ISL97634 Intersil Corporation, ISL97634 Datasheet - Page 8

• Manufacturer Part Number: ISL97634
• Description: White LED Driver
• Manufacturer: Intersil Corporation

V
The analog dimming circuit can be tailored to a desired
relative brightness for different V
Equation 8.
R
Where V
the minimum relative brightness (i.e., the brightness with
V
i.e., V
i.e., V
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, V
voltage for the boost circuit as shown in Figure 15 and the
efficiency improvement is shown in Figure 16.
Vs = 12V
IN
2
Dim_max
FIGURE 15. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER
C1
= 2.7V to 5.5V
=
C2
[
-------------------------------------------------------------------
Dim_max
Dim_max
(
V
1µ
0.1µ
Dim_max
[
Dim_max
V
applied).
FB
EFFICIENCY OPERATION
•
IN
= 5V, F
= 1V, F
1
VIN
PWM/EN
(
1 F
–
must be separated from the high supply
ISL97634
–
is the maximum V
V
22µ
L1
FB
min
min
min
FBSW
) R
VOUT
•
)
GND
]
2
FB
LX
= 10% (i.e., 0.1), R
= 10% (i.e., 0.1), R
1
]
8
Dim
Dim
ranges using
voltage and F
C3
2
2
0.22µ
R1
= 189k
= 35k
4Ω
D1
D2
D3
D4
D5
D6
(EQ. 8)
min
is
ISL97634
.
8 LEDs Operation
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 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 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.
FIGURE 16. EFFICIENCY IMPROVEMENT WITH 9 AND 12V
90
85
80
75
70
0
INPUTS
5
V
S
= 12V
10
ILED (mA)
15
20
V
S
= 9V
V
6 LEDs
L1 = 22µH
R1 = 4Ω
f
PWM
IN
25
= 4V
April 10, 2007
FN6264.2
30