NCV887001 Просмотр технического описания (PDF) - ON Semiconductor
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NCV887001 Datasheet PDF : 11 Pages
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NCV8870
The maximum average inductor current can be calculated
as follows:
IL,avg
+
VOUTIOUT(max)
VIN(min)
The Peak Inductor current can be calculated as follows:
IL,peak
+
IL,avg
)
VIN(min) 2 Dmax
LfsVOUT
Where: IL,peak: Peak inductor current value [A]
4. Select Output Capacitors
The output capacitors smooth the output voltage and
reduce the overshoot and undershoot associated with line
transients. The steady state output ripple associated with the
output capacitors can be calculated as follows:
VOUT(ripple) +
ǒ Ǔ IOUT(max) VOUT * VIN(min)
2
ǒ Ǔ COUTf
)
IOUT(max)VOUTRESR
VIN(min)
The capacitors need to survive an RMS ripple current as
follows:
Ǹ ICout(RMS) + IOUT
VOUT * VIN(min)
VIN(min)
The use of parallel ceramic bypass capacitors is strongly
encouraged to help with the transient response.
5. Select Input Capacitors
The input capacitor reduces voltage ripple on the input to
the module associated with the ac component of the input
current.
ICin(RMS)
+
VIN(WC) 2 DWC
LfsVOUT2 Ǹ3
6. Select Feedback Resistors
The feedback resistors form a resistor divider from the
output of the converter to ground, with a tap to the feedback
pin. During regulation, the divided voltage will equal Vref.
The lower feedback resistor can be chosen, and the upper
feedback resistor value is calculated as follows:
Rupper
+
Rlower
ǒVout * VrefǓ
Vref
The total feedback resistance (Rupper + Rlower) should be in
the range of 1 kW – 100 kW.
7. Select Compensator Components
Current Mode control method employed by the NCV8870
allows the use of a simple, Type II compensation to optimize
the dynamic response according to system requirements.
8. Select MOSFET(s)
In order to ensure the gate drive voltage does not drop out
the MOSFET(s) chosen must not violate the following
inequality:
Qg(total)
v
Idrv
fs
Where: Qg(total): Total Gate Charge of MOSFET(s) [C]
Idrv: Drive voltage current [A]
fs: Switching Frequency [Hz]
The maximum RMS Current can be calculated as follows:
ID(max)
+
Iout
ǸD
DȀ
The maximum voltage across the MOSFET will be the
maximum output voltage, which is the higher of the
maximum input voltage and the regulated output voltaged:
VQ(max) + VOUT(max)
9. Select Diode
The output diode rectifies the output current. The average
current through diode will be equal to the output current:
ID(avg) + IOUT(max)
Additionally, the diode must block voltage equal to the
higher of the output voltage and the maximum input voltage:
VD(max) + VOUT(max)
The maximum power dissipation in the diode can be
calculated as follows:
PD + Vf (max) IOUT(max)
Where: Pd: Power dissipation in the diode [W]
Vf(max): Maximum forward voltage of the diode [V]
Low Voltage Operation
If the input voltage drops below the UVLO or MOSFET
threshold voltage, another voltage may be used to power the
device. Simply connect the voltage you would like to boost
to the inductor and connect the stable voltage to the VIN pin
of the device. In boost configuration, the output of the
converter can be used to power the device. In some cases it
may be desirable to connect 2 sources to VIN pin, which can
be accomplished simply by connecting each of the sources
through a diode to the VIN pin.
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