ISL97632IRT14ZT 查看數據表（PDF） - Intersil
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ISL97632IRT14ZT Datasheet PDF : 9 Pages
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ISL97632
Applications
Efficiency Improvement
Figure 2 on page 5 shows the efficiency measurements. 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 from lowering the peak current is
greater than the impact of the resistance increase with larger
value of inductor. Efficiency can also be improved for
systems that have high supply voltages. Since the ISL97632
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 7 and the efficiency improvement is shown in
Figure 8.
Vs = 12V
C1 1µF
L1
1
2
22µH
VIN = 2.7V TO 5.5V
VIN
C2 0.1µF
LX
VOUT
ISL97632
EN
FBSW
SDIN
FB
GND
C3 0.22µF
D1
D2
D3
D4 25mA
D5
D6
R1 4Ω
FIGURE 7. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER
EFFICIENCY OPERATION
90
VS = 12V
85
80
VS = 9V
75
70
0
VIN = 4V
6 LEDs
L1 = 22µH
R1 = 4Ω
5
10
15
20
25
30
ILED (mA)
FIGURE 8. EFFICIENCY IMPROVEMENT WITH 9V AND 12V
INPUTS
9 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 ISL97632 is not sufficient. However, the ISL97632 can
be used as an LED controller with an external protection
MOSFET connected in cascode fashion to achieve higher
output voltage. A conceptual 9 LEDs driver circuit is shown
in Figure 9. A 40V 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 ISL97632 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 ISL97632 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 breaks down the gate oxide of M1. To
prevent this, VOUT should be connected to VIN, allowing the
internal Schottky 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 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 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 and via a 10k resistor to FB. A zener can be placed
from VOUT to the FB pin allowing an over voltage event to
pull up on FB with a low breakdown current (and thus low
power zener) as a result of the 10k resistor.
VIN = 2.7V TO 5.5V
C1
1µF
L1
1
2
2.2µH
M1
D0
10BQ100
C3
4.7µF
D1
C2
0.1µF
VIN VOUT
LX
ISL97632
FBSW
EN
FB
SDIN
GND
FQT13N06L
D2
SK011C226KAR
R1 6.3Ω
D8
D9
FIGURE 9. CONCEPTUAL 9 LEDS HIGH VOLTAGE DRIVER
7
FN9239.3
March 22, 2010
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