LM3508TLEV National Semiconductor, LM3508TLEV Datasheet - Page 11

• Manufacturer Part Number: LM3508TLEV
• Description: BOARD EVALUATION FOR LM3508TL
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3508TLEV

• Current - Output / Channel: 20mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 17V
• Features: Dimmable
• Voltage - Input: 2.7 ~ 5.5V
• Utilized Ic / Part: LM3508
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Once I
I
to DIM. This results in:
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 con-
verter 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 383kΩ 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 ac-
tion of the LM3508’s boost converter. For continuous inductor
current operation the input voltage ripple is composed of 2
primary components, the capacitor discharge (delta V
the capacitor’s equivalent series resistance (delta V
ripple due to strictly to the capacitor discharge is:
The ripple due to strictly to the capacitors ESR is:
In the typical application circuit, a 1µF ceramic input capacitor
works well. Since the ESR in ceramic capacitors is typically
less than 5mΩ and the capacitance value is usually small, the
input voltage ripple is primarily due to the capacitive dis-
charge. With larger value capacitors such as tantalum or
aluminum electrolytic the ESR can be greater than 0.5Ω. 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 ca-
pacitor 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 ca-
pacitor. Values of 2.2µF or 4.7µF can be used although start-
up current and start-up time will be increased. As with the
LED_MAX
LED_MAX
down to I
is set, the LED current can be adjusted from
LED_MIN
by applying a logic level PWM signal
ESR
Q
). The
) and
11
input capacitor, the output voltage ripple is composed of two
parts, the ripple due to capacitor discharge (delta V
ripple due to the capacitors ESR (delta V
the LM3508 will operate in continuous conduction mode. In
this mode the ripple due to capacitor discharge is given by:
The output voltage ripple component due to the output ca-
pacitors ESR is found by:
Table 1. Recommended Output Capacitor Manufacturers
Inductor Selection
The LM3508 is designed to operate with 10µH to 22µH
inductor’s. When choosing the inductor ensure that the in-
ductors saturation current rating is greater than
Additionally, the inductor’s value should be large enough such
that at the maximum LED current, the peak inductor current
is less than the LM3508’s peak switch current limit. This is
done by choosing L such that
Values for I
current limit vs. V
tics graphs.
Manufact
Murata
urer
TDK
PEAK
GRM39X5
R1E105M
R105K25
C1608X5
Number
D539
Part
and efficiency can be found in the plot of peak
IN
in the Typical Performance Characteris-
Value
1µF
1µF
Case Size
ESR
0603
0603
). Most of the time
www.national.com
Voltage
Q
Rating
) and the
25V
25V