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EL7630
The efficiency bench test results are shown in Figure 11. In
the test, the input voltage is 4V and 2, 3, 4, 5 and 6 LEDs are
used as the load (boost inductor L = 22μH Sumida
CDRH5D28R-220NC).
PCB Layout Considerations
The PCB layout is very important for the converter to
function properly. For the SC-70 6 pin package, Power
Ground and Signal Ground should be separated to ensure
the high pulse current in the power ground does not interfere
90
85
22μH,V IN =4V
with the sensitive signals connected to Signal Ground. Both
grounds should only be connected at one point right at the
80
75
70
65
60
4LED
6LED
3LED
2LED
5LED
chip. The heavy current loops (V IN -L1-LX-PGND, and V IN -
L1-D1-C2-PGND) should be as short as possible. For the
TSOT 5 pin package, there is no separated GND. All return
GNDs should be connected in GND pin but with no sharing
branch. Based on the signal level on each branch, the lower
power level of the branch, the closer the branch to GND pin
in order to minimize the branch interactive.
55
0
10 20
LED CURRENT (mA)
30
The FB pin is most important. The current sense resistor
R S ET should be very close to this pin. If a long trace is
required to the LEDs, a small decoupling capacitor should be
FIGURE 11. EFFICIENCY CURVE WITH 2, 3, 4, 5 AND 6 LEDs
LOAD
White LED Connections
One leg of LEDs connected in series will ensure brightness
uniformity. The 27V maximum output voltage specification
enables up to 6 LEDs to be placed in series.
In order to output more power to drive more LEDs, LEDs
should be in series/parallel connection. Due to the LED's
negative temperature coefficient, in each parallel branch, the
driving source should be high impedance, to balance the
LED current in each branch. One of the ways to ensure the
brightness uniformity is to add mirror current balance circuit,
built up with three transistors for the 15 LEDs series/parallel
connection application shown in Figure 12.
D1
L1
placed at this pin.
The heat of the IC is mainly dissipated through the PGND
pin. Maximizing the copper area connected to this pin is
preferable. In addition, a solid ground plane is always helpful
for the EMI performance.
The demo board is a good example of layout based on the
principle. Please refer to the EL7630 Application Brief for the
layout.
V IN
2.7V~5.5V
VDD
LX
C2
C1
EL7630
ENAB FB
OFF/ON
ENAB FB
GND
R SET
LEDs
FIGURE 12. LEDs IN SERIES/PARALLEL WITH MIRROR
CURRENT BALANCE
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
7
FN7371.1
February 22, 2006
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