AND8098/D

http://onsemi.com

2

through the output and a low-frequency ripple will be found

order to reduce output voltage ripple because some

efficiency is lost due to this low-frequency ripple.

Input

Output

D

L

C

FB

DS

1

(a) Buck

Input

Output

D

L

C

FB

DS

(b) Buck-boost

In Figure 2b it is noted that in the buck-boost topology the

charging current path is blocked by diode D and hence the

However, it still affects the output voltage indirectly and

slightly by adding some low-frequency noise on the

(a) Buck

(b) Buck-boost

Charging Current

to transfer the magnitude of output voltage to a voltage

Figure 3, when the main switch inside the IC is opened and

the diode D is closed. In buck, the potential of the IC

reference ground (pin S) becomes almost 0 V in this

moment. In buck-boost, the potential of the IC reference

hand, when main switch is closed and the diode D is opened,

normal operation of the buck and buck-boost converter.

possibly greater than the output voltage especially when

output current or output ripple is too large. It directly affects

the load regulation of the circuit since the IC regulates the

can be pulled up and a good regulation is made.

Larger value of L can help the load regulation but it

voltage. Hence, it is called as a “pull-up resistor” and it can

help to pull up the output voltage slightly.

feedback to the feedback (FB) pin of the NCP1052 through

high, there will be a greater-than-50

mA current inserting

into the feedback pin of the NCP1052. The NCP1052 will

stop switching when it happens. When output voltage is not

high enough, the current inserting into the feedback is

smaller than 50

mA. The NCP1052 enables switching and

power is delivered to the output until the output voltage is

too high again.

inserting into the feedback pin because the switching of

NCP1052 can also be stopped when there is a

greater-than-50

mA current sinking from the FB pin. The

threshold. The FB pin of NCP1052 with a condition of

50

mA sourcing current is about 4.3 V. The volt-drop of the

mA. Hence, the output

voltage can be loosely set as follows:

Vout + zener ) 4.3 V ) 0.7 V

(eq. 1)

+ zener ) 5V

According to (1), the possible minimum output voltage of

If there is no load, the IC will automatically minimize its

duty cycle to the minimum value but the output voltage is

still possible to be very high because there is no passive

component in the circuit try to absorb the energy. As a result,