RT8260A Datasheet, PDF (9/12 Pages) Richtek Technology Corporation – 1.8A, 24V, 1.4MHz Step-Down Converter

RT8260A
Table 3. Suggested Diode
Component
Supplier
Series
V_RRM
(V)
I_OUT
(A)
Package
DIODES B220A 20
2
SMA
DIODES B230A 30
2
SMA
PANJIT SK22 20
2 DO-214AA
PANJIT SK23 30
2 DO-214AA
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoidal
current at the source of the top 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.
The selection of C_OUT is determined by the required Effective
Series Resistance (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.
The output ripple, ΔV_OUT , is determined by :
ΔV_OUT
≤
ΔI_L
⎡⎢⎣_{ESR +}
1
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 recovery
time, V_OUT can be monitored for overshoot or 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
the high side MOSFET is turned-on/off, this spike voltage
on SW may impact 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 place them as close
as possible to the SW pin (see Figure 3). Another method
is to add a resistor in series with the bootstrap capacitor,
C_BOOT. But this method will decrease the driving capability
to the high side MOSFET. It is strongly recommended to
DS8260A-03 March 2011
www.richtek.com
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