LM3501 Datasheet, PDF (14/27 Pages) National Semiconductor (TI) – Synchronous Step-up DC/DC Converter for White LED Applications

LM3501
SNVS230C – DECEMBER 2003 – REVISED MAY 2013
APPLICATION INFORMATION
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ADJUSTING LED CURRENT
The maximum White LED current is set using the following equation:
I_LED = V_FB(MAX)/R_LED
(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 V_OUT and FB pins, the maximum number of LEDs that can be placed in series (N_MAX) is dependent on the
maximum LED forward voltage (V_F-MAX), the voltage of the LM3501 feedback pin (V_FB-MAX = 0.545V), and the
minimum output overvoltage protection level of the chosen LM3501 option (LM3501-16: OVP_MIN = 15V; LM3501-
21: OVP_MIN = 20V). For the circuit to function properly, the following inequality must be met:
(N_MAX x V_F-MAX) + 0.545V ≤ OVP_MIN
(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
V_OUT and FB pins).
# of LEDs
(in series)
3
4
5
6
7
LM3501-16
4.82V
3.61V
2.89V
X
X
Maximum LED V_F
LM3501-21
6.49V
4.86V
3.89V
3.24V
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.
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