CAT32 White LED Driver Efficiency and Inductor Value Tradeoffs

AND8423/D
Former Catalyst Document Number DN9
CAT32 White LED Driver
Efficiency and Inductor
Value Tradeoffs
http://onsemi.com
Prepared by: Fabien Franc,
Applications Manager
ON Semiconductor
APPLICATION NOTE
The inductors used are 3.3 mH and 6.8 mH from
Panasonic, ELJEA (type 1210). The setup uses the
ON Semiconductor CAT32EVAL1 evaluation board. The
four white LEDs are from Nichia, NSCW100.
This application brief shows the effect of the inductor
value on efficiency when the CAT32 is used as a White LED
driver. In addition, it shows the output voltage VOUT and the
switch SW pin waveforms and describes how efficiency is
defined and measured.
Panasonic ELJEA6R8
Zetex ZHCS400
L1
6.8mH
VIN
SW
VIN
3.7V
VOUT
White
LEDs
CAT32
+
C1
4.7mF
SHDN
LED
RSET
GND
C2
1mF
15mA
R1
Figure 1. Test Setup with CAT32 driving 4 LEDs
− for L = 6.8 mH, the efficiency is 82%
− for L = 3.3 mH, the efficiency is 78%
A 4% improvement in efficiency is gained with the larger
inductor value. This is true for a given setup and for this
specific type of inductor.
Efficiency
The CAT32 efficiency is defined as the ratio of the power
dissipated through the LEDs (not including the loss in the
Schottky diode) to the power from the input supply:
Efficiency + P LEDńP IN + ǒV OUT * V LEDǓ
I LEDńǒV IN
I INǓ
The efficiency is measured for an input supply of 3.7 V.
The efficiency for LED current between 15 mA and 20 mA
is as follows:
© Semiconductor Components Industries, LLC, 2009
August, 2009 − Rev. 0
1
Publication Order Number:
AND8423/D
AND8423/D
Signal Waveforms
(VOUT). The measurement is done for a 3.7 V supply and for
a 15 mA LED current flowing through the 4 LEDs.
The signal waveforms below show the switch pin voltage
(Vsw), the inductor current (IL), and the output voltage
Vsw, IL, VOUT with L = 6.8 mH
Vsw, IL, VOUT with L = 3.3 mH
For the smaller inductor value of 3.3 mH:
• The switching noise on VOUT is greater
• The charging and discharging time are shorter
• The peak inductor current is greater
• The ringing frequency on SW is higher (when there is
no current in the inductor)
In summary, reducing the inductor value has some impact
on the overall power efficiency, the inductor peak current,
the switching noise and the ringing.
Information on Nichia and their LEDs can be found at:
www.nichia.com.
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