RT8289
Table 2. Suggested Inductors for Typical
Application Circuit
Component
Dimensions
Series
Supplier
(mm)
TAIYO
NR10050
10 x 9.8 x 5
YUDEN
TDK
SLF12565
12.5 x 12.5 x 6.5
The output ripple,
ΔV
OUT
, is determined by :
1
⎤
Δ
V
OUT
≤ Δ
I
L
⎡
ESR
+
⎢
8fC
OUT
⎥
⎣
⎦
The output ripple will be highest at the maximum input
voltage since
ΔI
L
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.
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.
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
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
minimum voltage drop and recovery times. Schottky diode
is recommended and it should be able to handle those
current. The reverse voltage rating of the diode should be
greater than the maximum input voltage, and current rating
should be greater than the maximum load current. For
more detail please refer to Table
4.
C
IN
and C
OUT
Selection
The input capacitance, C
IN,
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 :
I
RMS
= I
OUT(MAX)
V
OUT
V
IN
V
IN
−
1
V
OUT
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not offer much relief.
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,two 4.7μF low ESR ceramic
capacitors are recommended. For the recommended
capacitor, please refer to table 3 for more detail.
The selection of C
OUT
is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for C
OUT
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.
DS8289-01 March 2011
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