rt8297a Richtek Technology Corporation, rt8297a Datasheet - Page 12

• Manufacturer Part Number: rt8297a
• Description: 1.5a, 17v, 340/800khz Synchronous Step-down Converter
• Manufacturer: Richtek Technology Corporation

RT8297A/B
C
The input capacitance, C
trapezoidal current at the source of the high side 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
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, a 10μF low ESR ceramic
capacitor is recommended. For the recommended
capacitor, please refer to table 3 for more detail. The
selection of C
minimize voltage ripple. Moreover, the amount of bulk
capacitance is also a key for C
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
determined by :
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
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12
I
Δ
OUT
RMS
IN
V
OUT
and C
/2. This simple worst-case condition is commonly used
= I
≤ Δ
OUT(MAX)
OUT
I ESR
L
⎡
⎢
⎣
OUT
Selection
L
increases with input voltage. Multiple
V
is determined by the required ESR to
V
+
OUT
IN
8fC
1
OUT
V
V
OUT
IN
IN
⎤
⎥
⎦
, is needed to filter the
−
OUT
IN
1
= 2V
selection to ensure
OUT
, where I
OUT
RMS
, is
=
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, VIN. 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 VIN 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
equal to ΔI
C
to return V
recovery time, V
ringing that would indicate a stability problem.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula:
P
where
the ambient temperature, and
thermal resistance.
For recommended operating condition specifications of
the RT8297A/B, the maximum junction temperature is
125°C and
ambient thermal resistance,
D(MAX)
OUT
generating a feedback error signal for the regulator
T
= (T
J(MAX)
LOAD
T
J(MAX)
OUT
A
is the ambient temperature. The junction to
is the maximum junction temperature,
(ESR) also begins to charge or discharge
to its steady-state value. During this
OUT
− T
OUT
A
can be monitored for overshoot or
) / θ
immediately shifts by an amount
JA
θ
θ
JA
DS8297A/B-00 March 2011
JA
, is layout dependent. For
is the junction to ambient
T
A
is