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參數(shù)資料
| 型號(hào)： | 400WA10M12.5X16 |
| 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(yè)數(shù)： | 5/10頁(yè) |
| 文件大?。?/td> | 93K |
| 代理商： | 400WA10M12.5X16 |
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.
VCC Capacitor
The VCC capacitor C2 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 VCC = 8.5 V to
7.5 V. The VCC capacitor directly affects this time duration.
In normal mode, the VCC follows a 8.5 V-7.5 V-8.5 V
hysteresis loop. When the circuit is in fault mode, the VCC
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
VCC = 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 VCC 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 VCC 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 VCC
capacitor is needed to be big enough to ensure sufficient time
for VCC 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 VCC Capacitor
Output waveforms with big enough VCC capacitor
time
FB current
VCC
Vout
Output waveforms with too small VCC capacitor
time
VCC
Vout
Desired level of Vout
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,
C
+ I dt
dV
+ 0.5
10-3
1
sampling time
(eq. 4)
+ 0.5
10-3 sampling time
For example, if sampling time or startup transient is
designed to be 20 ms, 10
F VCC capacitor is needed.
Inductor
The 300 mA current limit in the NCP1052 is measured
with a condition that the di/dt reaches 300 mA in 4
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
s
condition, the inductance value will be 5 mH by (5) and (6).
For buck,
di
dt
+
Vin * Vout
L
[
Vin
L
(eq. 5)
For buck-boost,
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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