Datasheet
T
D
t
D
t
SD
t
D
t
SU
t
SD
t
D
t
SU
t
SD
D
MIN
D
MAX
T T
DIM
I
F
f
PWM
T =
1
T
t
D
+ t
SU
D
MIN
=
T
T - t
SD
D
MAX
=
t
SU
R
SNS
=
2
V
IN
- V
O
0.2 x L
I
F
x L + V
O
x t
SNS
-
x t
ON
Setting the LED Current
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5 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 current setting resistor R
SNS
can be changed according to the following equation:
(1)
t
SNS
= 220 ns (2)
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.
6 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 minimum logic high level of 2.2V. The maximum PWM dimming
frequency, minimum PWM duty cycle and maximum duty cycle are illustrated in Figure 2. PWM frequency
should be at least one order of magnitude below the LM3404/04HV switching 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.
Figure 2. PWM Dimming Limits
The logic of DIM1 is direct, hence the LM3404/04HV will deliver regulated output current when the voltage
at DIM1 is high, and the current output is disabled when the voltage at DIM1 is low. Connecting a constant
logic low will disable the output, and the LM3404/04HV is enabled if the DIM pin is open-circuited. The
DIM1 function disables only the power MOSFET, leaving all other circuit blocks functioning to minimize the
converter response time.
2
AN-1545 LM3404/LM3404HV Evaluation Board SNVA193E–January 2007–Revised April 2013
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