LM3501TL-16 National Semiconductor Corporation, LM3501TL-16 Datasheet - Page 11

• Manufacturer Part Number: LM3501TL-16
• Description: Synchronous Step-up DC/DC Converter For White Led Applications
• Manufacturer: National Semiconductor Corporation

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 maximum feedback voltage is achieved when
CNTRL is 2.7V, typically FB = 0.515V. 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 100Hz
(greater than visible frequency spectrum) to 1kHz. For con-
trolling LED currents down to the µA levels, it is best to use
a PWM signal frequency between 200-500Hz. The LM3501
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.
Applying a voltage greater than 125mV to the CNTRL pin will
begin regulating current to the LEDs. A voltage below 75mV
will prevent application or regulation of the LED current.
The minimum number of LEDs the LM3501 can drive is 2 in
series and the maximum is 4. The LM3501 can also drive
multiple strings of white LEDs. When driving 4 LEDs in
series the application should use lower forward voltage drop
white LEDs to prevent the OVP function from activating
during normal operation.
OUTPUT OVERVOLTAGE PROTECTION
The LM3501 contains dedicated circuitry for monitoring the
output voltage. In the event that the primary LED network is
disconnected the output will increase and be limited to
15.5V. There is a 900mV hysteresis associated with this
circuitry which will cause the output to fluctuate between
15.5V and 14.6V 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 creat-
ing an overall small solution for white LED applications.
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 V
than 535mA (see INDUCTOR SELECTION section for aver-
age switch current). To preserve the long term reliability of
the device the average switch current should not exceed
535mA.
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.
SW
pin can see a higher DC current
11
INDUCTOR SELECTION
The inductor used with the LM3501 must have a saturation
current greater than the cycle by cycle peak inductor current.
Choosing inductors with low DCR decreases power losses
and increases efficiency.
The minimum inductor value required for the LM3501 can be
calculated using the following equation:
where L is in µH, V
R
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 0.5. An
inductor value in the range of 4.7µH to 22µH is good for most
applications, however the value must meet the requirements
of the above equation. The duty cycle, D, is given by the
following equation:
where V
cycle peak inductor current can be calculated from the fol-
lowing equation:
where I
frequency, L is the inductance and η is the converter effi-
ciency of the total driven load. A good typical number to use
for η is 0.8. The value of η can vary with load and duty cycle.
The average inductor current, which is also the average V
pin current, is given by the following equation:
The maximum output current capability of the LM3501 can
be estimated with the following equation:
where I
include but are not limited to:
Coilcraft DT1608C series
Coilcraft DO1608C series
TDK VLP4612 series
TDK VLP5610 series
TDK VLF4012A series
DSON
CL
OUT
is the ON resistance of the NMOS power switch
OUT
is the current limit. Some recommended inductors
is the total load current, F
is the voltage at pin C1. The typical cycle-by-
IN
is the input supply of the chip in Volts,
SW
is the switching
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SW