RT9167/A
Application Information
Capacitor Selection and Regulator Stability
Like any low-dropout regulator, the external capacitors used
with the RT9167/A must be carefully selected for regulator
stability and performance.
Using a capacitor whose value is > 1μF on the RT9167/A
input and the amount of capacitance can be increased
without limit. The input capacitor must be located a
distance of not more than 0.5" from the input pin of the IC
and returned to a clean analog ground. 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 LDOs
application. The RT9167/A 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
RT9167/A output ensures stability. The RT9167/A still
works well with output capacitor of other types due to the
wide stable ESR range. Figure 1. shows the curves of
allowable ESR range as a function of load current for various
output voltages and capacitor values. 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
OUT
pin of the RT9167/A 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 manufacture.
Use a 10nF bypass capacitor at BP for low output voltage
noise. The capacitor, in conjunction with an internal 200kΩ
resistor, which connects bypass pin and the band-gap
reference, creates an 80Hz low-pass filter for noise
reduction. Increasing the capacitance will slightly decrease
the output noise, but increase the start-up time. The
capacitor connected to the bypass pin for noise reduction
must have very low leakage. This capacitor leakage current
causes the output voltage to decline by a proportional
amount to the current due to the voltage drop on the internal
200kΩ resistor. Figure 2 shows the power on response.
Region of Stable C
OUT
ESR vs. Load Current
100.00
100
Unstable Region
10.00
10
C
OUT
= 1uF
C
BP
=
= 1nF
CBP
10nF
Stable Region
Voltage (0.5V/Div)
Voltage (0.5V / DIV)
C
OUT
ESR (Ω)
C
BP
=
= 10nF
BP
10nF
1.00
1
0.1
0.10
Unstable Region
0.01
0.01
0.001
0.00
0
50
100
150
200
250
300
V
V
=
=3.0V
3.0V
OUT
OUT
0
0
Load Current (mA)
5.0
10.0
15.0
15.0
Figure 1
Time (ms)
Time (ms)
Figure 2
DS9167/A-26 March 2007
www.richtek.com
8
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
