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ISL97687 数据表(PDF) 17 Page - Renesas Technology Corp

部件名 ISL97687
功能描述  4-Channel LED Driver with Phase Shift Control and 10-Bit Dimming Resolution
Download  24 Pages
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制造商  RENESAS [Renesas Technology Corp]
网页  http://www.renesas.com
标志 RENESAS - Renesas Technology Corp

ISL97687 数据表(HTML) 17 Page - Renesas Technology Corp

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ISL97687
FN7714 Rev.3.00
Page 17 of 24
Sep 13, 2017
Inductor
The selection of the inductor should be based on its maximum
current (ISAT) characteristics, power dissipation, EMI
susceptibility (shielded vs unshielded), and size. Inductor type
and value influence many key parameters, including the inductor
ripple current, current limit, efficiency, transient performance
and stability.
The inductor’s maximum current capability must be large enough
to handle the peak current at the worst case condition. If an
inductor core is chosen with a lower current rating, saturation in
the core will cause the effective inductor value to fall, leading to
an increase in peak to average current level, poor efficiency and
overheating in the core. The series resistance, DCR, within the
inductor causes conduction loss and heat dissipation. A shielded
inductor is usually more suitable for EMI susceptible
applications, such as LED backlighting.
The peak current can be derived from the voltage across the
inductor during the off period, as expressed in Equation 10:
The choice of 85% is just an average term for the efficiency
approximation. The first term is the average current, which is
inversely proportional to the input voltage. The second term is
the inductor current change, which is inversely proportional to L
and fSW. As a result, for a given switching frequency, minimum
input voltage must be used to calculate the input/inductor
current as shown in Equation 10. For a given inductor size, the
larger the inductance value, the higher the series resistance
because of the extra number of turns required, thus, higher
conductive losses. The ISL97687 current limit should be less
than the inductor saturation current.
Output Capacitors
The output capacitor acts to smooth the output voltage and
supplies load current directly during the conduction phase of the
power switch. Output ripple voltage consists of the discharge of
the output capacitor during the FET turn-on period and the
voltage drop due to load current flowing through the ESR of the
output capacitor. The ripple voltage is shown in Equation 11:
where IO represents the output current, CO is the output
capacitance, D is the duty ratio as described in Equation 9. ESR
is the equivalent series resistance of the output capacitance and
fsw is the switching frequency of the converter. Equation 11
shows the importance of using a low ESR output capacitor for
minimizing output ripple.
As shown in Equation 11, the output ripple voltage,
VCo, can be
reduced by increasing the output capacitance, CO or the
switching frequency, fSW, or using output capacitors with small
ESR. In general, ceramic capacitors are the best choice for
output capacitors in small to medium sized LCD backlight
applications due to their cost, form factor, and low ESR.
The choice of X7R over Y5V ceramic capacitors is highly
recommended because the X7R type capacitor is less sensitive
to capacitance change overvoltage. Y5V’s absolute capacitance
can be reduced to 10%~20% of its rated capacitance at the
maximum voltage. Because of this, Y5V type ceramic capacitors
should be avoided.
A larger output capacitor will also ease the driver response
during PWM dimming off period due to the longer sample and
hold effect of the output drooping. The driver does not need to
boost as much on the next on period, which minimizes transient
current. The output capacitor also plays an important role for
system compensation.
Channel Capacitor
It is recommended to use at least 1nF capacitors from CH pins to
VOUT. Larger capacitors will reduce LED current ripple at boost
frequency, but will degrade transient performance at high PWM
frequencies. The best value is dependant on PCB layout. Up to
4.7nF is sufficient for most configurations.
Schottky Diode
A high speed rectifier diode is necessary to prevent excessive
voltage overshoot, especially in the boost configuration. Low
forward voltage and reverse leakage current will minimize
losses, making Schottky diodes the preferred choice. Although
the Schottky diode turns on only during the boost switch off
period, it carries the same peak current as the inductor,
therefore, a suitable current rated Schottky diode must be used.
High Current Applications
Each channel of the ISL97687 can support up to 160mA. For
applications that need higher current, multiple channels can be
grouped to achieve the desirable current. For example, in
Figure 27, the cathodes of the last LEDs can be connected to
CH1/CH2 and CH3/CH4, this configuration can be treated as a
single string with up to 350mA current driving capability.
ILpeak VO
IO 85%
VI 12 VI VO
VI L
VO fSW

+
=
(EQ. 10)
VCO
I
O CO DfSw IO ESR

+
=
(EQ. 11)
FIGURE 27. GROUPING MULTIPLE CHANNELS FOR HIGH CURRENT
APPLICATIONS
CH1
CH2
CH3
CH4
BOOST OUTPUT


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