ISL97634(2007) データシートの表示（PDF） - Intersil
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ISL97634 Datasheet PDF : 10 Pages
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ISL97634
VIN = 2.7V to 5.5V
C1
1µ
L1
1
2
2.2µ
M1
D0
10BQ100 C3 4.7µ D1
C2
0.1µ
VIN VOUT
LX
ISL97634
FBSW
FQT13N06L
D2
SK011C226KAR
D3
D4
FB
PWM/EN
D5
GND
R1 6.3Ω
D6
D7
D8
FIGURE 17. CONCEPTUAL 8 LEDs HIGH VOLTAGE DRIVER
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.
VIN = 2.7V to 5.5V 1 L1 2
VA
C3
D0
VB
C1
1µ
C2
0.1µ
22µ
VIN
LX
VOUT
ISL97634
FBSW
1µ
L2 C4 0.22µ
22µ
D1
SDIN
FB
GND
R1 1Ω
FIGURE 18. SEPIC LED DRIVER
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 -VIN rather than 0V.
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. VB is therefore shorted to the ground and C3 is
charged to VIN with VA = VIN. When the ISL97634 internal
switch closes and the circuit is in the PWM on state, VA is now
pulled to ground. Since the voltage in C3 cannot be changed
instantaneously, therefore VB is shifted downward and
becomes -VIN. The next cycle when the ISL97634 switch
opens, VB boosts up to the targeted output like the standard
boost regulator operation, except the lowest reference point is
at -VIN. The output is approximated as:
VOUT = VIN (---1-----D–----D-----)
(EQ. 9)
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 VB is considered the level-shifted LX node of a standard
boost regulator. The higher the input voltage, the lower the VB
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.
76
VIN = 2.7V
72
VIN = 4.2V
68
1 LED
64
L1 = L2 = 22µH
C3 = 1µF
R1 = 4.7Ω
60 0
5
10
15
20
ILED (mA)
FIGURE 19. EFFICIENCY MEASUREMENT OF A SINGLE LED
SEPIC DRIVER
PCB Layout Considerations
The layout is very important for the converter to function
properly. RSET must be located as close as possible to the FB
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. Maximizing the copper area connected to this
pad if possible. In addition, a solid ground plane is always
helpful for the EMI performance.
9
FN6264.2
April 10, 2007
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