RT8280 RICHTEK [Richtek Technology Corporation], RT8280 Datasheet - Page 11

• Manufacturer Part Number: RT8280
• Description: 3A, 24V, 3MHz Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Diode Selection
When the power switch turns off, the path for the current
is through the diode connected between the switch output
and ground. This forward biased diode must have a minimal
voltage drop and recovery time. Schottky diodes are
recommended and should be able to handle those current.
The reverse voltage rating of the diode should be greater
than the maximum input voltage, and the current rating
should be greater than the maximum load current. For
details, please refer to Table 3.
C
The input capacitance, C
trapezoidal current at the source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
This formula has a maximum at V
I
commonly used for design.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design.
For the input capacitor, one 10μF low ESR ceramic
capacitors is recommended. For the recommended
Copyright
DS8280-02 March 2012
I
Component
RMS
RMS
IN
Component
Supplier
DIODES
DIODES
PANJIT
PANJIT
Supplier
and C
YUDEN
TAIYO
= I
TDK
TDK
= I
Table2. Suggested Inductors for Typical
©
OUT
OUT(MAX)
2012 Richtek Technology Corporation. All rights reserved.
OUT
/ 2. This simple worst-case condition is
Table 3. Suggested Diode
Series V
Selection
B330A
B340A
SK33
SK34
Application Circuit
V
V
OUT
SLF12565
IN
VLC6045
NR8040
Series
RRM
V
30
40
30
40
V
OUT
IN
IN
(V) I
, is needed to filter the
−
1
OUT
12.5 x 12.5 x 6.5
IN
3
3
3
3
(A)
Dimensions
= 2V
6 x 6 x 4.5
8 x 8 x 4
(mm)
DO-214AB
DO-214AB
Package
OUT
SMA
SMA
, where
capacitor, please refer to Table 4 below for more details.
The selection of C
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for C
Loop stability can be checked by viewing the load transient
response as described in a later section.
The output ripple, ΔV
The output ripple will be highest at the maximum input
voltage since ΔI
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors offer very low ESR value.
However, it provides lower capacitance density than other
types. Although Tantalum capacitors have the highest
capacitance density, it is important to only use types that
pass the surge test for use in switching power supplies.
Aluminum electrolytic capacitors have significantly higher
ESR. However, it can be used in cost sensitive applications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low
ESR characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing.
Nevertheless, high value low 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 the
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
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
part.
Δ
V
OUT
OUT
is a registered trademark of Richtek Technology Corporation.
≤ Δ
selection to ensure that the control loop is stable.
IN
I ESR
L
. At best, this ringing can couple to the output
⎡
⎢
⎣
L
increases with input voltage. Multiple
OUT
+
8fC
OUT
is determined by the required ESR
1
OUT
, is determined by :
IN
large enough to damage the
⎤
⎥
⎦
RT8280
www.richtek.com
11