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ISL97675
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 ISL97675
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 the notebook backlight application. The input power may
instantly change when the user switches from a drained battery
to a AC/DC adapter without causing any flicker in the display
backlight. The ISL97675 is capable of boosting up to 45V and
typically can drive 13 (3.2V/20mA) LEDs in series on each of the
4 channels.
The LED peak current is set by translating the R ISET current to the
output with a scaling factor of 392/R ISET . The drain terminals of
the current source MOSFETs are designed to run at ~ 500mV to
minimize power loss. The sources of errors for the channel-to-
channel current matching are due to internal mismatches,
offsets and the external R ISET resistor. To minimize this external
offset, a 1% tolerance resistor is recommended.
-
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OVP
The Overvoltage Protection (OVP) pin has a primary function of
+
REF
+
setting the overvoltage trip level.
The ISL97675 OVP threshold is set by R UPPER and R LOWER such
that:
R ISET
PWM DIMMING
V OUT _OVP = 1.21V × ( R UPPER + R LOWER ) ? R LOWER
(EQ. 1)
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
The ISL97675 has a patent pending switching architecture that
uses the OVP block for feedback monitoring, hence allowing very
low PWM dimming duty cycle operation. As a result, the
overvoltage trip level also limits the V OUT regulation range
between 64% and 100% of the V OUT _OVP and the equation is :
Dynamic Headroom Control
The ISL97675 features a proprietary Dynamic Headroom Control
circuit that detects the highest forward voltage string or
effectively the lowest voltage from any of the FB1-4 pins digitally.
This lowest FB voltage is used as the feedback signal for the
Allowable V OUT = 64% to 100% of V OUT _OVP
(EQ. 2)
boost regulator. Since all LED stacks are connected in parallel to
the same output voltage, the other FB pins will have a higher
I LEDmax = ---------------
( 392 )
R ISET
R ISET = ( 392 ) ? 0.02 = 19.6k Ω
For example, if 10 LEDs are used with the worst case V OUT of 35V, and
R UPPER and R LOWER are chosen such that the OVP level is set at 40V,
then the allowed V OUT range is between 25.6V and 40V. If the
requirement is changed to 6 LEDs/channel for a maximum V OUT of
21V, then the OVP level must be reduced according to Equation 2 to
accommodate the new reduced output voltage. Otherwise, the
headroom control will be disturbed and the channel voltage may
be higher and prevent the driver from operating properly.
The ratio of the OVP capacitors should be the inverse of the OVP
resistors. For example, if R UPPER /R LOWER = 33/1, then
C UPPER /C LOWER = 1/33. For example, if C UPPER = 100pF then
C LOWER = 3.3nF.
Enable
An EN signal is required to enable the internal regulator for normal
operation. If there is no signal longer than 28ms, the device will
enter shutdown.
Power Sequence
There is no specific power sequence requirement for the
ISL97675. The EN signal can be tied to V IN but not the VDDIO as
it will prevent the device from powering up.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 19.
12
voltage, but the regulated current source circuit on each channel
will ensure that each channel has the same current. The output
voltage will regulate cycle by cycle and it is always referenced to
the highest forward voltage string in the architecture.
Dimming Controls
The ISL97675 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 initial brightness should be set by choosing an appropriate
value for R ISET . This should be chosen to fix the maximum
possible LED current:
(EQ. 3)
For example, if the maximum required LED current
(I LED(max) ) is 20mA, rearranging Equation 3 yields Equation 4:
(EQ. 4)
FN7630.1
September 14, 2011
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