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.

circuit operate in normal mode or fault mode. The device

recognizes a fault condition when there is no feedback

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

to the output in fault mode.

A common and extreme case to enter fault condition is the

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.

7.5V, the circuit will remain in normal mode. Otherwise, the

circuit will enter the fault mode and cannot provide the

capacitor is needed to be big enough to ensure sufficient time

current in startup.

Figure 6. Startup Scenarios of the Circuits with

time

FB current

time

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

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

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