´

¶

where

´

¶

´

¶

D X 200 mV

200 mV

LM3508

www.ti.com

SNVS494C – APRIL 2007 – REVISED MAY 2013

APPLICATION INFORMATION

Brightness Adjustment

resistor from SET to GND) such that:

(2)

PWM signal to DIM. This results in:

(3)

where D is the duty cycle of the PWM pulse applied to DIM. The LM3508 can be brought out of shutdown while a

signal is applied to DIM, allowing the device to turn on into a low LED current mode. A logic low at DIM will shut

off the current source making ILED high impedance however, the boost converter continues to operate. Due to

an offset voltage at SET (approximately +/-2mV) the LED’s can faintly illuminate even with DIM pulled to GND. If

zero LED current is required then pulling EN low will shutdown the current source causing the LED current to

drop to zero. DIM has an internal 383k

Ω pull down to PGND.

Input Capacitor Selection

Choosing the correct size and type of input capacitor helps minimize the input voltage ripple caused by the

switching action of the LM3508’s boost converter. For continuous inductor current operation the input voltage

(4)

The ripple due to strictly to the capacitors ESR is:

(5)

In the typical application circuit, a 1µF ceramic input capacitor works well. Since the ESR in ceramic capacitors is

typically less than 5m

Ω and the capacitance value is usually small, the input voltage ripple is primarily due to the

capacitive discharge. With larger value capacitors such as tantalum or aluminum electrolytic the ESR can be

greater than 0.5

Ω. In this case the input ripple will primarily be due to the ESR.

Output Capacitor Selection

In a boost converter such as the LM3508, during the on time, the inductor is disconnected from OUT forcing the

output capacitor to supply the LED current. When the PMOS switch (synchronous rectifier) turns on the inductor

energy supplies the LED current and restores charge to the output capacitor. This action causes a sag in the

output voltage during the on time and a rise in the output voltage during the off time.

The LM3508’s output capacitor is chosen to limit the output ripple to an acceptable level and to ensure the boost

converter is stable. For proper operation use a 1µF ceramic output capacitor. Values of 2.2µF or 4.7µF can be

used although start-up current and start-up time will be increased. As with the input capacitor, the output voltage

mode the ripple due to capacitor discharge is given by:

(6)

The output voltage ripple component due to the output capacitors ESR is found by:

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