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ISL97675
Inductor
The selection of the inductor should be based on its maximum
For 20mA applications with V IN < 7V, 2 x 4.7μF (X7R type) is
required in some configurations.
current (I SAT ) characteristics, power dissipation (DCR), EMI
susceptibility (shielded vs unshielded), and size. Inductor type
and value influence many key parameters, including 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
3.0
2.5
2.0
1.5
1.0
0.5
POLY. (CERAMIC X7R 2.2μF 50V CAP)
POLY. (CERAMIC Y5V 2.2μF 50V CAP)
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
0
0
5
10
15 20 25 30
APPLIED VOLTAGE (V)
35
40
45
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 17:
IL peak = ( V O × I O ) ? ( 85% × V I ) + 1 ? 2 [ V I × ( V O – V I ) ? ( L × V O × f SW ) ]
(EQ. 17)
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 f SW . As a result, for a given switching frequency, minimum
input voltage must be used to calculate the input/inductor
current as shown in Equation 17. Fora 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 ISL97675 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 ton period and the voltage
drop due to load current flowing through the ESR of the output
capacitor. The ripple voltage is shown in Equation 18:
FIGURE 26. X7R AND V5Y TYPES CERAMIC CAPACITORS
Channel Capacitor
It is recommended to use at least 1.5nF capacitors from CH pins
to V OUT . 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.
Output Ripple
Δ V Co , can be reduced by increasing Co or f SW , or using small ESR
capacitors as shown in Equation 18. 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.
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 is also needed for compensation,
and, in general one to two 4.7μF/50V ceramic capacitors are
needed for netbook or notebook display backlight applications.
Schottky Diode
A high speed rectifier diode is necessary to prevent excessive
Δ V CO = ( I O ? C O × D ? f S ) + ( ( I O × ESR )
(EQ. 18)
voltage overshoot, especially in the boost configuration. Low
forward voltage and reverse leakage current will minimize
Equation 18 shows the importance of using a low ESR output
capacitor for minimizing output ripple.
The choice of X7R over Y5V ceramic capacitors is highly
recommended because the former capacitor is less sensitive to
capacitance change over voltage as shown in Figure 26. Y5V’s
absolute capacitance can be reduced to 10%~20% of its rated
capacitance at the maximum voltage. In any case, Y5V type of
ceramic capacitor should be avoided.
Here are some recommendations for various applications:
For 20mA applications with V IN > 7V, 1 x 4.7μF (X7R type) is
sufficient.
18
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.
FN7630.1
September 14, 2011
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