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ISL97682, ISL97683, ISL97684
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 ISL97682,
ISL97683, ISL97684 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
where the power can be a series of drained batteries or instantly
change to an AC/DC adapter without rendering a noticeable
visual nuisance. The number of LEDs that can be driven by the
ISL97682, ISL97683, ISL97684 depend on the type of LED
chosen in the application. The ISL97682, ISL97683, ISL97684
are capable of boosting up to 45V and typically driving 13 LEDs
in series for each of the 4 channels, enabling a total of 52 pieces
of the 3.2V/20mA type of LEDs.
OVP
The Overvoltage Protection (OVP) pin has a function of setting the
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current
source circuit, as shown in Figure 19.
The LED peak current is set by translating the R SET current to the
output with a scaling factor of 401.8/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 amps offset, internal layout, reference, and current source
resistors. These parameters are optimized for current matching
and absolute current accuracy. However, the absolute accuracy is
additionally determined by the external R SET . A 1% tolerance
resistor is recommended.
-
overvoltage trip level as well as limiting the V OUT regulation
range.
The ISL97682, ISL97683, ISL97684 OVP threshold is set by
R UPPER and R LOWER shown by Equation 1:
(EQ. 1)
V OUT_OVP = 1.22V * (R UPPER + R LOWER )/R LOWER
RSET
+
REF
+
-
V OUT can only regulate between 42% and 100% of the V OUT_OVP
such that:
Allowable V OUT = 42% to 100% of V OUT_OVP
For example, if 10 LEDs are used with the worst case being V OUT
of 35V. If R 1 and R 2 are chosen such that the OVP level is set at
40V, then the V OUT is allowed to operate between 16.8V and 40V.
If the requirement is changed to 4 LEDs of 14V V OUT application,
then the OVP level must be reduced and users should follow the
V OUT = (42% ~100%) OVP level requirement. Otherwise, the
headroom control will be disturbed such that the channel voltage
can be much higher than expected and sometimes can prevent
the driver from operating properly.
The ratio of the OVP capacitor should be the inverse of the OVP
resistor. For example:
if R UPPER /R LOWER = 33/1, then C UPPER /C LOWER = 1/33 with
C UPPER = 100pF and C UPPER = 3.3nF.
Enable
An EN signal is required to enable the internal regulator for normal
operation. If there is no signal for longer than 28ms, the device will
enter shutdown.
Power Sequence
There is no specific power sequence requirement for the
ISL97682, ISL97683, ISL97684. The EN signal can be tied to V IN
but not the VDC that will prevent the device from powering up.
10
PWM DIMMING
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
Dynamic Headroom Control
The ISL97682, ISL97683, ISL97684 feature a proprietary
Dynamic Headroom Control circuit that detects the highest
forward voltage string or the lowest voltage from any of the CH
pins digitally. When the lowest CH voltage is lower than the short
circuit threshold (V SC ), such voltage will be used as the feedback
signal for the boost regulator. The boost makes the output to the
correct level such that the lowest CH is at the target headroom
voltage. Since all LED stacks are connected to the same output
voltage, the other CH pins will have a higher 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 ISL97682, ISL97683, ISL97684 allow 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.
There are various ways to achieve DC or PWM current control,
which will be described in the following.
FN7689.1
May 16, 2012
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ISL97684IRTZ-T
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ISL97687IBZEV1Z
制造商:Intersil 功能描述:ISL97687 EVAL