RT8253A データシートの表示（PDF） - Richtek Technology
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RT8253A Datasheet PDF : 15 Pages
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RT8253A
shutdown mode is activated, the RT8253A will stop
switching. The accurate 0.4V falling edge threshold on
the EN pin can be used to detect a specific analog voltage
level and to shutdown the device. Once in shutdown, the
2.7V rising edge threshold must be triggered to reactivate
the power up sequence. For external timing control (e.g.
RC), the EN pin can also be externally pulled high by
adding a REN* resistor and CEN* capacitor from the VIN
pin. For general applications, the EN pin is externally pulled
high by adding a 100kΩ from the VIN pin (see Figure 3).
UVLO Protection
The RT8253A has an input under voltage lockout protection
(UVLO). If the input voltage exceeds the UVLO rising
threshold voltage (4.2V typ.), the converter will reset and
prepare the PWM for operation. If the input voltage falls
below the UVLO falling threshold voltage during normal
operation, the device will stop switching. The UVLO rising
and falling threshold voltage has a hysteresis to prevent
noise caused reset.
External Soft-Start (SS)
It is highly recommended to program the soft start time
externally because it is not included internally. The
RT8253A effectively uses the lower voltage of the internal
voltage reference or the SS pin voltage as the power
supply's reference voltage which is fed into the error
amplifier and regulate accordingly. A capacitor (CSS)
between the SS pin and ground implements a soft start
time. The RT8253A has an internal pull up current source
of 6μA that charges the external soft start capacitor. The
equation for the soft start time is shown below :
tSS
=
CSS x VREF
ISS
where VREF is 0.8V and ISS current is 6μA.
Generally, a 0.1μF capacitor is used and the soft start
time will be 13.5ms (typ.)
Under Voltage Protection
Hiccup Mode
For the RT8253AH, Hiccup Mode Under Voltage Protection
(UVP) function is provided. When the FB voltage drops
below half of the feedback voltage, VFB, the UVP function
will be triggered and the RT8253AH will shut down for a
period of time and then recover automatically. The Hiccup
Mode UVP can reduce input current in short circuit
conditions.
Latch Off Mode
For the RT8253AL, Latch Off Mode Under Voltage
Protection (UVP) function is provided. When the FB voltage
drops below half of the feedback voltage, VFB, the UVP
function will be triggered and the RT8253AL will shut down
in Latch Off Mode. In shutdown condition, the RT8253AL
can only be reset through EN or power input VIN.
Input Inrush Current
To calculate the input inrush current, the following
equation can be used :
IINRUSH
=
COUT x VOUT
tSS
where IINRUSH is the input current during start up, COUT is
the total output capacitance, VOUT is the desired output
voltage, and tSS is the soft start time. If the inrush current
is higher than the current limit level, current limit will be
triggered.
Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as shown
below :
L=
VOUT x(VIN − VOUT )
fSW x LIR x ILOAD(MAX) x VIN
where LIR is the ratio of the peak to peak ripple current to
the average inductor current.
Find a low loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice, although powdered iron is
inexpensive and can work well at 200kHz. The core must
be large enough not to saturate at the peak inductor current
(IPEAK) :
IPEAK = ILOAD(MAX) + ⎡⎣(LIR/2) xILOAD(MAX) ⎤⎦
The calculation above shall serve as a general reference.
To further improve transient response, the output inductor
can be reduced further. This needs to be considered along
with the selection of the output capacitor.
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DS8253A-02 March 2011
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