FR9886 Просмотр технического описания (PDF) - Unspecified
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FR9886 Datasheet PDF : 14 Pages
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Application Information
FR9886
85T
Output Voltage Setting
The output voltage VOUT is set using a resistive
divider from the output to FB. The FB pin regulated
voltage is 0.925V. Thus the output voltage is:
VOUT=0.925V
1+
R1
R2
Table 2 lists recommended values of R1 and R2 for
most used output voltage.
Table 2 Recommended Resistance Values
VOUT
R1
R2
5V
44.2kΩ
10kΩ
3.3V
26.1kΩ
10kΩ
2.5V
16.9kΩ
10kΩ
1.8V
9.53kΩ
10kΩ
1.2V
3kΩ
10kΩ
Place resistors R1 and R2 close to FB pin to prevent
stray pickup.
Input Capacitor Selection
The use of the input capacitor is filtering the input
voltage ripple and the MOSFETS switching spike
voltage. Because the input current to the step-down
converter is discontinuous, the input capacitor is
required to supply the current to the converter to
keep the DC input voltage. The capacitor voltage
rating should be 1.25 to 1.5 times greater than the
maximum input voltage. The input capacitor ripple
current RMS value is calculated as:
IIN RMS =IOUT D 1-D
Where D is the duty cycle of the power MOSFET.
This function reaches the maximum value at D=0.5
and the equivalent RMS current is equal to IOUT/2.
The following diagram is the graphical representation
of above equation.
2.5A
2A
1.5A
1A
A low ESR capacitor is required to keep the
noise minimum. Ceramic capacitors are
better, but tantalum or low ESR electrolytic
capacitors may also suffice. When using
tantalum or electrolytic capacitors, a 0.1uF
ceramic capacitor should be placed as close to
the IC as possible.
Output Capacitor Selection
The output capacitor is used to keep the DC
output voltage and supply the load transient
current. When operating in constant current
mode, the output ripple is determined by four
components:
VRIPPLE t VRIPPLE(C) t VRIPPLE(ESR) t
VRIPPLE(ESL) t VNOISE
The following figures show the form of the ripple
contributions.
VRIPPLE(ESR)(t)
+
(t)
VRIPPLE(ESL) (t)
+
(t)
VRIPPLE(C) (t)
+
(t)
VNOISE
=
VRIPPLE(t)
(t)
FR9886- 1.0-APR-2011
10
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