NCP5173 데이터 시트보기 (PDF) - ON Semiconductor

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NCP5173

1.5 A 560 kHz−1.0 MHz Boost Regulator

ON Semiconductor 

NCP5173

A simple diode clamp, as shown in Figure 30, clamps the

VC voltage to a diode drop above the voltage on resistor R3.

Unfortunately, such a simple circuit is not generally

acceptable if VIN is loosely regulated.

VIN

R2

VCC

D1

VC

R3

R1

C1

C2

Figure 30. Current Limiting Using a Diode Clamp

Another solution to the current limiting problem is to

externally measure the current through the switch using a

sense resistor. Such a circuit is illustrated in Figure 31.

VCC

PGND AGND VC

VIN

R2

Q1

C3

RSENSE

R1

C1

C2

Output

Ground

Figure 31. Current Limiting using a Current Sense

Resistor

The switch current is limited to:

ISWITCH(PEAK)

+

VBE(Q1)

RSENSE

where:

VBE(Q1) = the base−emitter voltage drop of Q1, typically

0.65 V.

The improved circuit does not require a regulated voltage

to operate properly. Unfortunately, a price must be paid for

this convenience in the overall efficiency of the circuit. The

designer should note that the input and output grounds are

no longer common. Also, the addition of the current sense

resistor, RSENSE, results in a considerable power loss which

increases with the duty cycle. Resistor R2 and capacitor C3

form a low−pass filter to remove noise.

Subharmonic Oscillation

Subharmonic oscillation (SHM) is a problem found in

current−mode control systems, where instability results

when duty cycle exceeds 50%. SHM only occurs in

switching regulators with a continuous inductor current.

This instability is not harmful to the converter and usually

does not affect the output voltage regulation. SHM will

increase the radiated EM noise from the converter and can

cause, under certain circumstances, the inductor to emit

high−frequency audible noise.

SHM is an easily remedied problem. The rising slope of

the inductor current is supplemented with internal “slope

compensation” to prevent any duty cycle instability from

carrying through to the next switching cycle. In the

NCP5173, slope compensation is added during the entire

switch on−time, typically in the amount of 180 mA/ms.

In some cases, SHM can rear its ugly head despite the

presence of the onboard slope compensation. The simple

cure to this problem is more slope compensation to avoid the

unwanted oscillation. In that case, an external circuit, shown

in Figure 32, can be added to increase the amount of slope

compensation used. This circuit requires only a few

components and is “tacked on” to the compensation

network.

VSW

VSW

VC

R1

R2

C1

C2

C3

R3

Figure 32. Technique for Increasing Slope

Compensation

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