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ISL97671
Components Selections
According to the inductor Voltage-Second Balance principle, the
change of inductor current during the switching regulator On
time is equal to the change of inductor current during the
switching regulator Off time. Since the voltage across an inductor
The peak current can be derived from the voltage across the
inductor during the Off period, as expressed in Equation 23:
IL pk = ( V O × I O ) ? ( 85% × V I ) + 1 ? 2 [ V I × ( V O – V I ) ? ( L × V O × f SW ) ]
(EQ. 23)
is:
V L = L × Δ I L ? Δ t
and Δ I L @ On = Δ I L @ Off, therefore:
( V I – 0 ) ? L × D × t S = ( V O – V D – V I ) ? L × ( 1 – D ) × t S
(EQ. 19)
(EQ. 20)
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 and
minimum input voltage on which the system operates, the
V O ? V I = 1 ? ( 1 – D )
where D is the switching duty cycle defined by the turn-on time
over the switching period. V D is Schottky diode forward voltage
that can be neglected for approximation.
Rearranging the terms without accounting for V D gives the boost
ratio and duty cycle respectively as:
(EQ. 21)
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 ISL97671
current limit should also have to be taken into account.
Output Capacitors
The output capacitor acts to smooth the output voltage and
D = ( V O – V I ) ? V O
(EQ. 22)
supplies load current directly during the conduction phase of the
power switch. Output ripple voltage consists of the discharge of
Input Capacitor
Switching regulators require input capacitors to deliver peak
charging current and to reduce the impedance of the input
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:
supply. This reduces interaction between the regulator and input
supply, thereby improving system stability. The high switching
Δ V CO = ( I O ? C O × D ? f S ) + ( ( I O × ESR )
(EQ. 24)
frequency of the loop causes almost all ripple current to flow in
the input capacitor, which must be rated accordingly.
A capacitor with low internal series resistance should be chosen
to minimize heating effects and improve system efficiency, such
as X5R or X7R ceramic capacitors, which offer small size and a
lower value of temperature and voltage coefficient compared to
other ceramic capacitors.
In Boost mode, input current flows continuously into the inductor;
AC ripple component is only proportional to the rate of the
inductor charging, thus, smaller value input capacitors may be
used. It is recommended that an input capacitor of at least 10μF
be used. Ensure the voltage rating of the input capacitor is
suitable to handle the full supply range.
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.
The inductor’s maximum current capability must be adequate
enough 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.
24
The conservation of charge principle in Equation 24 also brings
up the 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 ESR and
enough input ripple current capability.
Output Ripple
Δ V Co , can be reduced by increasing Co or f SW , or using small ESR
capacitors. 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 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
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, and
therefore, a suitable current rated Schottky diode must be used.
FN7631.3
August 14, 2012
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