NCV887601D1R2G データシートの表示（PDF） - ON Semiconductor
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NCV887601D1R2G Datasheet PDF : 17 Pages
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NCV8876
ROSC
+
2859
(Fsw * 170)
Where: fsw: switching frequency [kHz]
ROSC: resistor from ROSC pin to GND [k]
Note: The ROSC resistor ground return to the NCV8876 pin
3 must be independent of power grounds.
3. Select Current Sense Resistor
Current sensing for peak current mode control and current
limit relies on the MOSFET current signal, which is
measured with a ground referenced amplifier. The easiest
method of generating this signal is to use a current sense
resistor from the source of the MOSFET to device ground.
The sense resistor should be selected as follows:
RS
+
VCL
ICL
Where: RS: sense resistor [W]
VCL: current limit threshold voltage [V]
ICL: desire current limit [A]
4. Select Output Inductor
The output inductor controls the current ripple that occurs
over a switching period. A high current ripple will result in
excessive power loss and ripple current requirements. A low
current ripple will result in a poor control signal and a slow
current slew rate in case of load steps. A good starting point
for peak to peak ripple is around 20−40% of the inductor
current at the maximum load at the worst case VIN, but
operation should be verified empirically. The worst case VIN
is half of VOUT, or whatever VIN is closest to half of VOUT.
After choosing a peak current ripple value, calculate the
inductor value as follows:
L
+
VIN(WC) DWC
DIL,max fs
Where: VIN(WC): VIN value as close as possible to
half of VOUT [V]
DWC: duty cycle at VIN(WC)
DIL,max: maximum peak to peak ripple [A]
The maximum average inductor current can be calculated
as follows:
IL,AVG
+
VOUTIOUT(max)
VIN(min)h
The Peak Inductor current can be calculated as follows:
IL,peak
+
IL,avg
)
DIL,max
2
Where: IL,peak: Peak inductor current value [A]
5. 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 DIOUT(max)
fCOUT
)
IOUT(max)
1*D
)
VIN(min)D
2fL
RESR
The capacitors need to survive an RMS ripple current as
follows:
Ǹ ǒ Ǔ ICout(RMS) + IOUT
DWC
DȀWC
)
DWC
12
2
DȀWC
L
ROUT TSW
The use of parallel ceramic bypass capacitors is strongly
encouraged to help with the transient response.
6. 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
7. Select Compensator Components
Current Mode control method employed by the NCV8876
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:
IQ(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)
NVMFS5844NL 12 mW, 60 V SO−8FL package
MOSFET is a recommended device.
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)
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