ISL97632 データシートの表示（PDF） - Renesas Electronics
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ISL97632 Datasheet PDF : 10 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
C2 0.1µF
VIN
LX
VOUT
ISL97632
EN
FBSW
SDIN
FB
GND
C3 0.22µF
D1
D2
D3
D4 25mA
D5
D6
R1 4
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 = 4
5
10
15
20
25
30
ILED (mA)
FIGURE 8. EFFICIENCY IMPROVEMENT WITH 9V AND 12V
INPUTS
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
1 L1 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 6.3
D8
D9
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
FIGURE 9. CONCEPTUAL 9 LEDS HIGH VOLTAGE DRIVER
FN9239 Rev 4.00
March 22, 2010
Page 7 of 10
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