SC453 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC453' PDF : 23 Pages View PDF
SC453
POWER MANAGEMENT
Applications Information (Cont.)
To calculate the maximum DC value of current we must
add the maximum DC current and the maximum ripple
value to obtain peak current:
IRIPPLE_MAX
S.
IPEAK := IMAX_FL +
2
IPEAK = 23.389 A
We set the current limit at ICLIM = ICLMAX, and then solve
for RCL, which is R6 in the typical applications circuit. For
balance, R8, in series with the CLRF pin is kept the same
value as R6.
ICLIM⋅RHYS⋅RCS
V.
RCL := 2.5⋅VREF
It is recommended that the current limit be set at 120%
of the peak value to allow for inductor current overshoot
during load transients:
RCL = 673.59 Ω R6 := 681Ω
T.
ICLIM := 120%⋅IPEAK
R8 := R6
R8 = 681 Ω
ICLIM = 28.066 A
The Current Limit Comparator internal to the SC453 moni-
tors the output current and turns the high side switch off
when the current exceeds the upper current limit thresh-
old, I and re-enables only if the load current drops be-
CLMAX
low the lower current limit threshold, ICLMIN. The current is
sensed by monitoring the voltage drop across the current
sense resistor RCS.
Step 7: Small Capacitors/Resistor Selection
Several small capacitors are required for signal filtering.
Use SMT ceramic capacitors with an X7R or better tem-
perature coefficient. COG is preferred.
C11, which filters the output voltage feedback, is sized to
provide filtering beyond the 5th harmonic of the funda-
mental.
Current limiting will cycle from ICLMAX to ICLMIN for 32 switch-
ing cycles to allow for short term transients, then the con-
verter is latched off. ICLMAX and ICLMIN are set according to
W.
the following equations:
1
C11 :=
2⋅ Π ⋅ R7⋅ FS⋅ 5
C11
=
9.095
×
−
10
11
F
U.
RCL
ICLMAX := 3⋅VREF⋅ RHYS⋅RCS
RCL
ICLMIN := 2⋅VREF⋅ RHYS⋅RCS
In the evaluation board design, we use 100pF, 603, X7R
ceramic caps for C11. The DAC output requires a 1nF,
X7R or COG capacitor (C23) for high frequency noise fil-
tering. The values for C12 and C13 are calculated in a
similar manner, though they are returned to the CORE pin
because that is the reference point for the current limit
comparator.
1
X.
C12 := 2⋅Π⋅R6⋅FS⋅5
C13 := C12
C12
=
1.335
×
−
10
10
F
C13
=
1.335
×
−
10
10
F
© 2006 Semtech Corp.
19
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