NCP1547(2007) 데이터 시트보기 (PDF) - ON Semiconductor
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NCP1547 Datasheet PDF : 13 Pages
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NCP1547
When the power switch is turned on, the voltage on the
BOOST pin is equal to
VBOOST + VIN ) VO * VF
where:
VF = diode forward voltage.
The anode of the diode can be connected to any DC
voltage as well as the regulated output voltage (Figure 1).
However, the maximum voltage on the BOOST pin shall not
exceed 40 V.
As shown in Figure 7, the BOOST pin current includes a
constant 7.0 mA pre−driver current and base current
proportional to switch conducting current. A detailed
discussion of this current is conducted in Thermal
Consideration section. A 0.1 mF capacitor is usually
adequate for maintaining the Boost pin voltage during the on
time.
30
25
20
15
10
5
00
0.5
1.0
1.5
SWITCHING CURRENT (A)
Figure 7. The Boost Pin Current Includes 7.0 mA
Pre−Driver Current and Base Current when the
Switch is Turned On. The Beta Decline of the
Power Switch Further Increases the Base
Current at High Switching Current
Shutdown
The internal power switch will not turn on until the VIN
pin rises above the Startup Voltage. This ensures no
switching will occur until adequate supply voltage is
provided to the IC.
The IC enters a sleep mode when the SHDNB pin is pulled
below the Shutdown Threshold Voltage. In sleep mode, the
power switch is kept open and the supply current reduces to
Shutdown Quiescent Current (1 mA typically). This pin has
an internal pull−down current. When not in use, pull this pin
up to VCC with a resistor (See Figure 1).
Startup
During power up, the regulator tends to quickly charge up
the output capacitors to reach voltage regulation. This gives
rise to an excessive in−rush current which can be detrimental
to the inductor, IC and catch diode. In V2 control , the
compensation capacitor provides Soft−Start with no need
for extra pin or circuitry. During the power up, the Output
Source Current of the error amplifier charges the
compensation capacitor which forces VC pin and thus output
voltage ramp up gradually. The Soft−Start duration can be
calculated by
TSS
+
VC CCOMP
ISOURCE
where:
VC = VC pin steady−state voltage, which is approximately
equal to error amplifier’s reference voltage.
CCOMP = Compensation capacitor connected to the VC pin
ISOURCE = Output Source Current of the error amplifier.
Using a 0.1 mF CCOMP, the calculation shows a TSS over
5.0 ms which is adequate to avoid any current stresses.
Figure 8 shows the gradual rise of the VC, VO and envelope
of the VSW during power up. There is no voltage over−shoot
after the output voltage reaches the regulation. If the supply
voltage rises slower than the VC pin, output voltage may
over−shoot.
Figure 8. The Power Up Transition of NCP1547
Regulator
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