SC4626XEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4626XEVB' PDF : 22 Pages View PDF
SC4626
Applications Information (continued)
Over-Voltage Protection
In the event of a 15% over-voltage on the output, the
PWM drive is disabled with LX pin floating.
Soft-Start
The soft-start mode is activated after VIN reaches its
UVLO and EN signal is set high to enable the part. An over
temperature shutdown event will also activate the soft
start sequence. Soft-start mode controls the maximum
current during startup thus limiting in-rush current. The
PMOS current limit is stepped through four soft start
levels of approximately 20%, 25%, 40%, & 100%. Each
step is maintained for 20μs following internal reference
start up of 20μs giving the total nominal startup period
of 100μs. During startup, the chip operates in controlling
the inductor current swings between 0A and current
limit. If VOUT reaches 90% of the target within the first 2
current levels, the chip continues in hysteretic mode till
the end of the soft-start time period before switching to
PWM mode. If VOUT does not reach 90% by the end of the
second current limit level, soft start will continue to level 3
or level 4 till the output voltage reaches 96% and will then
transition into PWM mode. After the full soft start time
period, the SC4626 will switch into PWM mode operation
regardless of the VOUT level.
The SC4626 is capable of starting up into a pre-biased
output. When the output is precharged by another supply
rail, the SC4626 will not discharge the output during the
soft start interval.
Shut Down
When the EN pin voltage goes low, the SC4626 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.
of output filter can be defined by the equation
1
fC 2S L COUT
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.
Inductor Selection
The SC4626 converter has internal loop compensation.
The compensation is designed to work with a output filter
corner frequency is less than 100kHz over any operating
condition, tolerance and bias effect. The corner frequency
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
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.
© 2009 Semtech Corp.
16
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