ISL97632
MOSFET connected in cascode fashion to achieve higher
output voltage. A conceptual 8 LEDs driver circuit is shown
in Figure 9. A 60V logic level N-Channel MOSFET is
Applications
Efficiency Improvement
Figure 2 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 seperated from the high supply
voltage for the boost circuit as shown in Figure 7 and the
efficiency improvement is shown in Figure 8.
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.
C3
0.22µ
D1
D2
D3
D4
D5
D6
L1
Vs = 12V
C1
1
2
22µ
1µ
V
= 2.7V TO 5.5V
25mA
IN
VIN
LX
VOUT
ISL97632
FBSW
0.1µ
C2
EN
FB
SDIN
GND
R1
4Ω
FIGURE 7. SEPERATE HIGH INPUT VOLTAGE FOR HIGHER
EFFICIENCY OPERATION
90
L1
D0
V
= 2.7V TO 5.5V
V
= 12V
IN
S
1
2
C3
C1
1µ
2.2µ
10BQ100
FQT13N06L
D1
D2
D3
D4
D5
D6
D7
D8
4.7µ
M1
85
80
V
= 9V
S
SK011C226KAR
VIN
VOUT
LX
ISL97632
C2
0.1µ
FBSW
EN
V
= 4V
75
70
IN
FB
6 LEDs
L1 = 22µH
R1 = 4Ω
SDIN
GND
6.3Ω
R1
5
10
15
ILED (mA)
20
25
30
0
FIGURE 8. EFFICIENCY IMPROVEMENT WITH 9V AND 12V
INPUTS
8 LEDs Operation
FIGURE 9. CONCEPTUAL 8 LEDS HIGH VOLTAGE DRIVER
For medium size LCDs that need more than 6 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
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
FN9239.2
April 10, 2007
7
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