ISL97632 Intersil Corporation, ISL97632 Datasheet - Page 8

• Manufacturer Part Number: ISL97632
• Description: White LED Driver
• Manufacturer: Intersil Corporation

Converter) topology, (shown in Figure ), can be considered
for such an application. A single cell Li-Ion battery
operating a cellphone backlight or flashlight is one
example. The battery voltage is between 2.5V and 4.2V
depending on the state of charge. On the other hand, the
output may require only one 3V to 4V medium power LED
for illumination because the light guard of the backlight
assembly is optimized or it is a cost efficiency trade off
reason.
In fact, a SEPIC configured LED driver is flexible enough to
allow the output to be well above or below the input voltage,
unlike the previous example. Another example is when the
number of LEDs and input requirements are different from
platform to platform, a common circuit and PCB that fit all the
platforms, in some cases, may be beneficial enough that it
outweights the disadvantage of adding additional component
cost. L1 and L2 can be a coupled inductor in one package.
The simplest way to understand SEPIC topology is to think
about it as a boost regulator in which the input voltge is level
shifted downward at the same magnitude and the lowest
reference level starts at -V
The SEPIC works as follows; assume the circuit in Figure 10
operates normally when the ISL97632 internal switch opens
and it is in the PWM off state after a short duration where few
LC time constants elapsed, the circuit is considered in the
steady-state within the PWM off period that L1 and L2 are
shorted. V
charged to V
switch closes and the circuit is in the PWM on state, V
now pulled to ground. Since the voltage in C3 cannot be
changed instantenously, V
becomes -V
opens, V
VIN = 2.7V to 5.5V
C1
1µ
C2
0.1µ
B
B
boosts up to the targetted output like the standard
IN
is therefore shorted to the ground and C3 is
IN
. The next cycle when the ISL97632 switch
FIGURE 10. SEPIC LED DRIVER
with V
VIN
EN
SDIN
1
22µ
L1
A
FBSW
VOUT
= V
2
GND
FB
LX
IN
B
IN
is shifted downward and
rather than 0V.
. When the ISL97632 internal
8
V
A
R1
C3
1µ
1Ω
V
B
22µ
L2
C4
0.22µ
A
is
D1
ISL97632
boost regulator operation, except the lowest reference point
is at -V
where D is the on-time of the PWM duty cycle.
The convenience of SEPIC comes with some trade off in
addition to the additional L and C costs. The efficiency is
usually lowered because of the relatively large efficiency
loss through the Schottky diode if the output voltage is low.
The L2 series resistance also contributes additional loss.
Figure 11 shows the efficiency measurement of a single LED
application as the input varies between 2.7V and 4.2V.
Note, V
standard boost regulator. The higher the input voltage, the
lower the V
result is that the efficiency will be lower at higher input
voltages because the SEPIC has to work harder to boost up
to the required level. This behavior is the opposite to the
standard boost regulator’s and the comparison is shown in
Figure 11.
PCB Layout Considerations
The layout is very important for the converter to function
properly. R
and GND pins. Longer traces to the LEDs are acceptable.
Similarly, the supply decoupling cap and the output filter cap
should be as close as possible to the VIN and VOUT pins.
The heat of the IC is mainly dissipated through the thermal
pad of the package. Maximize the copper area connected to
this pad if possible. In addition, a solid ground plane is always
helpful for the EMI performance.
V
FIGURE 11. EFFICIENCY MEASUREMENT OF A SINGLE LED
OUT
IN
=
76
72
68
64
60
B
. The output is approximated as:
V
0
is considered the level-shifted LX node of a
IN
SET
B
----------------- -
(
voltage will be during PWM on period. The
1 D
SEPIC DRIVER
D
–
must be located as close as possible to the FB
)
5
ILED (mA)
V
V
IN
IN
10
= 4.2V
= 2.7V
1 LED
L1 = L2 = 22µH
C3 = 1µF
R1 = 4.7Ω
15
April 10, 2007
(EQ. 5)
FN9239.2
20