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ISL97682 データシート(PDF) 10 Page - Renesas Technology Corp

部品番号 ISL97682
部品情報  Compact 2-3-4-Ch LED Drivers with Phase Shift Control
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メーカー  RENESAS [Renesas Technology Corp]
ホームページ  http://www.renesas.com
Logo RENESAS - Renesas Technology Corp

ISL97682 データシート(HTML) 10 Page - Renesas Technology Corp

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ISL97682, ISL97683, ISL97684
FN7689 Rev.2.00
Page 10 of 18
Sep 19, 2017
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, and ISL97684 employ 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 depends 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
overvoltage trip level as well as limiting the VOUT regulation
range.
The ISL97682, ISL97683, ISL97684 OVP threshold is set by
RUPPER and RLOWER shown by Equation 1:
VOUT can only regulate between 42% and 100% of the VOUT_OVP
such that:
Allowable VOUT = 42% to 100% of VOUT_OVP
For example, if 10 LEDs are used with the worst case being VOUT
of 35V. If R1 and R2 are chosen such that the OVP level is set at
40V, then the VOUT is allowed to operate between 16.8V and 40V.
If the requirement is changed to 4 LEDs of 14V VOUT application,
then the OVP level must be reduced and users should follow the
VOUT = (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 RUPPER/RLOWER = 33/1, then CUPPER/CLOWER = 1/33 with
CUPPER = 100pF and CUPPER =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 VIN
but not the VDC that 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.
The LED peak current is set by translating the RSET current to the
output with a scaling factor of 401.8/RSET. The source terminals
of the current source MOSFETs are designed to operate within a
range at about 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 RSET. A 1% tolerance resistor is recommended.
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 (VSC), 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.
(EQ. 1)
VOUT_OVP = 1.22V * (RUPPER + RLOWER)/RLOWER
FIGURE 19. SIMPLIFIED CURRENT SOURCE CIRCUIT
REF
+
-
PWM DIMMING
RSET
+-


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