RT8203GA Richtek USA Inc, RT8203GA Datasheet - Page 21

• Manufacturer Part Number: RT8203GA
• Description: IC CTRLR SMPS STPDN DUAL 28SSOP
• Manufacturer: Richtek USA Inc

Specifications of RT8203GA

• Output Isolation: Isolated
• Frequency Range: Adjustable, Selectable
• Voltage - Input: 7 V ~ 24 V
• Voltage - Output: Selectable
• Power (watts): 1.05W
• Operating Temperature: -10°C ~ 85°C
• Package / Case: 28-SSOP (0.150", 3.90mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

In non-CPU applications, the output capacitor's size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple :
where V
Organic semiconductor capacitor(s) or specialty polymer
capacitor(s) are recommended.
For low input-to-output voltage differentials (VIN / VOUTx
< 2), additional output capacitance is required to maintain
stability and good efficiency in ultrasonic mode.
The amount of overshoot due to stored inductor energy
can be calculated as :
where I
Output Capacitor Stability
The output capacitor stability is determined by the value of
the ESR zero relative to the switching frequency. The point
of instability is given by the following equation :
Do not put high-value ceramic capacitors directly across
the outputs without taking precautions to ensure stability.
Large ceramic capacitors can have a high ESR zero
frequency and cause erratic, unstable operation. However,
it is easy to add enough series resistance by placing the
capacitors a couple of inches downstream from the inductor
and connecting VOUTx or the FBx divider close to the
inductor.
Unstable operation manifests itself in two related and
distinctly different ways : double-pulsing and feedback loop
instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there is not enough voltage
ramp in the output voltage signal. This “fools” the error
comparator into triggering a new cycle immediately after
the 400ns minimum off-time period has expired. Double-
pulsing is more annoying than harmful, resulting in nothing
worse than increased output ripple. However, it may
DS8203-04 March 2011
ESR
V
f
ESR
SOAR
=
≤
PEAK
=
P-P
2
LIR I
× ×
2 C
is the peak-to-peak output voltage ripple.
π
is the peak inductor current.
×
(I
×
PEAK
ESR C
V
LOAD(MAX)
OUT
P-P
1
)
×
×
2
V
×
OUT
OUT
L
≤
f
SW
4
indicate the possible presence of loop instability, which is
caused by insufficient ESR.
Loop instability can result in oscillations at the output after
line or load perturbations that can trip the overvoltage
protection latch or cause the output voltage to fall below
the tolerance limit.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient and carefully observe
the output-voltage-ripple envelope for overshoot and ringing.
It helps to simultaneously monitor the inductor current with
an AC current probe. Do not allow more than one cycle of
ringing after the initial step-response under- or overshoot.
Layout Considerations
Layout is very important in high frequency switching
converter design. If designed improperly, the PCB could
radiate excessive noise and contribute to the converter
instability. Certain points must be considered before
starting a layout using the RT8203.
Gather ground terminal of VIN capacitor(s), VOUTx
Connect RC low pass filter from LDO5 to VCC, 1-mF and
10Ω are recommended. Place the filter capacitor close
to the IC, within 12mm(0.5 inch) if possible.
Keep current limit setting network as close as possible
to the IC. Routing of the network should avoid coupling
to high voltage switching node.
Connections from the drivers to the respective gate of
the high side or the low side MOSFET should be as short
as possible to reduce stray inductance. Use 0.65-mm
(25 mils) or wider trace.
All sensitive analog traces and components such as
VOUTx, FBx, GND, ONx, PGOOD, ILIMx, VCC, and TON
should be placed away from high-voltage switching nodes
such as PHASEx, LGATEx, UGATEx, or BOOTx nodes
to avoid coupling. Use internal layer(s) as ground plane(s)
and shield the feedback trace from power traces and
components.
capacitor(s), and source of low side MOSFETs as close
as possible. PCB trace defined as PHASEx node, which
connects to source of high side MOSFET, drain of low
side MOSFET and high voltage side of the inductor, should
be as short and wide as possible.
RT8203
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21