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

vestigating a little more deeply reveals that t

proportional to the current flowing into the R

dition of a single, general purpose PNP transistor forces t

ON

to be proportional to (V

are particularly useful to LED drivers: improved tolerance of

the average LED current, I

Δi

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

point, increasing I

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

V

30064501

FIGURE 1. Constant Ripple LED Driver Using the LM3404 Buck Regulator

The maximum voltage that can be achieved is then:

V

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

single infrared LED (V

LEDs in series, yielding a V

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

© 2008 National Semiconductor Corporation

300645

www.national.com