RT9181-18PB RICHTEK [Richtek Technology Corporation], RT9181-18PB Datasheet - Page 9

• Manufacturer Part Number: RT9181-18PB
• Description: 150mA LDO Regulator with PGOOD
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Any good quality ceramic or tantalum can be used for this
capacitor. The capacitor with larger value and lower ESR
(equivalent series resistance) provides better PSRR and
line-transient response.
The output capacitor must meet both requirements for
minimum amount of capacitance and ESR in all LDO
applications. The RT9181 is designed specifically to work
with low-ESR ceramic output capacitor in space-saving
and performance consideration. Using a ceramic
capacitor whose value is at least 1μF with ESR is > 5mΩ
on the RT9181 output ensures stability. The RT9181 still
works well with output capacitor of other types due to the
wide stable ESR range. Output capacitor of larger
capacitance can reduce noise and improve load-transient
response, stability, and PSRR. The output capacitor
should be located not more than 0.5" from the V
of the RT9181 and returned to a clean analog ground.
Note that some ceramic dielectrics exhibit large
capacitance and ESR variation with temperature. It may
be necessary to use 2.2μF or more to ensure stability at
temperatures below -10°C in this case. Also, tantalum
capacitors, 2.2μF or more may be needed to maintain
capacitance and ESR in the stable region for strict
application environment.
Tantalum capacitors maybe suffer failure due to surge
current when it is connected to a low-impedance source
of power (like a battery or very large capacitor). If a
tantalum capacitor is used at the input, it must be
guaranteed to have a surge current rating sufficient for
the application by the manufacturer.
Load-Transient Considerations
The RT9181 load-transient response graphs show two
components of the output response: a DC shift from the
output impedance due to the load current change, and
the transient response. The DC shift is quite small due
to the excellent load regulation of the IC. Typical output
voltage transient spike for a step change in the load
current from 0mA to 50mA is tens mV, depending on the
ESR and ESL of the output capacitor. Increasing the
output capacitor's value and decreasing the ESR and ESL
attenuates the overshoot.
DS9181-14 March 2007
OUT
pin
Reverse Current Path
The power transistor used in the RT9181 has an inherent
diode connected between each regulator input and output
(see Figure 1). If the output is forced above the input by
more than a diode-drop, this diode will become forward
biased and current will flow from the V
V
the input voltage to a value below the output voltage. To
prevent regulator mis-operation, a Schottky diode could
be used in the applications where input/output voltage
conditions can cause the internal diode to be turned on
(see Figure 2). As shown, the Schottky diode is
connected in parallel with the internal parasitic diode and
prevents it from being turned on by limiting the voltage
drop across it to about 0.3V @ 100mA to prevent damage
to the part.
IN
. This diode will also be turned on by abruptly stepping
VIN
Figure 1
Figure 2
RT9181
www.richtek.com
OUT
terminal to
VOUT
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