NCP1547

http://onsemi.com

7

duty cycle of the sync pulses can vary from 10% to 90%. The

frequency foldback feature is disabled during the sync

mode.

Figure 4. A NCP1547 Buck Regulator is

Synchronized to an External 443 kHz Pulse Signal

Power Switch and Current Limit

The collector of the built−in NPN power switch is

when the power switch turns off, and the inductor current is

commutated to the catch diode. Due to the presence of high

and diode should be kept as short as possible to minimize the

noise and radiation. For the same reason, the input capacitor

diode.

The saturation voltage of the power switch is dependent

on the switching current, as shown in Figure 5.

Figure 5. The Saturation Voltage of the Power Switch

Increases with the Conducting Current

0

0.5

1.0

1.5

SWITCHING CURRENT (A)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

The NCP1547 contains pulse−by−pulse current limiting

to protect the power switch and external components. When

the peak of the switching current reaches the Current Limit,

the power switch turns off after the Current Limit Delay. The

switch will not turn on until the next switching cycle. The

current limit threshold is independent of switching duty

cycle. The maximum load current, given by the following

formula under continuous conduction mode, is less than the

Current Limit due to the ripple current.

IO(MAX) + ILIM *

VO(VIN * VO)

2(L)(VIN)(fs)

where:

L = inductor value.

When the regulator runs under current limit, the

subharmonic

oscillation

may

cause

low

frequency

oscillation, as shown in Figure 6. Similar to current mode

control, this oscillation occurs at the duty cycle greater than

50% and can be alleviated by using a larger inductor value.

The current limit threshold is reduced to Foldback Current

when the FB pin falls below Foldback Threshold. This

feature protects the IC and external components under the

power up or over−load conditions.

Figure 6. The Regulator in Current Limit

BOOST Pin

The BOOST pin provides base driving current for the

headroom to turn on the power switch. This in turn reduces

IC power dissipation and improves overall system

efficiency. The BOOST pin can be connected to an external

boost−strapping circuit which typically uses a 0.1

mF capacitor

and a 1N914 or 1N4148 diode, as shown in Figure 1.