Datasheet > LM3404_2

LM3404_2

更新时间： 2024-09-26 11:34:55

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COT Drivers Control LED Ripple Current

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National Semiconductor

Application Note 1853

Chris Richardson

COT Drivers Control LED

Ripple Current

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_SWconstant. In-

vestigating a little more deeply reveals that t_ONis in fact

proportional to the current flowing into the R_ONpin. 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.

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_Fcauses 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.

Benefits of Constant Ripple

Circuit Performance

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.

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_Fin series, the output voltage is:

V_O= 0.2 + n x V_F

Controlling LED ripple current implies control over peak LED

current, which in turn affects the luminous flux of an LED. All

30064501

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

The maximum voltage that can be achieved is then:

V_O-MAX= V_IN-MINx (1 - f_SWx 300 ns)

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_Ocan now power anything from a

single infrared LED (V_F-TYPof ~1.8V) to as many as five white

LEDs in series, yielding a V_Oof ~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

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

300645

www.national.com

12 3 4

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与LM3404_2相关器件

型号	品牌	获取价格	描述	数据表
LM3404HV	NSC	获取价格	1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HV	TI	获取价格	LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMA	NSC	获取价格	1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMA	TI	获取价格	LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMA/NOPB	TI	获取价格	1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMAX	NSC	获取价格	1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMAX	TI	获取价格	LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMAX/NOPB	TI	获取价格	1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMR	NSC	获取价格	1.0A Constant Current Buck Regulator for Driving High Power LEDs
LM3404HVMR	TI	获取价格	LM3404/04HV 1.0A Constant Current Buck Regulator for Driving High Power LEDs

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