SC189ASKTRT Semtech, SC189ASKTRT Datasheet - Page 19

• Manufacturer Part Number: SC189ASKTRT
• Description: 2.5MHz, 1.5A Synchronous Step Down Regulator In SOT-23_5
• Manufacturer: Semtech

Applications Information (continued)
compensation is designed to work with a output filter
corner frequency of less than 100kHz over any operating
condition, tolerance and bias effect. The corner frequency
of output filter can be defined by the equation
Values outside this range may lead to instability,
malfunction, or out-of-specification performance.
When choosing an inductor, it is important to consider
the change in inductance with DC bias current. The
inductor saturation current is specified as the current at
which the inductance drops a specific percentage from
the nominal value. This is approximately 30%. Except for
short-circuit or other fault conditions, the peak current
must always be less than the saturation current specified
by the manufacturer. The peak current is the maximum
load current plus one half of the inductor ripple current at
the maximum input voltage. Load and/or line transients
can cause the peak current to exceed his level for short
durations. Maintaining the peak current below the
inductor saturation specification keeps the inductor ripple
current and the output voltage ripple at acceptable levels.
Manufacturers often provide graphs of actual inductance
and saturation characteristics versus applied inductor
current. The saturation characteristics of the inductor can
vary significantly with core temperature. Core and ambient
temperatures should be considered when examining the
core saturation characteristics.
When the inductance has been determined, the DC
resistance (DCR) must be examined. The efficiency that
can be achieved is dependent on the DCR of the inductor.
The lower values give higher efficiency. The RMS DC
current rating of the inductor is associated with losses in
the copper windings and the resulting temperature rise of
the inductor. This is usually specified as the current which
produces a 40˚C temperature rise. Most copper windings
are rated to accommodate this temperature rise above
maximum ambient.
Magnetic fields associated with the output inductor can
interfere with nearby circuitry. This can be minimized by
the use of low noise shielded inductors which use the
© 2009 Semtech Corp.
f
C
2
L
1
C
OUT
19
minimum gap possible to limit the distance that magnetic
fields can radiate from the inductor. However shielded
inductors typically have a higher DCR and are thus less
efficient than a similar sized non-shielded inductor.
The SC189 is compatible with small shielded chip inductors
for low cost, low profile applications. The inductance roll
off characteristic of chip inductor is worse resulting in
high ripple current and increased output voltage ripple
at heavy load operation. SC189 has OCP peak inductor
current threshold of 2.0A minimum, to support 1.5A DC
load current, the inductor ripple current at 1.5A DC load
current needs to be less than 1A.
Final inductor selection depends on various design
considerations such as efficiency, EMI, size, and cost. Table
2a and 2b list the manufacturers of recommended inductor
and output capacitors. Chip inductors provide smaller
footprint and height with lower efficiency and increased
output voltage ripple. Transient load performance is
equivalent to wire wound inductors. Figure 6 shows the
typical efficiency curves for different inductors.
C
The internal voltage loop compensation in the SC189 limits
the minimum output capacitor value to 10μF if using the
inductor of 2.2μH. This is due to its influence on the the
loop crossover frequency, phase margin, and gain margin.
Increasing the output capacitor above this minimum
value will reduce the crossover frequency and provide
greater phase margin. A total output capacintance should
not exceed 30uF to avoid any start-up problems. For most
typical applications, it is recommended to use output
capacitance of 10uF to 22uF. When choosing output
OUT
Selection
Figure 6 — Typical efficiency curves
100%
95%
90%
85%
80%
75%
70%
65%
60%
0.0
(V
L=1071AS-1R0N (33m_typ)
V
V
T
L=MDT2520-CR1R0M (60m_typ)
IN
OUT
A
IN
=25° C
= 5.0V
0.3
=5.0V, V
= 3.3V
L=1071AS-2R2N (50m_typ)
L=LQM2HP1R0MG0 (55m_typ)
Output Current (A)
Efficiency
0.6
OUT
=3.3V)
0.9
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1.2
SC189
1.5