RT9178
Application Information
The RT9178 is ideal for mobile phone and similar battery-
powered wireless applications. It provides up to 200mA,
from a 2.5V to 6V input. Like any low-dropout regulator,
C
OUT
ESR (Ω)
Region of Stable C
OUT
ESR vs. Load Current
100.00
100
Unstable
10
10.00
the device requires input and output decoupling capacitors.
These capacitors must be correctly selected for good
performance (see Capacitor Characteristics Section).
Please note that linear regulators with a low dropout
voltage have high internal loop gains which require care in
guarding against oscillation caused by insufficient
decoupling capacitance.
Input Capacitor
An input capacitance of
≅1μF
is required between the
device input pin and ground directly (the amount of the
capacitance may be increased without limit). The input
capacitor
MUST
be located less than 1 cm from the device
to assure input stability (see PCB Layout Section). A lower
ESR capacitor allows the use of less capacitance, while
higher ESR type (like aluminum electrolytic) require more
capacitance.
Capacitor types (aluminum, ceramic and tantalum) can
be mixed in parallel, but the total equivalent input
capacitance/ESR must be defined as above to stable
operation.
There are no requirements for the ESR on the input
capacitor, but tolerance and temperature coefficient must
be considered when selecting the capacitor to ensure the
capacitance will be
≅
1μF over the entire operating
temperature range.
Output Capacitor
The RT9178 is designed specifically to work with very
small ceramic output capacitors. The recommended
minimum capacitance (temperature characteristics
X7R, X5R, Z5U or Y5V) is 1μF to 10μF range with 5mΩ to
50mΩ range ceramic capacitor between LDO output and
GND for transient stability, but it may be increased without
limit. Higher capacitance values help to improve transient.
The output capacitor's ESR is critical because it forms a
zero to provide phase lead which is required for loop
stability.
1.00
Stable
0.10
0.01
Unstable
0.00
0
40
80
120
160
200
Load Current (mA)
Reference Bypass Capacitor (BP)
Connecting a 10nF between the BP (reference bypass)
pin and GND significantly reduces noise on the regulator
output. It should be noted that the capacitor is connected
directly to a high impedance circuit in the band gap
reference. Because this circuit has only a few micro-
amperes flowing into it, any significantly loading on this
node will cause a change on the regulated output voltage.
For this reason, DC leakage current through the noise
bypass capacitor must never exceed 100nA, and should
be kept as low as possible for best output voltage
accuracy. The type of capacitors best suited for the noise
bypass capacitor with either NP0 or C0G dielectric
typically have very low leakage. 10nF polypropylene and
polycarbonate film capacitors are available in small
surface mount packages and typically have extremely low
leakage current.
No Load Stability
The device will remain stable and in regulation with no
external load. This is specially important in CMOS RAM
keep-alive applications
Shutdown Input Operation
The RT9178 is shutdown by pulling the EN pin low, and
turned on by driving the input high. If the shutdown feature
is not required, the EN pin should be tied to VIN to keep
the regulator on at all times (the EN pin
MUST NOT
be
left floating).
www.richtek.com
7
DS9178-16 April 2008
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
