RT8279 RICHTEK [Richtek Technology Corporation], RT8279 Datasheet - Page 10

• Manufacturer Part Number: RT8279
• Description: 5A, 36V, 500kHz Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT8279
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
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 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
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.
www.richtek.com
10
I
RMS
RMS
IN
Component
and C
Supplier
OUT
YUDEN
= I
TAIYO
= I
Table 2. Suggested Inductors for Typical
TDK
OUT(MAX)
OUT
selection to ensure that the control loop is stable.
OUT
/2. This simple worst-case condition is
Selection
Application Circuit
V
OUT
V
OUT
IN
SLF12565
NR10050
is determined by the required ESR
Series
V
V
OUT
IN,
IN
is needed to filter the
−
4.
1
12.5 x 12.5 x 6.5
IN
Dimensions
= 2V
10 x 9.8 x 5
(mm)
OUT
, where
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.
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
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.
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
equal to ΔI
C
to return V
Δ
OUT
V
OUT
generating a feedback error signal for the regulator
IN
≤ Δ
. At best, this ringing can couple to the output
LOAD
I ESR
L
OUT
⎡
⎢
⎣
(ESR) also begins to charge or discharge
to its steady-state value. During this
L
increases with input voltage. Multiple
+
OUT
8fC
OUT
immediately shifts by an amount
1
OUT
, is determined by :
IN
large enough to damage the
DS8279-01
⎤
⎥
⎦
December 2011