ISL97634 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
These equations are valid for values of R1 and R2 such that
both R1>>RSET and R2>>RSET.
The analog dimming circuit can be tailored to a desired
relative brightness for different VDim ranges using
Equation 8.
R2
=
-[--(--V-----D----i-m-----_---m----a---x-----–----V----F----B----)---•-----R----1----]
[VFB • (1 – Fmin)]
(EQ. 8)
Where VDim_max is the maximum VDim voltage and Fmin is
the minimum relative brightness (i.e., the brightness with
VDim_max applied).
i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k
i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k
Efficiency Improvement
Figure 1 shows the efficiency measurements during PWM
operation. The choice of the inductor has a significant impact
on the power efficiency. As shown in Equation 4, the higher
the inductance, the lower the peak current, therefore, the
lower the conduction and switching losses. On the other hand,
it has also a higher series resistance. Nevertheless, the
efficiency improvement effect by lowering the peak current is
greater than the resistance increases with larger value of
inductor. Efficiency can also be improved for systems that
have high supply voltages. Since the ISL97634 can only
supply from 2.4V to 5.5V, VIN must be separated from the
high supply voltage for the boost circuit as shown in Figure 15
and the efficiency improvement is shown in Figure 16.
Vs = 12V
C1 1µF
L1
1
2
22µH
C3 0.22µF
D1
D2
D3
VIN = 2.7V TO 5.5V
VIN
C2 0.1µF
LX
VOUT
ISL97634
FBSW
FB
PWM/EN
GND
D4
D5
D6
R1 4Ω
FIGURE 15. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER
EFFICIENCY OPERATION
.
90
85
VS = 12V
VS = 9V
80
75
70
0
VIN = 4V
7 LEDs
L1 = 22µH
R1 = 4Ω
fPWM
5
10
15
20
25
30
ILED (mA)
FIGURE 16. EFFICIENCY IMPROVEMENT WITH 9 AND 12V
INPUTS
8 LEDs Operation
For medium size LCDs that need more than 7 low power LEDs
for backlighting, such as a portable media player or automotive
navigation panel displays, the voltage range of the ISL97634 is
not sufficient. However, the ISL97634 can be used as an LED
controller with an external protection MOSFET connected in
cascode fashion to achieve higher output voltage. A conceptual
8 LEDs driver circuit is shown in Figure 17. A 60V logic level N-
Channel MOSFET is configured such that its drain ties between
the inductor and the anode of Schottky diode, its gate ties to the
input, and its source ties to the ISL97634 LX node connecting
to the drain of the internal switch. When the internal switch
turns on, it pulls the source of M1 down to ground and LX
conducts as normal. When the internal switch turns off, the
source of M1 will be pulled up by the follower action of M1,
limiting the maximum voltage on the ISL97634 LX pin to below
VIN, but allowing the output voltage to go much higher than the
breakdown limit on the LX pin. The switch current limit and
maximum duty cycle will not be changed by this setup, so input
voltage will need to be carefully considered to make sure that
the required output voltage and current levels are achievable.
Because the source of M1 is effectively floating when the
internal LX switch is off, the drain-to-source capacitance of M1
may be sufficient to capacitively pull the node high enough to
break down the gate oxide of M1. To prevent this, VOUT should
be connected to VIN, allowing the internal Schottky diode to
limit the peak voltage. This will also hold the VOUT pin at a
known low voltage, preventing the built in OVP function from
causing problems. This OVP function is effectively useless in
this mode as the real output voltage is outside its intended
range. If the user wants to implement their own OVP protection
(to prevent damage to the output capacitor), they should insert
a zener diode from VOUT to the FB pin. In this setup, it would
be wise not to use the FBSW to FB switch, as otherwise, the
zener diode will have to be a high power one capable of
dissipating the entire LED load power. Then the LED stack can
then be connected directly to the sense resistor via a 10kΩ
resistor to FB. A zener can be placed from VOUT to the FB pin
allowing an overvoltage event to pull-up on FB with a low
breakdown current (and thus low power zener diode) as a
result of the 10kΩ resistor.
8
FN6264.3
March 7, 2008
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