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Ripple Reduction Capacitor C3

4 Ripple Reduction Capacitor C3

The component C3 is optional and not installed on the standard demo board. This capacitor is used to

reduce LED ripple current without changing average LED current or inductor current. Without C3, the LED

ripple current is equal to the inductor ripple current. The peak-to-peak ripple current values shown in

Table 2 are without C3 installed. A typical C3 value of 1 µF is recommended for most applications. Any

type of low ESR capacitor can be used. However, the working voltage must be rated higher than the

maximum input voltage. C3 can also be used to reduce switching noise which could affect the SNS and

DIM pins, especially at higher LED current. For noise reduction purposes, a C3 value of 220 pF is

recommended.

Table 2. Typical Operating Conditions

# of Series LED Type Average LED Freq.

(1)

p-p Ripple

(1)

Minimum VIN

LEDs Current

1 Osram golden dragon 340 mA 425 kHz 110 mA 4.5V

2 Osram golden dragon 340 mA 655 kHz 110 mA 7.0V

3 Luxeon Star 340 mA 800 kHz 110 mA 10.4V

4 Luxeon Star 340 mA 845 kHz 110 mA 13.8V

1 Luxeon V Star 700 mA 415 kHz 190 mA 7.0V

2 Luxeon V Star 700 mA 500 kHz 190 mA 14.5V

1 Luxeon K2 Emitter 1A 230 kHz 280 mA 4.5V

(1)

at 24Vin

5 PWM Dimming

The DIM terminal on the PCB provides an input for a pulse width modulated (PWM) signal to control

dimming of the LED string. The PWM signal should have a minimum logic high level of 2.25V, and

minimum low and high periods of 100 ns. For the default demo board circuit, a dimming frequency less

than 1kHz is recommended to maintain an adequate contrast ratio. The maximum contrast ratio is shown

in Figure 10, and assumes a minimum of 2 switching cycles. The maximum contrast ratio depends largely

on the rise time of the inductor current, and therefore is dependant on input voltage, inductor size, and

LED string forward voltage. Additionally, if C3 is installed, the maximum contrast ratio or DIM frequency

will be further reduced.

6 Open Circuit/Short Circuit

If the LED array is disconnected or fails open, the LM3401 will operate at 100% duty cycle. The output

voltage (at LED+) and CS node will rise to equal the input voltage; the circuit is rated to 40V and will not

suffer damage. However, care should be taken not to connect an LED string when the demo board is

powered up and the output is high as excessive surge current may damage the LEDs.

The default current limit threshold is typically 1.5A, set by R3. The current limit will protect the circuit from

a short at the LED anode or cathode. Current limit is sensed across the R

DSon

of the PFET and will

therefore vary with temperature. R3 can be adjusted if a different current limit threshold is required. For

the current limit details, refer to LM3401 Hysteretic PFET Controller for High Power LED Drive

(SNVS516).

SNVA254C–August 2007–Revised April 2013 AN-1657 LM3401 Demo Board

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