ISL97634 Intersil Corporation, ISL97634 Datasheet - Page 9

• Manufacturer Part Number: ISL97634
• Description: White LED Driver
• Manufacturer: Intersil Corporation

SEPIC Operation
For applications where the output voltage is not always
above the input voltage, a buck or boost regulation is
needed. A SEPIC (Single Ended Primary Inductance
Converter) topology, shown in Figure 18, can be considered
for such application. A single cell Li-Ion battery operating a
cellular phone 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 for 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
outweighs 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 where the input voltage is level
shifted downward at the same magnitude and the lowest
reference level starts at -V
V
IN
V
FIGURE 17. CONCEPTUAL 8 LEDs HIGH VOLTAGE DRIVER
IN
= 2.7V to 5.5V
C1
1µ
C1
1µ
= 2.7V to 5.5V
0.1µ
C2
0.1µ
C2
FIGURE 18. SEPIC LED DRIVER
VIN
PWM/EN
VIN
SDIN
1
ISL97634
ISL97634
1
22µ
L1
FBSW
2.2µ
VOUT
FBSW
L1
VOUT
GND
2
GND
FB
LX
FB
IN
LX
2
rather than 0V.
9
V
M1
A
R1
R1
C3
1µ
FQT13N06L
6.3Ω
1Ω
10BQ100
V
B
SK011C226KAR
22µ
D0
L2
D0
C4
C3 4.7µ
0.22µ
D1
D1
D2
D3
D4
D5
D6
D7
D8
ISL97634
The SEPIC works as follows; assume the circuit in Figure 18
operates normally, when the ISL97634 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
pulled to ground. Since the voltage in C3 cannot be changed
instantaneously, therefore V
becomes -V
opens, V
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 19
shows the efficiency measurement of a single LED application
as the input varies between 2.7V and 4.2V.
Note V
boost regulator. The higher the input voltage, the lower the V
voltage will be during PWM on period. The 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 19.
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 V
The heat of the IC is mainly dissipated through the thermal pad
of the package. Maximizing the copper area connected to this
pad if possible. In addition, a solid ground plane is always
helpful for the EMI performance.
V
FIGURE 19. EFFICIENCY MEASUREMENT OF A SINGLE LED
OUT
IN
76
72
68
64
60
. The output is approximated as:
B
0
=
is considered the level-shifted LX node of a standard
B
V
B
SET
boosts up to the targeted output like the standard
IN
IN
is therefore shorted to the ground and C3 is
IN
----------------- -
(
SEPIC DRIVER
. The next cycle when the ISL97634 switch
1 D
must be located as close as possible to the FB
with V
D
–
V
IN
5
)
= 2.7V
A
= V
IN
ILED (mA)
B
. When the ISL97634 internal
is shifted downward and
10
V
IN
IN
and V
= 4.2V
1 LED
L1 = L2 = 22µH
C3 = 1µF
R1 = 4.7Ω
15
OUT
pins.
April 10, 2007
A
(EQ. 9)
FN6264.2
is now
20
B