NCP5173(2004) View Datasheet(PDF) - ON Semiconductor
Part Name
Description
Manufacturer
NCP5173 Datasheet PDF : 18 Pages
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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
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
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.
VC
R1
R2
C1
C2
C3
R3
Figure 32. Technique for Increasing Slope
Compensation
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