(PIN/(2V

LED*ILED

))

3 LED Efficiency vs LED Current

L = Coilcraft DT1608C-223,

Efficiency = 100*(PIN/(3V

LED*ILED

4 LED Efficiency vs LED Current

L = Coilcraft DT1608C-223,

Efficiency = 100*(PIN/(4V

LED*ILED

20065757

))

20065758

))

3 LED Efficiency vs LED Current

L = TDK VLP4612T-220MR34,

Efficiency = 100*(PIN/(3V

LED*ILED

4 LED Efficiency vs LED Current

L = TDK VLP4612T-220MR34,

Efficiency = 100*(PIN/(4V

LED*ILED

20065779

))

20065780

))

20065759

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20065781

LM3500

2 LED Efficiency vs V

L = Coilcraft DT1608C-223,

Efficiency = 100*(PIN/(2V

4 LED Efficiency vs V

L = Coilcraft DT1608C-223,

Efficiency = 100*(PIN/(4V

IN

LED*ILED

IN

LED*ILED

))

20065769

))

3 LED Efficiency vs V

L = Coilcraft DT1608C-223,

Efficiency = 100*(PIN/(3V

IN

LED*ILED

))

20065770

SHDN Pin Current vs SHDN Pin Voltage

Output Power vs VIN: LM3500-16

(L = Coilcraft DT1608C-223)

20065773

20065784

20065761

Output Power vs Temperature: LM3500-16

(L = Coilcraft DT1608C-223)

20065785

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LM3500

Switch Current Limit vs VIN: LM3500-16

Switch Current Limit vs Temperature

LM3500-16, V

OUT

=8V

20065762

Switch Current Limit vs Temperature

LM3500-16, V

OUT

=12V

20065776

Switch Current Limit vs Temperature

LM3500-21, V

OUT

=8V

20065763

Switch Current Limit vs VIN: LM3500-21

20065791

Switch Current Limit vs Temperature

LM3500-21, V

OUT

=12V

20065792

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20065793

LM3500

Switch Current Limit vs Temperature

V

DC Bias vs V

OUT

LM3500-21, V

OUT

=18V

OUT

Voltage: LM3500-16

20065794

Oscillator Frequency vs V

V

DC Bias vs V

OUT

IN

Voltage: LM3500-21

OUT

20065764

FB Voltage vs Temperature

20065765

20065766

FB Voltage vs V

20065795

IN

20065767

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LM3500

NMOS R

DSON

(ISW = 300mA)

vs V

IN

20065774

PMOS R

vs Temperature

DSON

20065775

LM3500-16, 3 LEDs, R

1) SW, 10V/div, DC

3) IL, 100mA/div, DC

4) VIN, 100mV/div, AC

T = 250ns/div

Typical VIN Ripple

= 22Ω, VIN = 3.0V

LED

Start-Up: LM3500-21

20065768

3 LEDs, R

= 22Ω, VIN = 3.0V

LED

1) SHDN, 1V/div, DC

2) IL, 100mA/div, DC

3) I

, 20mA/div, DC

LED

T = 100µs/div

SHDN Pin Duty Cycle Control Waveforms

Start-Up: LM3500-16

20065771

3 LEDs, R

= 22Ω, VIN = 3.0V

LED

20065796

1) SHDN, 1V/div, DC

4) IL, 100mA/div, DC

2) V

, 10/div, DC

OUT

T = 200µs/div

V

= 2.7V

CONT

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LM3500-16, 3 LEDs, R

1) SHDN, 1V/div, DC

2) IL, 100mA/div, DC

3) I

, 20mA/div, DC

LED

4) V

, 10V/div, DC

OUT

T = 1ms/div

= 22Ω, VIN = 3.0V, SHDN frequency = 200Hz

LED

20065772

LM3500

Typical V

V

open circuit and equals approximately 15V DC, VIN = 3.0V

OUT

3) V

, 200mV/div, AC

OUT

Ripple, OVP Functioning: LM3500-16

OUT

T = 1ms/div

20065782

Typical V

V

open circuit and equals approximately 20V DC, VIN = 3.0V

OUT

1) V

, 200mV/div, AC

OUT

Ripple, OVP Functioning: LM3500-21

OUT

T = 400µs/div

20065797

11 www.national.com

Operation

LM3500

20065704

FIGURE 1. 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 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 1. 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 out-

put 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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LM3500

