LM3500 Datasheet, PDF (14/26 Pages) National Semiconductor (TI) – Synchronous Step-up DC/DC Converter for White LED

LM3500
SNVS231G – AUGUST 2003 – REVISED MAY 2013
APPLICATION INFORMATION
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ADJUSTING LED CURRENT
The White LED current is set using the following equation:
I_LED = V_FB/R_LED
(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 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 LM3500 feedback pin (V_FB-MAX = 0.53V), and the
minimum output over-voltage protection level of the chosen LM3500 option (LM3500-16: OVP_MIN = 15V;
LM3500-21: OVP_MIN = 20V). For the circuit to function properly, the following inequality must be met:
(N_MAX × V_F-MAX) + 0.53V ≤ OVP_MIN
(2)
When inserting a value for maximim LED V_F, 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_OUT and FB pins).
# of LEDs
(in series)
3
4
5
6
7
LM3500-16
4.82V
3.61V
2.89V
X
X
Maximum LED V_F
LM3500-21
6.49V
4.86V
3.89V
3.24V
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_OUT 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 V_SW 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.
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