LM3404_08 Datasheet, PDF (1/10 Pages) National Semiconductor (TI) – Evaluation Board

LM3404/LM3404HV
Evaluation Board
National Semiconductor
Chris Richardson
January 7, 2008
Introduction
The LM3404/04HV is a buck regulator derived controlled cur-
rent source designed to drive a series string of high power,
high brightness LEDs (HBLEDs) such as the Luxeon™ K2
Emitter at forward currents of up to 1.0A. The board can ac-
cept an input voltage ranging from 6V to 42V when using the
LM3404. When using the pin-for-pin compatible LM3404HV
the upper bound of input voltage is 75V. The converter output
voltage adjusts as needed to maintain a constant current
through the LED array. The LM3404/04HV is a true step-down
regulator with an output voltage range extending from a V_O
(MIN) ^{of 200 mV (the reference voltage) to a V}O(MAX) ^deter-
mined by the minimum off time (typically 300 ns). It can
maintain regulated current through any number of LEDs as
long as the combined forward voltage of the array does not
exceed V_O(MAX)..
Circuit Performance LM3404
The LM3404 circuit and BOM have been designed to provide
a constant forward current of 1.0A to a single LED with a for-
ward voltage of approximately 3.7V (Typical of white, blue,
and green LEDs using InGaN technology) from an input of
24V ±10%. Over the input voltage range of 6V to 42V the av-
erage LED current, I_F, is 1.0A ±10%, the ripple current, Δi_F,
will not exceed 400 mA_P-P, and the switching frequency is 450
kHz ± 10%.
Circuit Performance LM3404HV
The LM3404HV circuit and BOM have been designed to pro-
vide a constant forward current of 1.0A to a single LED with
a forward voltage of approximately 3.7V from an input of 48V
±10%. Over the input voltage range of 6V to 75V the average
LED current, I_F, is 1.0A ±10%, the ripple current, Δi_F, will not
exceed 400 mA_P-P, and the switching frequency is 200 kHz ±
10%.
Connecting to LED Array
The LM3404/04HV Evaluation Board includes a female 6-pin
SIP, J1, connector as well as two standard 94mil turret con-
nectors for the cathode and anode connections of the LED
array. Figure 1 shows the pinout of J1. Solid 18 or 20 gauge
wire with about 1 cm of insulation stripped away makes a
convenient, solderless connection to J1.
FIGURE 1.
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Setting the LED Current
The default forward current I_LED delivered to the LED array is
1.0A, typical of many 3W LEDs. To adjust this value the cur-
rent setting resistor R_SNS can be changed according to the
following equation:
t_SNS = 220 ns
This resistor should be rated to handle the power dissipation
of the LED current. For example, the closest 5% tolerance
resistor to set an LED current of 1.0A is 0.22 Ω. In steady state
this resistor will dissipate (1.0 x 0.22) = 220 mW, indicating
that a resistor with a 1/4W power rating is appropriate.
PWM Dimming
The DIM1 terminal on the PCB provides an input for a pulse
width modulation signal for dimming of the LED array. In order
to fully enable and disable the LM3404/04HV the PWM signal
should have a maximum logic low level of 0.8V and a mini-
mum logic high level of 2.2V. The maximum PWM dimming
frequency, minimum PWM duty cycle and maximum duty cy-
cle are illustrated in Figure 2. PWM frequency should be at
least one order of magnitude below the LM3404/04HV switch-
ing frequency. The interval t_D represents the delay from a
logic high at the DIM pin to the onset of the output current.
The quantities t_SU and t_SD represent the time needed for the
output current to slew up to steady state and slew down to
zero, respectively. Typical response times for the standard
LM3404 and LM3404HV demo boards circuits are shown in
the Typical Performance Characteristics section.
© 2008 National Semiconductor Corporation 202159
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