NCV898031(2012) 查看數據表（PDF） - ON Semiconductor
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NCV898031 Datasheet PDF : 13 Pages
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NCV898031
Internal Soft-Start
To insure moderate inrush current and reduce output
overshoot, the NCV898031 features a soft start which
charges a capacitor with a fixed current to ramp up the
reference voltage.
VDRV
An internal regulator provides the drive voltage for the
gate driver. Bypass with a ceramic capacitor to ground to
ensure fast turn on times. The capacitor should be between
0.1 mF and 1 mF, depending on switching speed and charge
requirements of the external MOSFET.
Oscillator
PWM Comparator S Q
R
+
CSA
Slope
Compensation
Voltage Error
Gate
Drive
GDRV
ISNS
L1
VIN
CCPL
L2
VFB
Co
RL
NCV898031
VEA
Compensation
Figure 10. SEPIC Current Mode Schematic
SEPIC Design Methodology
This section details an overview of the component
selection process for the NCV898031 in continuous
conduction mode SEPIC. It is intended to assist with the
design process but does not remove all engineering design
work. Many of the equations make heavy use of the small
ripple approximation. This process entails the following
steps:
1. Define Operational Parameters
2. Select Current Sense Resistor
3. Select SEPIC Inductors
4. Select Coupling Capacitor
5. Select Output Capacitors
6. Select Input Capacitors
7. Select Feedback Resistors
8. Select Compensator Components
9. Select MOSFET(s)
10. Select Diode
Define Operational Parameters
Before beginning the design, define the operating
parameters of the application. These include:
VIN(min): minimum input voltage [V]
VIN(max): maximum input voltage [V]
VOUT: output voltage [V]
IOUT(max): maximum output current [A]
ICL: desired typical cycle−by−cycle current limit [A]
From this the ideal minimum and maximum duty cycles
can be calculated as follows:
Dmin
+
VOUT
VIN(max) ) VOUT
Dmax
+
VOUT
VIN(min) ) VOUT
Both duty cycles will actually be higher due to power loss
in the conversion. The exact duty cycles will depend on
conduction and switching losses.
If the calculated DWC (worst case) is higher than the Dmax
limit of the NCV898031, the conversion will not be
possible. It is important for a SEPIC converter to have a
restricted Dmax, because while the ideal conversion ratio of
a SEPIC converter goes up to infinity as D approaches 1, a
real converter’s conversion ratio starts to decrease as losses
overtake the increased power transfer. If the converter is in
this range it will not be able to regulate properly.
If the following equation is not satisfied, the device will
skip pulses at high VIN:
Dmin
fs
w
ton(min)
Where: fs: switching frequency [Hz]
ton(min): minimum on time [s]
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