NCP1442, NCP1443, NCP1444, NCP1445
V
IN
current I . Summing up, the output voltage peak−peak
IN
ripple can be calculated by:
V
CC
(I * I
(1 * D)
(f)
IN
OUT)
OUT)
V
+
OUT(RIPPLE)
(C
R2
R3
I
D
OUT
(C
)
) I ESR
IN
V
C
)(f)
OUT
D1
The equation can be expressed more conveniently in
terms of V , V
and I
for design purposes as
CC
OUT
OUT
follows:
I
(V
(C
* V
)(f)
)
CC
OUT OUT
1
OUT
V
+
OUT(RIPPLE)
(C
)(f)
R1
OUT
C1
(I )(ESR)
)(V
OUT OUT
)
V
CC
C2
The capacitor RMS ripple current is:
Ǹ
2
2
) (D)
OUT
I
+
(I * I
) (1 * D))(I
RIPPLE
IN
OUT
Figure 36. Current Limiting using a Diode Clamp
V
* V
OUT
V
CC
OUT Ǹ
+ I
CC
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 37.
Although the above equations apply only for boost
circuits, similar equations can be derived for flyback
circuits.
V
CC
Reducing the Current Limit
In some applications, the designer may prefer a lower
limit on the switch current than 4.0 A. An external shunt can
V
C
PGND
AGND
be connected between the V pin and ground to reduce its
C
+
−
V
IN
clamp voltage. Consequently, the current limit of the
internal power transistor current is reduced from its nominal
value.
R1
C2
The voltage on the V pin can be evaluated with the
equation:
C
C1
R2
C3
Q1
V
+ I R A
SW E V
C
Output
Ground
where:
R = .015 W, the value of the internal emitter resistor;
R
SENSE
E
Figure 37. Current Limiting using a Current Sense
Resistor
A = 5.0 V/V, the gain of the current sense amplifier.
V
Since R and A cannot be changed by the end user, the
E
V
only available method for limiting switch current below
The switch current is limited to:
4.0 A is to clamp the V pin at a lower voltage. If the
C
V
R
BE(Q1)
SENSE
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 36, clamps the
I
+
SWITCH(PEAK)
where:
V voltage to a diode drop above the voltage on resistor R3.
C
V
BE(Q1)
= the base−emitter voltage drop of Q1, typically
Unfortunately, such a simple circuit is not generally
0.65 V.
acceptable if V is loosely regulated.
IN
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