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FN7371.0

June 28, 2005

The efficiency bench test results are shown in Figure 12. 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).

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 13.

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

with the sensitive signals connected to Signal Ground. Both

grounds should only be connected at one point right at the

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.

The FB pin is most important. The current sense resistor

required to the LEDs, a small decoupling capacitor should be

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.

FIGURE 12. EFFICIENCY CURVE WITH 2, 3, 4, 5 AND 6 LEDs

LOAD

55

60

65

70

75

80

85

90

010

20

30

LED CURRENT (mA)

3LED

4LED

5LED

2LED

6LED

EL7630

C1

VDD

LX

ENAB

FB

GND

L1

D1

C2

2.7V~5.5V

OFF/ON

LEDs

EL7630

VDD

LX

ENAB

FB

GND

L1

D1

FIGURE 13. LEDs IN SERIES/PARALLEL WITH MIRROR

CURRENT BALANCE

EL7630