RT8258 RICHTEK [Richtek Technology Corporation], RT8258 Datasheet - Page 8

• Manufacturer Part Number: RT8258
• Description: 1.2A, 24V, 700kHz Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT8258
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materials
are small and do not radiate energy. However, they are
usually more expensive than the similar powdered iron
inductors. The rule for inductor choice mainly depends on
the price vs. size requirement and any radiated field/EMI
requirements.
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 3.
C
The input capacitance, C
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 :
This formula has a maximum at V
I
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
Series Resistance (ESR) 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
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8
I
RMS
RMS
IN
OUT
and C
= I
= I
selection to ensure that the control loop is stable.
OUT
OUT(MAX)
/2. This simple worst-case condition is commonly
OUT
Selection
OUT
V
V
OUT
is determined by the required Effective
IN
OUT
IN,
, is determined by :
is needed to filter the trapezoidal
V
V
OUT
IN
−
1
IN
= 2V
OUT
, where
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
mistaken as loop instability. At worst, a sudden inrush of
current through the long wires can potentially cause a
voltage spike at V
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
discharge C
regulator to return V
this recovery time, V
ringing that would indicate a stability problem.
Δ
IN
V
OUT
. At best, this ringing can couple to the output and be
≤ Δ
I ESR
L
LOAD
OUT
⎡
⎢
⎣
generating a feedback error signal for the
L
(ESR) and also begins to charge or
IN
increases with input voltage. Multiple
+
large enough to damage the part.
OUT
OUT
OUT
8fC
can be monitored for overshoot or
to its steady-state value. During
immediately shifts by an amount
1
OUT
⎤
⎥
⎦
DS8258-02 March 2011