Application Information

ADJUSTING LED CURRENT

The White LED current is set using the following equation:

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
V

and FB pins, the maximum number of LEDs that can be

OUT

placed in series (N
forward voltage (V
pin (V
protection level of the chosen LM3500 option (LM3500-16:
OVP
function properly, the following inequality must be met:

(N

= 0.53V), and the minimum output over-voltage

FB-MAX

= 15V; LM3500-21: OVP

MIN

× V

MAX

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 V

# of LEDs
(in series)

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 V

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

) is dependent on the maximum LED

MAX

), the voltage of the LM3500 feedback

F-MAX

= 20V). For the circuit to

MIN

) + 0.53V ≤ OVP

F-MAX

MIN

OUT

Maximum LED V

LM3500-16 LM3500-21

pins.

OUT

and FB pins).

F

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 micro
SMD 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.

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 Typical Peak Inductor Currents 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:

The minimum inductor value required for the LM3500-21 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, R
power switch found in the Typical Performance Characteris-

is the ON resistance of the NMOS

DSON

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 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:

where V

is the voltage at pin C1.

OUT

13 www.national.com

Typical Peak Inductor Currents (mA)

V

LM3500

(V)

IN

# LEDs

(in

series)

15mA20mA30mA40mA50mA60

LED Current

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

4.2 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

CIN = C
L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223

2 and 3 LED applications: LM3500-16 or LM3500-21; LED VF = 3.77V at
20mA; TA = 25°C
4 LED applications: LM3500-16 or LM3500-21; LED VF = 3.41V at 20mA;
TA = 25°C
5 LED applications: LM3500-21 only; LED VF = 3.28V at 20mA; TA = 25°C

OUT

= 1 µF

The typical cycle-by-cycle peak inductor current can be cal­culated from the following equation:

where I
quency, L is the inductance and η is the converter efficiency

is the total load current, FSW is the switching fre-

OUT

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 maximum output current capability of the LM3500 can be
estimated with the following equation:

where ICL is the current limit. Some recommended inductors
include but are not limited to:

mA

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 1, must
be placed close to the device and connect between the V
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, C
the LM3500 and connected directly between the V
GND pins. Any copper trace connections for the C
itor can increase the series resistance, which directly effects

, should also be placed close to

OUT

OUT

and

OUT

capac-

output voltage ripple and efficiency. The current setting re­sistor, R
copper trace connections that can inject noise into the sys-

, should be kept close to the FB pin to minimize

LED

tem. 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 V
routing away from sensitive pins such as the FB pin. Failure

SW

to do so may inject noise into the FB pin and affect the regu­lation of the device. See Figure 2 and Figure 3 for an example
of a good layout as used for the LM3500 evaluation board.

IN

www.national.com 14

20065777

LM3500

FIGURE 2. Evaluation Board Layout (2X Magnification)

Top Layer

20065778

FIGURE 3. Evaluation Board Layout (2X Magnification)

Bottom Layer (as viewed from the top)

15 www.national.com

LM3500

20065709

FIGURE 4. 2 White LED Application

FIGURE 5. Multiple 2 LED String Application

www.national.com 16

20065754

FIGURE 6. Multiple 3 LED String Application

LM3500

20065783

FIGURE 7. LM3500-21 5 LED Application

17 www.national.com

20065790

LM3500

www.national.com 18

Physical Dimensions inches (millimeters) unless otherwise noted

LM3500

For Ordering, Refer to Ordering Information Table

X1 = 1.92mm (±0.03mm), X2 = 1.92mm (±0.03mm), X3 = 0.6mm (±0.075mm)

8-Bump Micro SMD Package (TL)

NS Package Number TLA08SSA

19 www.national.com

Notes

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Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and

LM3500 Synchronous Step-up DC/DC Converter for White LED Applications

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. A critical component is any component in a life support device or system whose failure to perform
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