SC283 SEMTECH [Semtech Corporation], SC283 Datasheet - Page 14

• Manufacturer Part Number: SC283
• Description: Dual Channel 2.5MHz, 1.8A Synchronous Step-Down Regulator
• Manufacturer: SEMTECH [Semtech Corporation]

© 2010 Semtech Corp.
Applications Information (continued)
Over-Voltage Protection
In the event of a 15% over-voltage on the output, the
PWM drive is disabled leaving the LX pin floating.
Soft-Start
The soft-start mode is activated after VIN reaches its UVLO
and one or more CTL pins are set high to enable the part.
A thermal shutdown event will also activate the soft start
sequence. Soft-start mode controls the maximum current
during startup thus limiting inrush current. The PMOS
current limit is stepped through four soft start levels of
approximately 20%, 25%, 40%, & 100%. Each step is main-
tained for 200μs following an internal reference start up
duration of 50μs giving a total nominal startup period of
850μs. During startup, the chip operates by controlling
the inductor current swings between 0A and current limit.
If at any time V
of the soft-start period, the SC283 will switch to PWM
mode operation. Figure 5 shows the typical diagram of
soft start operation.
The SC283 is capable of starting up into a pre-biased
output. When the output is precharged by another supply
rail, the SC283 will not discharge the output during the
soft start interval.
Shut Down
When all CTL pins of each channel are low, the channel will
run in shutdown mode, drawing less than 1μA from the
input power supply. The internal switches and bandgap
voltage will be immediately turned off.
Thermal Shutdown
The device has a thermal shutdown feature to protect
the SC283 if the junction temperature exceeds 160°C.
During thermal shutdown, the on-chip power devices are
disabled, tri-stating the LX output. When the temperature
drops by 10°C, it will initiate a soft start cycle to resume
normal operation.
Inductor Selection
The SC283 converter has internal loop compensation. The
compensation is designed to work with an output filter
corner frequency of less than 40kHz for a V
50KHz for a V
IN
OUT
of 3.3V over any operating condition. The
reaches 86% of the target or at the end
vs. Output Voltage
IN
of 5V and
14
corner frequency of the output filter is shown in Equation
3.
Values outside this range may lead to instability,
malfunction, or out-of-specification performance.
In general, the inductance is chosen by making the
inductor ripple current to be less than 30% of maximum
load current. 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 this
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
f
C
=
2
π
L
1
⋅
C
OUT
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