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參數(shù)資料
| 型號： | VJ1206Y224KXXAT |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Low-Cost 100 mA High-Voltage Buck and Buck-Boost Using NCP1052 |
| 中文描述： | 低成本百毫安高電壓降壓和降壓升壓使用NCP1052 |
| 文件頁數(shù)： | 4/10頁 |
| 文件大?。?/td> | 93K |
| 代理商： | VJ1206Y224KXXAT |
AND8098/D
http://onsemi.com
4
In non-isolated topologies such as buck or buck-boost,
the circuits are mainly designed for CCM. The CCM
burst-mode waveform is different to the PWM waveform in
Figure 5. Because of this characteristic, burst mode requires
a higher peak value of the inductor current in order to have
the same level of averaged inductor current (or output
current).
Figure 5. CCM Inductor Currents in Burst Mode
and traditional PWM Control
Burst mode
PWM
As shown in Figure 4 and 5 burst-mode control produces
low-frequency waveform comparing to the switching
frequency. Part of the power loss in this low frequency
becomes audible noise. Therefore, burst-mode control is
not suitable for high power applications such as more than
20 W.
V
CC
Capacitor
The V
CC
capacitor C
2
is the key component to make the
circuit operate in normal mode or fault mode. The device
recognizes a fault condition when there is no feedback
current in the FB pin during the time from V
CC
= 8.5 V to
7.5 V. The V
CC
capacitor directly affects this time duration.
In normal mode, the V
CC
follows a 8.5 V-7.5 V-8.5 V
hysteresis loop. When the circuit is in fault mode, the V
CC
follows a 8.5 V-7.5 V-4.5 V-8.5 V hysteresis loop. The
device keeps its MOSFET opened except for the time from
V
CC
= 8.5 V to 7.5 V and delivers a little amount of power
to the output in fault mode.
A common and extreme case to enter fault condition is the
startup. The MOSFET begins switching at the V
CC
is firstly
charged to 8.5 V and hence output voltage rises. The output
voltage needs some time to build up the output voltage from
0 V to a desired value. When the desired level is reached, a
feedback current flows into the device to stop its switching.
If the feedback current is determined before V
CC
reaches
7.5V, the circuit will remain in normal mode. Otherwise, the
circuit will enter the fault mode and cannot provide the
output voltage at its desired level. Therefore, the V
CC
capacitor is needed to be big enough to ensure sufficient time
for V
CC
going from 8.5 V to 7.5 V to sample feedback
current in startup.
Figure 6. Startup Scenarios of the Circuits with
Big Enough or Too Small V
CC
Capacitor
Output waveforms with big enough V
CC
capacitor
time
FB current
V
CC
V
out
Output waveforms with too small V
CC
capacitor
time
V
CC
V
out
Desired level of V
out
Practically, the NCP1052 consumes approximately 0.5
mA in normal operation. The concerned fault sampling time
for feedback signal is from 8.5 V to 7.5V. Hence,
10- 3
1
0.5
10- 3· sampling time
For example, if sampling time or startup transient is
designed to be 20 ms, 10
μ
F V
CC
capacitor is needed.
C
Idt
dV
0.5
· sampling time
(eq. 4)
Inductor
The 300 mA current limit in the NCP1052 is measured
with a condition that the di/dt reaches 300 mA in 4
μ
s. When
the buck or buck-boost circuit is designed for universal ac
input voltage (85 to 265 Vac), the rectified input voltage will
be possibly as high as 375 Vdc. In order to keep the 4
μ
s
condition, the inductance value will be 5 mH by (5) and (6).
For buck,
Vin
L
For buck-boost,
di
dt
Vout
Vin
L
(eq. 5)
di
dt
Vin
L
(eq. 6)
The 5 mH is practically too high and hence not very
practical. Therefore, the inductor is basically selected by
market available inductor models which is with a normally
smaller inductance (but not too small). It must have enough
saturation current level (>300 mA). If inductance is too
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| VJ1206Y224KXXMC | 制造商:Vishay Vitramon 功能描述:CAP .22UF 25V X7R 10% SMD 1206 - Tape and Reel 制造商:Vishay Intertechnologies 功能描述:MLCC CAPACITOR |
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| VJ1206Y224KXXRW1BC | 功能描述:多層陶瓷電容器MLCC - SMD/SMT 1206 0.22uF 25volts X7R 10% RoHS:否 制造商:American Technical Ceramics (ATC) 電容:10 pF 容差:1 % 電壓額定值:250 V 溫度系數(shù)/代碼:C0G (NP0) 外殼代碼 - in:0505 外殼代碼 - mm:1414 工作溫度范圍:- 55 C to + 125 C 產品:Low ESR MLCCs 封裝:Reel |
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