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ISL97672B
(EQ. 1)
I LEDmax = ---------------
R SET = 410.5 ? 0.02 = 20.52k Ω
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 ISL97672B
employs current mode control boost architecture that has a fast
current sense loop and a slow voltage feedback loop. Such
architecture achieves a fast transient response that is essential
for notebook backlight applications in which drained batteries
can be instantly changed to an AC/DC adapter without
noticeable visual disturbance. The number of LEDs that can be
driven by ISL97672B depends on the type of LED chosen in the
application. The ISL97672B is capable of boosting up to 45V and
typically driving 13 LEDs in series for each of the 6 channels,
enabling a total of 78 pieces of the 3.2V/20mA type of LEDs.
Enable
Device is enabled if the Enable pin voltage is high. If EN is pulled
low for longer than 30μs, the device will be shutdown. The Enable
pin should not float; a 10k or higher pull-down resistor should be
connected between EN and GND.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 20.
The LED DC current is set by translating the R SET current to the
output, with a scaling factor of 410.5/R SET . The source terminals
of the current source MOSFETs are designed to run at 500mV to
optimize power loss versus accuracy requirements. The sources
of errors of the channel-to-channel current matching come from
the op amp’s offset, internal layout, reference and current source
resistors. These parameters are optimized for current matching
and absolute current accuracy. The absolute accuracy is also
affected by the external R SET . A 1% tolerance resistor should be
used.
Dynamic Headroom Control
The ISL97672B features a proprietary Dynamic Headroom
Control circuit that detects the highest forward voltage string or
effectively the lowest voltage from any of the CH0 through CH5
pins. When this lowest channel voltage is lower than the
short-circuit threshold, V SC , this voltage is used as the feedback
signal for the boost regulator. The boost adjusts the output to the
correct level such that the lowest channel pin is at the target
headroom voltage. Since all LED stacks are connected to the
same output voltage, the other channel pins will have a higher
voltage, but the regulated current source circuit on each channel
ensures that each channel has the same current. The output
voltage regulates cycle by cycle, and it is always referenced to the
highest forward voltage string in the architecture.
Dimming Controls
The ISL97672B allows two ways of controlling the LED current,
and therefore, the brightness. They are:
1. DC current adjustment
2. PWM chopping of the LED current defined in Step 1.
MAXIMUM DC CURRENT SETTING
The LED DC current of each channel can be calculated as shown
in Equation 1:
410.5
R SET
For example, if the maximum required LED current
(I LED(max) ) is 20mA, rearranging Equation 1 yields Equation 2:
(EQ. 2)
PWM CURRENT CONTROL
The ISL97672B employs direct PWM dimming such that the output
PWM dimming follows directly with the input PWM signal without
modifying the input frequency. The average LED current of each
channel can be calculated as shown in Equation 3:
I LED ( ave ) = I LED × PWM
(EQ. 3)
Switching Frequency
The boost switching frequency can be adjusted by connecting a
resistor between FSW pin and GND. The calculation of the
resistance is shown in Equation 4:
( 5 × 10
)
f SW = -----------------------
+
-
REF
+
-
10
R FSW
(EQ. 4)
RSET
+
where f SW is the desirable boost switching frequency, and R FSW
is the setting resistor
PWM DIMMING
-
5V Low Dropout Regulator
There is an internal 5V low dropout (LDO) regulator to develop the
necessary low-voltage supply, which is used by the chip’s internal
FIGURE 20. SIMPLIFIED CURRENT SOURCE CIRCUIT
9
control circuitry. VDC is the output of this LDO regulator which
requires a bypass capacitor of 1μF or more for the regulation.
The VDC pin can be used as a coarse reference as long as it is
sourcing only a few milliamps.
FN7995.1
November 22, 2013
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