ISL97634 View Datasheet(PDF) - Renesas Electronics
Part Name
Description
MFG CO.
ISL97634 Datasheet PDF : 12 Pages
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ISL97634
Efficiency Improvement
Figure 2 shows the efficiency measurements during PWM
operation. The choice of the inductor has a significant impact on
the power efficiency. Equation 4 shows 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 16 and
the efficiency improvement is shown in Figure 17.
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 4
FIGURE 16. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER
EFFICIENCY OPERATION
.
90
85
VS = 12V
VS = 9V
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
10k 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 10k resistor.
VIN = 2.7V TO 5.5V
C1
1µF
L1
1
2
10µH or 22µH
M1
D0
10BQ100 C3 4.7µF
C2
0.1µF
VIN VOUT
LX
ISL97634
FBSW
FQT13N06L
SK011C226KAR
FB
PWM/EN
GND
R1 6.3
80
75
70
0
VIN = 4V
7 LEDs
L1 = 22µH
R1 = 4
fPWM
5
10
15
20
25
30
ILED (mA)
FIGURE 17. EFFICIENCY IMPROVEMENT WITH 9 AND 12V INPUTS
Operation with VOUT above 26V
For LED backlighting applications that need an output voltage above
26V, 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 as shown in Figure 18. 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
FN6264 Rev 4.00
August 27, 2013
FIGURE 18. HIGH VOLTAGE LED DRIVER USING A CASCODE
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 Converter) topology, shown in
Figure 19, can be considered for such an application. A single
cell Li-ion battery operating a cellular phone 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 for 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
Page 9 of 12
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