AND8098/D
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).
V
out
V
CC
FB current
time
Output waveforms with big enough V
CC
capacitor
Burst mode
Desired level of V
out
V
CC
PWM
V
out
time
Output waveforms with too small V
CC
capacitor
Figure 5. CCM Inductor Currents in Burst Mode
and traditional PWM Control
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
Figure 6. Startup Scenarios of the Circuits with
Big Enough or Too Small V
CC
Capacitor
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,
-3
C
+
I dt
+
0.5 10
· sampling time
1
dV
+
0.5 10- 3 · sampling time
(eq. 4)
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.
For example, if sampling time or startup transient is
designed to be 20 ms, 10
µF
V
CC
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
µs.
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
µs
condition, the inductance value will be 5 mH by (5) and (6).
For buck,
di
+
Vin
*
Vout
[
Vin
dt
L
L
(eq. 5)
For buck-boost,
di
+
Vin
dt
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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