NCP1546

http://onsemi.com

6

APPLICATIONS INFORMATION

THEORY OF OPERATION

The NCP1546 buck regulator provides a high level of

integration and high operating frequencies allowing the

layout of a switch−mode power supply in a very small board

as the ramp signal, providing an ease of use not generally

associated with voltage or current mode control. Improved

line regulation, load regulation and very fast transient

response are also major advantages.

Buck

Controller

FFB

+

Duty Cycle

Error

Amplifier

PWM

Comparator

R1

Oscillator

−

SFB

Latch

Slope

Comp

L1

C1

D1

R2

S

R

S1

As shown in Figure 3, there are two voltage feedback

SFB(Slow Feedback). In FFB path, the feedback voltage

connects directly to the PWM comparator. This feedback

path carries the ramp signal as well as the output DC voltage.

Artificial ramp derived from the oscillator is added to the

feedback signal to improve stability. The other feedback

path, SFB, connects the feedback voltage to the error

PWM comparator. In a constant frequency mode, the

oscillator signal sets the output latch and turns on the switch

S1. This starts a new switch cycle. The ramp signal,

composed of both artificial ramp and output ripple,

resets the latch to turn off the switch. The switch S1 will turn

on again at the beginning of the next switch cycle. In a buck

converter, the output ripple is determined by the ripple

current of the inductor L1 and the ESR (equivalent series

resistor) of the output capacitor C1.

The slope compensation signal is a fixed voltage ramp

provided by the oscillator. Adding this signal eliminates

subharmonic oscillation associated with the operation at

duty cycle greater than 50%. The artificial ramp also ensures

the proper PWM function when the output ripple voltage is

inadequate. The slope compensation signal is properly sized

to serve it purposes without sacrificing the transient

response speed.

Under load and line transient, not only the ramp signal

changes, but more significantly the DC component of the

feedback voltage varies proportionally to the output voltage.

FFB path connects both signals directly to the PWM

comparator. This allows instant modulation of the duty cycle

to counteract any output voltage deviations. The transient

response time is independent of the error amplifier

bandwidth. This eliminates the delay associated with error

amplifier and greatly improves the transient response time.

The error amplifier is used here to ensure excellent DC

accuracy.

Error Amplifier

The NCP1546 has a transconductance error amplifier,

whose non−inverting input is connected to an Internal

Reference Voltage generated from the on−chip regulator.

frequency compensation requires only a 0.1

mF capacitor

Figure 1. This capacitor and error amplifier’s output

resistance (approximately 8.0 M

W) create a low frequency

require a high bandwidth error amplifier, the frequency

compensation is greatly simplified.

allows the regulator to recover quickly from over current or

short circuit conditions.

Oscillator and Sync Feature

The on−chip oscillator is trimmed at the factory and

requires no external components for frequency control. The

high

switching

frequency

allows

smaller

external

components to be used, resulting in a board area and cost

savings. The tight frequency tolerance simplifies magnetic

components selection. The switching frequency is reduced

below Frequency Foldback Threshold. In short circuit or

over−load conditions, this reduces the power dissipation of

the IC and external components.

An external clock signal can sync the NCP1546 to a higher

frequency. The rising edge of the sync pulse turns on the

power switch to start a new switching cycle, as shown in

Figure 4. There is approximately 0.5

ms delay between the

voltage. The sync threshold is TTL logic compatible, and