SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
load to no load at the exact moment when inductor
current is at the peak, determines the required capaci-
tance. If the load release is instantaneous (load changes
from maximum to zero in < 1µs), the output capacitor
must absorb all the inductor’s stored energy. This will
cause a peak voltage on the capacitor according to the
following equation.
COUTMIN
L¨§
©
IOUT
1
2
u IRIPPLEMAX
¸·
¹
2
2
2
VPEAK VOUT
Assuming a peak voltage VPEAK of 1.98 (180mV rise upon
load release), and a 10A load release, the required capaci-
tance is shown by the next equation.
&2870,1
ȝ+¨§ u ¸·
© ¹
COUTMIN = 344µF
If the load release is relatively slow, the output capacitance
can be reduced. At heavy loads during normal switching,
when the FB pin is above the 600mV reference, the DL
output is high and the low-side MOSFET is on. During this
time, the voltage across the inductor is approximately
(-VOUT). This causes a down-slope or falling di/dt in the
inductor. If the load di/dt is not faster than the -di/dt in
the inductor, then the inductor current will tend to track
the falling load current. This will reduce the excess induc-
tive energy that must be absorbed by the output capaci-
tor, therefore a smaller capacitance can be used.
The following can be used to calculate the needed capaci-
tance for a given dILOAD/dt. Peak inductor current is shown
by the next equation.
ILPK = IMAX + 1/2 x IRIPPLEMAX
ILPK = 10 + 1/2 x 5 = 12.5A
Rate of change of Load Current dlLOAD
dt
IMAX = maximum load release = 10A
COUT
Lu ILPK IMAX u dt
ILPK u
VOUT dlLOAD
2 VPK VOUT
Example
dlLOAD
dt
2.5A
Ps
This would cause the output current to move from 10A to
zero in 4µs.
ȝ+u u ȝV
&287 u
u
COUT = 223µF
Note that COUT is much smaller in this example, 223µF
compared to 344µF based on a worst-case load release. To
meet the two design criteria of minimum 336µF and
maximum 14.4mΩ ESR, use two capacitors rated at
220µF/15mΩ.
It is recommended that an additional small capacitor with
a value of 1 to 10µF be placed in parallel with COUT in order
to filter high frequency switching noise.
Stability Considerations
Unstable operation is possible with adaptive on-time con-
trollers, and usually takes the form of double-pulsing or
ESR loop instability.
Double-pulsing occurs due to switching noise seen at the
FB input or because the FB ripple voltage is too low. This
causes the FB comparator to trigger prematurely after the
250ns minimum off-time has expired. In extreme cases
the noise can cause three or more successive on-times.
Double-pulsing will result in higher ripple voltage at the
output, but in most applications it will not affect opera-
tion. This form of instability can usually be avoided by
providing the FB pin with a smooth, clean ripple signal
that is at least 10mVp-p, which may dictate the need to
increase the ESR of the output capacitors. It is also impera-
tive to provide a proper PCB layout as discussed in the
Layout Guidelines section.
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