AND8098 Datasheet, PDF (5/10 Pages) ON Semiconductor – Low-Cost 100 mA High-Voltage Buck and Buck-Boost Using NCP1052

AND8098/D
small, the di/dt becomes too high and the NCP1052 will
have a very high current limit effectively because there is a
propagation delay (typically 135 ns) to turn off the switch.
The current flowing through the inductor L includes three
parts. First, there is a V_CC charging current I_start in Figure 2.
It happens when V_CC needs charging. Its magnitude is 6.3
mA. It is noted that the V_CC discharging current does not
flow through the inductor. Second, it is the main inductor
current to deliver the output current. It is noted that the peak
of burst-mode inductor current is higher than PWM one as
in Figure 5 for the same level of averaged inductor current
(or output current). Finally, there is a current flowing
through diode D₁ to charge up C₁. It also flows through the
inductor as shown in Figure 3. Its magnitude is a
greater-than-50 µA current and practically it is about 1 mA.
Hence, the saturation current of the inductor L is needed to
be bigger than their sum.
Another consideration on the inductor is the low-pass
filtering capability for the V_CC hysteresis low frequency
(and the 50/ 60 Hz rectified AC line voltage ripple). As
shown in Figure 2, there is a low-frequency charging current
with magnitude 6.3 mA flowing through the inductor and
causes low-frequency ripple in the output voltage. A higher
value of the inductance can help to reduce the output ripple.
It is noted that when the output power is higher, the startup
time becomes longer. It needs bigger V_CC capacitor and
makes lower V_CC charging frequency. As a result, a bigger
inductance is needed.
The last consideration is the effect of load regulation.
Large inductor can limit the inrush current flowing into
capacitor C₁ as shown in Figure 3. High inrush current is not
desirable because it can make the C₁ voltage higher than the
output voltage. It makes load regulation poor. If there is no
pull-up resistor R₁, inductor value L is chosen to be as large
as possible, say 2 mH.
Output Capacitor
Because of the burst-mode characteristic and the
low-frequency V_CC charging current, the output ripple is
larger than those in PWM. Hence, a relatively bigger output
capacitor is needed to keep output ripple small. However,
big output capacitor needs a long time to build up the output
voltage initially and hence the circuit may enter into fault
mode in the startup in Figure 6.
Buffering Capacitor
Buffering capacitor C₂ is to provide a greater-than-50 µA
to the feedback pin of NCP1052. It is relatively much
smaller than the output capacitor because the current
consumption in this capacitor is much smaller and the output
voltage cannot copy to this buffering capacitor if the
buffering capacitor voltage is higher than the output voltage.
Diodes
D and D₁ are recommended to be the same part for
compatibility in speed and voltage drop. It helps the voltage
in the capacitor C₁ to be similar to the output voltage. The
reverse blocking voltage of D and D₁ is needed to be large
enough to withstand the input voltage in buck and input
voltage plus output voltage in buck-boost respectively.
D₂ is not a critical component. Its function is to make sure
that feedback current is only in one direction. The accuracy
of its voltage drop used in (1) is not important since the 4.3V
reference voltage in the NCP1052 is loosely set.
Zener Diodes
Z₁ is to clamp the output voltage when there is light load
or no load. Hence, the accuracy of Z₁ helps the regulation
accuracy in the light load or no load condition. It is also the
main component to consume energy when the circuit is in no
load condition. The output voltage is clamped and hence the
output capacitor is protected.
Z₂ and R₁ are to set the output voltage at the nominal load
current. Hence, their accuracy affects the regulation
accuracy at the nominal load condition. The relationship
between zener voltage and output voltage is shown in (1).
Higher value of R₁ helps to pull up the output voltage higher
by reducing the charging rate of the buffering capacitor C₁.
Standby Condition
The standby ability of the proposed buck converter is not
good. It is because there is a V_CC charging current I_start flows
through the output capacitor in Figure 2(a). This charging
current is a low-frequency pulsating signal. As a result, the
voltage in the output capacitor continuously rises up by the
charging current pulses. In order to prevent over voltage in
the output capacitor, the zener Z₁ absorbs the charging
current. It consumes main portion of energy in standby.
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