10
LT1500/LT1501
APPLICATIONS INFORMATION
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mode. Normal operation resumes for one or more switch
cycles and the output voltage increases until the error
amplifier output falls below threshold, initiating a new
adaptive bias shutdown.
DESIGN GUIDE
Selecting Inductor Value
Inductor value is chosen as a compromise between size,
switching frequency, efficiency and maximum output cur-
rent. Larger inductor values become physically larger but
provide higher output current and give better efficiency
(because of the lower switching frequency). Low induc-
tance minimizes size but may limit output current and the
higher switching frequency reduces efficiency.
The simplest way to handle these trade-offs is to study the
graphs in the Typical Performance Characteristics sec-
tion. A few minutes with these graphs will clearly show the
trade-offs and a value can be quickly chosen that meets the
requirements of frequency, efficiency and output current.
This leaves only physical size as the final consideration.
The concern here is that for a given inductor value, smaller
size usually means higher series resistance. The graphs
showing efficiency loss vs inductor series resistance will
allow a quick estimate of the additional losses associated
with very small inductors.
One final consideration is inductor construction. Many
small inductors are “open frame ferrites” such as rods or
barrels. These geometries do not have a closed magnetic
path, so they radiate significant B fields in the vicinity of the
inductor. This can affect surrounding circuitry that is
sensitive to magnetic fields. Closed geometries such as
toroids or E-cores have very low stray B fields, but they are
larger and more expensive (naturally).
Catch Diode
The catch diode in a boost converter has an average
current equal to output current, but the peak current can
be significantly higher. Maximum reverse voltage is equal
to output voltage. A 0.5A Schottky diode like MBR0520L
works well in nearly all applications.
Input Capacitor
Input capacitors for boost regulators are less critical than
the output capacitor because the input capacitor ripple
current is a simple triwave without the higher frequency
harmonics found in the output capacitor current. Peak-to-
peak current is less than 200mA and worst-case RMS
ripple current in the input capacitor is less than 70mA.
Input capacitor series resistance (ESR) should be low
enough to keep input ripple voltage to less than 100mV
P-P
.
This assumes that the capacitor is an aluminum or tanta-
lum type where the capacitor reactance at the switching
frequency is small compared to the ESR.
C
f ESR
≥
()( )
2
π
A typical input capacitor is a 33µF, 6V surface mount solid
tantalum type TPS from AVX. It is a “C” case size, with
0.15Ω maximum ESR. Some caution must be used with
solid tantalum input capacitors because they can be dam-
aged with turn-on surge currents that occur when a low
impedance power source is hot-switched to the input of
the regulator. This problem is mitigated by using a capaci-
tor with a voltage rating at least twice the highest expected
input voltage. Consult with the manufacturer for additional
guidelines.
If a ceramic input capacitor is used, different design
criteria are used because these capacitors have extremely
low ESR and are chosen for a minimum number of
microfarads.
C Ceramic
f
()
=
1
4
f = switching frequency
A typical unit is an AVX or Tokin 3.3µF or 4.7µF.
Output Capacitor
Output ripple voltage is determined by the impedance of
the output capacitor at the switching frequency. Solid
tantalum capacitors rated for switching applications are
recommended. These capacitors are essentially resistive
at frequencies above 50kHz, so ESR is the important factor
in determining ripple voltage. A typical unit is a 220µF, 10V
