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ISL97691
Theory of Operation
PWM Boost Converter
The current mode PWM boost converter produces the minimal
voltage needed to enable the LED stack with the highest forward
voltage drop to run at the programmed current. The ISL97691
employs current mode control boost architecture that has a fast
current sense loop and a slow voltage feedback loop. This
architecture achieves the fast transient response which is
essential for portable product backlight applications where the
backlight must not flicker when the power source is changed
from a drained battery to an AC/DC adapter.
ANALOG DIMMING
The channel peak current for the two groups may be reduced
from the maximum LED current set by R SET by analog dimming
through two 6-bit DACs. The DACs are updated with simple
negative pulse duration and pulse counting 1-Wire interfaces.
REF
The number of LEDs that can be driven by the ISL97691 depends
on the type of LED chosen in the application. The maximum
1-WIRE DATA
6-BIT DAC
+
-
output is 26V at 40mA from 2.7V input, or 21V at 60mA from
2.7V input with 30% dimming duty.
AutoShutdown
The ISL97691 simplifies the implementation of low power
shutdown by detecting that both PWM inputs 1-WIRE_1 and
1-WIRE_2 have been low for 120ms (typical). When this timeout
is detected on both 1-WIRE_1 and 1-WIRE_2, the boost converter
and LED string drivers are turned off while retaining the value of
the two internal 6-bit 1-Wire registers which store the analog
dimming values.
The 1-Wire interfaces may be written during shutdown mode
subject to the following constraints:
? Any interface (1-WIRE_1 or 1-WIRE_2) to be written during
shutdown should idle high between transmissions
? The first bit of every 6-bit 1-Wire transmission must be sent
with a tighter timing tolerance than normal (25μs minimum
for logic 1, 100μs minimum for logic 0). The extra 10μs allows
the ISL97691 to wakeup and measure the transmission.
It is recommended that applications requiring 1-Wire
transmissions during shutdown simply apply the constraints
described above to all transmissions.
Dimming Controls
The ISL97691 provides two groups, channels 1 and 2 as one
group and channels 3 and 4 as the other group, with
independent analog and PWM dimming.
The maximum LED current is set globally for all channels by R SET
in the range 15mA to 60mA per Equation 1:
+
-
PWM SIGNAL
FIGURE 18.
Channels 1 and 2 are set through the 1-WIRE_1 pin, and
channels 3 and 4 are set through the 1-WIRE_2 pin. The two
1-Wire interfaces are independent of the two PWM inputs
PWMI_1 and PWMI_2, and any combination of these 4 inputs
may be active at the same time.
The 1-Wire interface provides 6-bit (64-level) analog dimming
resolution. The interface uses a normally high connection for use
with open-drain driving schemes and Intersil’s 1-Wire interface.
When held low for between 15μs and 45μs, the interface reads a
logic 1. When held low for between 90μs and 120μs the
interface reads a logic 0. When held low for greater that 215μs,
the interface loads the last 6 bits into the brightness control
register and updates the peak current. The required minimum
high time is 3μs.
The 1-Wire programming is summarized as follows:
? Logic 0 = Negative pulse >90μs and <120μs
? Logic 1 = Negative pulse >15μs and <45μs
? Load = Negative pulse >215μs
Figure 18 shows an example of transmitting and loading the
value b’100101’.
The serial interface defaults to b’111111’ (63) on power-up. The
I LEDmax = ---------------
I LED = I LEDmax × ------
(EQ. 3)
1066
R SET
Where:
R SET is resistor from ISET pin to GND ( ? )
I LEDmax is the peak current set by resistor R SET (A)
For example, if the maximum required LED current
(I LEDmax ) is 60mA, then the R SET value needed is:
R SET = 1066 ? 0.06 = 17.8k Ω
Choose nearest standard resistor: 17.8k ? , 0.1%
11
(EQ. 1)
(EQ. 2)
maximum LED current I LEDmax set by resistor R SET for each of
the two channel groups 1 and 2 is digitally adjusted by each
channel’s brightness control register per Equation 3:
N
63
Where:
N is the integer value 1 to 63 in the channel groups’ brightness
control register
I LEDmax is the peak current set by resistor R SET (A)
FN7840.1
February 1, 2013
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