RT8280 데이터 시트보기 (PDF) - Richtek Technology
부품명
상세내역
제조사
RT8280 Datasheet PDF : 16 Pages
| |||
RT8280
Table2. Suggested Inductors for Typical
Application Circuit
Component
Supplier
Series
Dimensions
(mm)
TDK
VLC6045
6 x 6 x 4.5
TDK
SLF12565 12.5 x 12.5 x 6.5
TAIYO
YUDEN
NR8040
8x8x4
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.
Table 3. Suggested Diode
Component
Supplier
Series
VRRM (V) IOUT (A)
Package
DIODES B330A 30
3
SMA
DIODES B340A 40
3
SMA
PANJIT SK33 30
3 DO-214AB
PANJIT SK34 40
3 DO-214AB
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
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 :
IRMS
=
IOUT(MAX)
VOUT
VIN
VIN
VOUT
−1
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT / 2. This simple worst-case condition is
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 ©2012 Richtek Technology Corporation. All rights reserved.
DS8280-02 March 2012
capacitor, please refer to Table 4 below for more details.
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response as described in a later section.
The output ripple, ΔVOUT , is determined by :
ΔVOUT
≤
ΔIL
⎡⎢⎣ESR
+
1
8fCOUT
⎤
⎥⎦
The output ripple will be highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
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, VIN. 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 VIN large enough to damage the
part.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
Share Link: 