RT8259A Datasheet, PDF (9/11 Pages) Richtek Technology Corporation – The RT8259A is a monolithic step-down switch mode

RT8259A
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.
External Bootstrap Diode
When the operating input voltage is lower than 5.5V, it is
recommended to add an external bootstrap diode for
efficiency improvement. The bootstrap diode can be a low
cost one such as IN4148 or BAT54. For higher operating
input voltage between 5.5V and 24V, the external diode
must be removed.
V_IN
4.5V to 5.5V
BOOT
RT8259A
PHASE
10nF
Figure 3
DS8259A-02 March 2011
Output Voltage Setting
The resistive divider allows the FB pin to sense a fraction
of the output voltage as shown in Figure 4.
V_OUT
R1
FB
RT8259A
R2
GND
Figure 4. Setting the Output Voltage
For adjustable voltage mode, the output voltage is set by
an external resistive divider according to the following
equation :
V_OUT
=
V_REF
⎛⎜⎝₁₊
R1
R2
⎞⎟⎠
Where V_REF is the internal reference voltage (0.8V typ.).
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.
Thermal Considerations
For continuous operation, do not exceed the maximum
operation junction temperature 125°C. The maximum power
dissipation depends on the thermal resistance of IC
package, PCB layout, the rate of surroundings airflow and
temperature difference between junction to ambient. The
maximum power dissipation can be calculated by following
formula :
P_D(MAX) = ( T_J(MAX) - T_A ) / θ_JA
where T_J(MAX) is the maximum operation junction
temperature 125°C, T_A is the ambient temperature and the
θ_JA is the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT8259A, where T_J(MAX) is the maximum junction
temperature of the die (125°C) and T_A is the maximum
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