RT8282B RICHTEK [Richtek Technology Corporation], RT8282B Datasheet - Page 11

• Manufacturer Part Number: RT8282B
• Description: 2A, 23V, 1.2MHz Synchronous Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

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.
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
DS8282B-01 March 2011
Δ
V
OUT
IN
≤ Δ
. At best, this ringing can couple to the output
I ESR
Chip Enable
4.5V to 23V
L
⎡
⎢
⎣
* : Optional
L
increases with input voltage. Multiple
V
+
IN
8fC
OUT
1
OUT
, is determined by :
C
0.1µF
Figure 5. Reference Circuit with Snubber and Enable Timing Control
R
SS
C
EN
EN
*
⎤
⎥
⎦
*
9 (Exposed Pad)
10µF
C
IN
4,
2
7
8
VIN
EN
SS
GND
RT8282B
COMP
BOOT
SW
FB
6
1
3
5
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
recovery time, V
ringing that would indicate a stability problem.
EMI Consideration
Since parasitic inductance and capacitance effects in PCB
circuitry
high side MOSFET is
SW may impact on EMI performance in the system. In
order to enhance EMI performance, there are two methods
to suppress the spike voltage. One is to place an R-C
snubber between SW and GND and make them as close
as possible to the SW pin
is adding a resistor in series with
capacitor,
capability to the high side MOSFET. It is strongly
recommended to reserve the R-C snubber during PCB
layout for EMI improvement. Moreover, reducing the SW
trace area and keeping the main power in a small loop will
be helpful on EMI performance. For detailed PCB layout
guide, please refer to the section of Layout Consideration.
OUT
R
BOOT
R
C
generating a feedback error signal for the regulator
S
S
*
*
0.82nF
*
would cause a spike voltage on SW pin when
C
C
C
LOAD
BOOT
OUT
C
100nF
C
NC
BOOT
P
32k
R
(ESR) also begins to charge or discharge
. But this method will decrease the driving
C
to its steady-state value. During this
OUT
3.6µH
L
OUT
can be monitored for overshoot or
turned-on/off, this spike voltage on
immediately shifts by an amount
(see Figure 5). Another method
IN
75k
R1
R2
24k
large enough to damage the
C
22µFx2
OUT
RT8282B
V
3.3V/2A
OUT
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