LM3404_2 Datasheet, PDF (1/4 Pages) National Semiconductor (TI) – COT Drivers Control LED Ripple Current

COT Drivers Control LED
Ripple Current
National Semiconductor
Application Note 1853
Chris Richardson
September 23, 2008
The constant on-time (COT) control method used by the
LM3402 and LM3404 constant-current buck regulators pro-
vides a balance between control over switching frequency
and fast transient response. Normally this "quasi-hysteretic"
control senses the input voltage and adjusts the on-time t_ON
of the power MOSFET as needed to keep f_SW constant. In-
vestigating a little more deeply reveals that t_ON is in fact
proportional to the current flowing into the R_ON pin. The ad-
dition of a single, general purpose PNP transistor forces t_ON
to be proportional to (V_IN - V_O) and provides two benefits that
are particularly useful to LED drivers: improved tolerance of
the average LED current, I_F, and constant LED ripple current,
Δi_F.
Benefits of Constant Ripple
The luminous flux and dominant wavelength (or color tem-
perature for white LEDs) of LED light are controlled by aver-
age current. The constant-ripple LED driver in Figure 1 is
much better at controlling average LED current over changes
in both input voltage and changes in output voltage because
it fixes the valley of the inductor current and also fixes the
current ripple.
Controlling LED ripple current implies control over peak LED
current, which in turn affects the luminous flux of an LED. All
LEDs have a relationship between their luminous flux and
forward current, I_F, that is linear up to a point. Beyond that
point, increasing I_F causes more heat than light. High ripple
current forces the LED to spend half of the time at a high peak
current, putting it in the lower lm/W region of the flux curve.
This reduces the light output when compared to a purely DC
drive current even though the average forward current re-
mains the same.
Close inspection of LED datasheets also reveals that the ab-
solute maximum ratings for peak current are close to or often
equal to the ratings for average current. High current density
in the LED junction lowers lumen maintenance, providing yet
another incentive for keeping the ripple current under control.
Circuit Performance
The circuit of Figure 1 uses the PNP-based constant ripple
concept to take an input voltage of 24VDC ±10% and drive
1A through as many LEDs in series as the maximum output
voltage will allow. For a circuit with 'n' LEDs of forward voltage
V_F in series, the output voltage is:
V_O = 0.2 + n x V_F
FIGURE 1. Constant Ripple LED Driver Using the LM3404 Buck Regulator
30064501
The maximum voltage that can be achieved is then:
V_O-MAX = V_IN-MIN x (1 - f_SW x 300 ns)
In the above equation, the 300 ns term reflects the minimum
off -time of the LM3402 and LM3404 buck regulators.
Making a "Universal" Current
Source
Figure 2 and Figure 3 show the dependence of ripple current
and switching frequency against output voltage. This change
in output voltage is effectively a change in the number of se-
ries-connected LEDs that the circuit drives.
One circuit with both average current and ripple current con-
trolled independently of V_O can now power anything from a
single infrared LED (V_F-TYP of ~1.8V) to as many as five white
LEDs in series, yielding a V_O of ~18V. Such a circuit would be
ideal for an LED-driving power-supply module. Many of the
existing, commercial AC-input 'brick' modules for driving
LEDs are specified to provide a constant current of 'x' mA at
a voltage up to 'y' volts. Depending on the need for galvanic
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