ISL97632(2007) Ver la hoja de datos (PDF) - Intersil
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ISL97632 Datasheet PDF : 9 Pages
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ISL97632
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.
VIN = 2.7V to 5.5V 1 L1 2
VA C3 VB
C1
1µ
22µ
1µ
L2 C4 0.22µ
22µ
D1
VIN
LX
C2
0.1µ
VOUT
EN
SDIN
FBSW
FB
GND
R1 1Ω
boost regulator operation, except the lowest reference point
is at -VIN. The output is approximated as:
VOUT
=
VI
N
--------D---------
(1 – D)
(EQ. 5)
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, 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 11.
76
VIN = 2.7V
72
FIGURE 10. SEPIC LED DRIVER
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 -VIN rather than 0V.
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. VB is therefore shorted to the ground and C3 is
charged to VIN with VA = VIN. When the ISL97632 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 instantenously, VB is shifted downward and
becomes -VIN. The next cycle when the ISL97632 switch
opens, VB boosts up to the targetted output like the standard
68
VIN = 4.2V
1 LED
64
L1 = L2 = 22µH
C3 = 1µF
R1 = 4.7Ω
60
0
5
10
15
20
ILED (mA)
FIGURE 11. 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. Maximize the copper area connected to
this pad if possible. In addition, a solid ground plane is always
helpful for the EMI performance.
8
FN9239.2
April 10, 2007
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