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Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high
voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these
capacitors are used at input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can induce ringing at the input, V
IN
. At best, this ringing can couple to
the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially
cause a voltage spike at V
IN
large enough to damage the part.
Output Rectifier Diode
The output rectifier diode supplies the current to the inductor when the high‐side switch is off. To reduce losses due to the
diode forward voltage and recovery times, use a Schottky diode.
Choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating
is greater than the maximum load current.
Choose a rectifier who’s maximum reverse voltage rating is greater than the maximum input voltage, and who’s current
rating is greater than the maximum load current.
Checking Transient Response
The regulator loop response can be checked by looking at the load transient response. Switching regulators take several
cycles to respond to a step in load current. When a load step occurs, V
OUT
immediately shifts by an amount equal to ΔI
LOAD (ESR)
also begins to charge or discharge C
OUT
generating a feedback error signal for the regulator to return V
OUT
to its steady‐state
value. During this recovery time, V
OUT
can be monitored for overshoot or ringing that would indicate a stability problem.
Table 1. Suggested Inductors for Typical Application Circuit
Component Supplier
MAGLAYERS
SUMIDA
TOKO
Table 2. Suggested Capacitors for C
IN
and C
OUT
Component Supplier
MURATA
TDK
MURATA
TDK
Part No.
GRM31CR61E106K
C3225X5R1E106K
GRM32ER71C226M
C3225X5R1C226M
Capacitance (uF)
10
10
22
22
Case Size
1206
1206
1200
1200
Series
MSCDRI‐124‐150M
CDRH104R
D104C
Dimensions (mm)
12 x 12 x 5.0
10.1 x 10 x 3.0
10 x 10 x 4.3
Table 3. Schottky Rectifier Selection Guide
V
IN
(Max.)
20V
Part No.
B320
SK33
SS32
B330
B340L
SK33
MBRD330
SS33
3A Load Current
Vendor
Diodes, Inc. (www.diodes.com)
Pan Jit International (www.panjit.com.tw)
General Semiconductor (www.gensemi.com)
Diodes, Inc. (www.diodes.com)
Diodes, Inc. (www.diodes.com)
Diodes, Inc. (www.diodes.com)
On Semiconductor (www.onsemi.com)
Fairchild Semiconductor (www.fairchildsemi.com)
General Semiconductor (www.gensemi.com)
26V
DS‐RS6509‐07
February, 2010
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
