AND8098/D

http://onsemi.com

5

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

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

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

(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

inductance is needed.

The last consideration is the effect of load regulation.

Large inductor can limit the inrush current flowing into

output voltage. It makes load regulation poor. If there is no

as possible, say 2 mH.

Output Capacitor

Because of the burst-mode characteristic and the

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

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

compatibility in speed and voltage drop. It helps the voltage

enough to withstand the input voltage in buck and input

voltage plus output voltage in buck-boost respectively.

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

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.

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).

Standby Condition

The standby ability of the proposed buck converter is not

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

current. It consumes main portion of energy in standby.