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ISL97636
Inductor
The selection of the inductor should be based on its
maximum 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.
Its maximum current capability must be adequate to handle
the peak current at the worst case condition. If an inductor
core is chosen with too low a 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 fact that the
voltage across the inductor during the Off period can be
shown as:
IL peak = ( V O × I O ) ? ( 85% × V I ) + 1 ? 2 [ V I × ( V O – V I ) ? ( L × V O × f S ) ]
(EQ. 9)
The choice of 85% is just an average term for the efficiency
approximation. The first term is average current that is
inversely proportional to the input voltage. The second term
is inductor current change that is inversely proportional to L
and f S . As a result, for a given switching frequency and
minimum input voltage the system operates, the inductor
I SAT must be chosen carefully. At a given inductor size,
usually the larger the inductance, the higher the series
resistance because of the extra winding of the coil. Thus the
higher the inductance, the lower the peak current capability.
The ISL97636 current limit may also have to be taken into
account.
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 for I LPEAK during FET On
and the voltage drop due to flowing through the ESR of the
output capacitor. The ripple voltage can be shown as:
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 harder in the next On period that minimizes
transient current. The output capacitor is also needed for
compensation, and in general, 2x4.7μF/50V ceramic
capacitors are suitable for the notebook display backlight
applications.
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 ’s, and therefore, a suitable current
rated Schottky diode must be used.
Applications
High Current Applications
Each channel of the ISL97636 can support up to 35mA. For
applications that need higher current, multiple channels can
be grouped to achieve the desirable current. For example,
the cathode of the last LED can be connected to IIN0 to IIN2;
this configuration can be treated as a single string with
105mA current driving capability.
V OUT
IIN0
IIN1
IIN2
FIGURE 19. GROUPING MULTIPLE CHANNELS FOR HIGH
Δ V CO = ( I O ? C O × D ? f S ) + ( ( I O × ESR )
(EQ. 10)
CURRENT APPLICATIONS
The conservation of charge principle in Equation 8 also
brings up a fact that during the boost switch Off period, the
output capacitor is charged with the inductor ripple current
minus a relatively small output current in boost topology. As
a result, the user needs to select an output capacitor with
low ESD and with a enough input ripple current capability.
Output Ripple
Δ V Co can be reduced by increasing C O or f S , or using small
ESR capacitors. In general, ceramic capacitors are the best
choice for output capacitors in small to medium sized LCD
15
Compensation
The ISL97636 has two main elements in the system; the
Current Mode Boost Regulator and the op amp based
multi-channel current sources. The ISL97636 incorporates a
transconductance amplifier in its feedback path to allow the
user some levels of adjustment on the transient response
and better regulation. The ISL97636 uses current mode
control architecture, which has a fast current sense loop and
a slow voltage feedback loop. The fast current feedback loop
does not require any compensation. The slow voltage loop
must be compensated for stable operation. The
FN6570.0
May 9, 2008
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