NCV5171_10 Datasheet, PDF (13/19 Pages) ON Semiconductor – 1.5 A 280 kHz/560 kHz Boost Regulators

NCV5171, NCV5173
Reducing the Current Limit
In some applications, the designer may prefer a lower
limit on the switch current than 1.5 A. An external shunt can
be connected between the V_C pin and ground to reduce its
clamp voltage. Consequently, the current limit of the
internal power transistor current is reduced from its nominal
value.
The voltage on the V_C pin can be evaluated with the
equation
VC + ISWREAV
where:
R_E = .063W, the value of the internal emitter resistor;
A_V = 5 V/V, the gain of the current sense amplifier.
Since R_E and A_V cannot be changed by the end user, the
only available method for limiting switch current below
1.5 A is to clamp the V_C pin at a lower voltage. If the
maximum switch or inductor current is substituted into the
equation above, the desired clamp voltage will result.
A simple diode clamp, as shown in Figure 31, clamps the
V_C voltage to a diode drop above the voltage on resistor R3.
Unfortunately, such a simple circuit is not generally
acceptable if V_IN is loosely regulated.
V_IN
V_CC
R2
D1
V_C
R3
R1
C1
C2
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 32.
V_CC
PGND AGND VC
V_IN
R1
R2
Q1
C1
C2
C3
R_SENSE
Output
Ground
Figure 32. Current Limiting using a Current Sense
Resistor
The switch current is limited to
ISWITCH(PEAK)
+
VBE(Q1)
RSENSE
where:
V_BE(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, R_SENSE, 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.
Figure 31. Current Limiting using a Diode Clamp
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