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參數(shù)資料
| 型號： | RT9184A |
| 英文描述： | Fast Transient Response |
| 中文描述： | 快速瞬態(tài)響應(yīng) |
| 文件頁數(shù)： | 7/10頁 |
| 文件大?。?/td> | 221K |
| 代理商： | RT9184A |
Preliminary
RT9182
DS9182-05 February 2003
www.richtek.com
7
Applications Information
Capacitor Selection and Regulator Stability
Like any low-dropout regulator, the external
capacitors used with the RT9182 must be carefully
selected for regulator stability and performance.
Using a capacitor whose value is > 1
μ
F on the
RT9182 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
LDO applications (see Fig.1). The RT9182 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 1
μ
F with ESR is > 5m
on the
RT9182 output ensures stability. The RT9182 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
OUT
pin of the RT9182 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 2.2
μ
F or more to ensure
stability at temperatures below -10°C in this case.
Also, tantalum capacitors, 2.2
μ
F 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
manufacture.
sufficient
for
the
application
by
the
Load-Transient Considerations
The RT9182 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 of the output capacitor.
Increasing
the
output
decreasing the ESR attenuates the overshoot.
capacitor’s
value
and
Input-Output (Dropout) Voltage
A
differential (or dropout voltage) determines the lowest
usable supply voltage. In battery-powered systems,
this will determine the useful end-of-life battery
voltage. Because the RT9182 uses a P-channel
MOSFET pass transistor, the dropout voltage is a
function of drain-to-source on-resistance [R
DS(ON)
]
multiplied by the load current.
regulator’s
minimum
input-output
voltage
Fig. 1
C
OUT
= 4.7
μ
F
C
OUT
= 1
μ
F
C
O
Region of Stable C
OUT
ESR vs. Load Current
100
Load Current (mA)
0 40 80 120 160 200
10
1
0.1
0.01
0.001
(
)
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