RT8253B RICHTEK [Richtek Technology Corporation], RT8253B Datasheet - Page 11

• Manufacturer Part Number: RT8253B
• Description: 3A, 23V, 1.2MHz Synchronous Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

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 10μF 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.
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.
DS8253B-02 March 2011
I
Δ
RMS
RMS
V
OUT
OUT
= I
= I
≤ Δ
OUT(MAX)
OUT
selection to ensure that the control loop is stable.
I ESR
L
/2. This simple worst-case condition is
⎡
⎢
⎣
L
increases with input voltage. Multiple
V
OUT
V
+
OUT
IN
8fC
OUT
is determined by the required ESR
1
OUT
, is determined by :
V
V
OUT
IN
⎤
⎥
⎦
−
1
IN
= 2V
OUT
, where
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
recovery time, V
ringing that would indicate a stability problem.
EMI Consideration
Since parasitic inductance and capacitance effects in PCB
circuitry would cause a spike voltage on the SW pin when
high side MOSFET is turned-on/off, this spike voltage on
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 (see Figure 5). Another method
is adding a resistor in series with the bootstrap
capacitor, C
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
generating a feedback error signal for the regulator
IN
. At best, this ringing can couple to the output
LOAD
BOOT
OUT
(ESR) also begins to charge or discharge
. But this method will decrease the driving
to its steady-state value. During this
OUT
OUT
can be monitored for overshoot or
immediately shifts by an amount
IN
large enough to damage the
RT8253B
www.richtek.com
11