SC414EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC414EVB' PDF : 29 Pages View PDF
SC414/SC424
Applications Information (continued)
This causes the output current to move from 6A to 0A in
CTOP
4.8μs, giving the minimum output capacitance require-
ment shown in the following equation.
COUT
COUT
1.5PHu 7.26A 6A u1Ps
7.26A u
1V 1.25A
2 1.05V 1V
443PF
Note that COUT is much smaller in this example, 443μF
compared to 772μF based on a worst-case load release. To
meet the two design criteria of minimum 443μF and
maximum 15mΩ ESR, select two capacitors rated at 220μF
and 15mΩ ESR or less.
It is recommended that an additional small capacitor be
placed in parallel with C in order to filter high frequency
OUT
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
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 operation. 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 imperative to provide a
proper PCB layout as discussed in the Layout Guidelines
section.
Another way to eliminate doubling-pulsing is to add a
small (~ 10pF) capacitor across the upper feedback resis-
tor, as shown in Figure 13. This capacitor should be left
unpopulated unless it can be confirmed that double-
pulsing exists. Adding the C capacitor will couple more
TOP
ripple into FB to help eliminate the problem. An optional
connection on the PCB should be available for this
capacitor.
VOUT
R1
To FB pin
R2
Figure 13 — Capacitor Coupling to FB Pin
ESR loop instability is caused by insufficient ESR. The
details of this stability issue are discussed in the ESR
Requirements section. The best method for checking sta-
bility is to apply a zero-to-full load transient and observe
the output voltage ripple envelope for overshoot and
ringing. Ringing for more than one cycle after the initial
step is an indication that the ESR should be increased.
One simple way to solve this problem is to add trace resis-
tance in the high current output path. A side effect of
adding trace resistance is a decrease in load regulation.
ESR Requirements
A minimum ESR is required for two reasons. One reason is
to generate enough output ripple voltage to provide
10mVp-p at the FB pin (after the resistor divider) to avoid
double-pulsing.
The second reason is to prevent instability due to insuffi-
cient ESR. The on-time control regulates the valley of the
output ripple voltage. This ripple voltage is the sum of the
two voltages. One is the ripple generated by the ESR, the
other is the ripple due to capacitive charging and dis-
charging during the switching cycle. For most applica-
tions, the total output ripple voltage is dominated by the
output capacitors, typically SP or POSCAP devices. For
stability the ESR zero of the output capacitor should be
lower than approximately one-third the switching fre-
quency. The formula for minimum ESR is shown by the
following equation.
ESRMIN
3
2 u S u COUT u fsw
24
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