RT9166/A
Application Information
Input-Output (Dropout) Voltage
Like any low-dropout regulator, the RT9166/A series
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.
A regulator's minimum input-to-output voltage differential
(dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this determines the
useful end-of-life battery voltage. Because the device uses
a PMOS, its dropout voltage is a function of drain-to-source
on-resistance, RDS(ON), multiplied by the load current :
VDROPOUT = VIN - VOUT = RDS(ON) x IOUT
Input Capacitor
Current Limit
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).Alower
ESR capacitor allows the use of less capacitance, while
higher ESR type (like aluminum electrolytic) require more
capacitance.
The RT9166/A monitors and controls the PMOS' gate
voltage, minimum limiting the output current to 300mA for
RT9166 and 600mA for RT9166A. The output can be
shorted to ground for an indefinite period of time without
damaging the part.
Short-Circuit Protection
The device is short circuit protected and in the event of a
peak over-current condition, the short-circuit control loop
will rapidly drive the output PMOS pass element off. Once
the power pass element shuts down, the control loop will
rapidly cycle the output on and off until the average power
dissipation causes the thermal shutdown circuit to respond
to servo the on/off cycling to a lower frequency. Please
refer to the section on thermal information for power
dissipation calculations.
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
Capacitor Characteristics
The RT9166/A is designed specifically to work with very
small ceramic output capacitors. The recommended
minimum capacitance (temperature characteristics X7R or
X5R) is 1μF to 4.7μF range with 10mΩ 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.
(When using the Y5V dielectric, the minimum value of the
input/output capacitance that can be used for stable over
full operating temperature range is 3.3μF.)
It is important to note that capacitance tolerance and
variation with temperature must be taken into consideration
when selecting a capacitor so that the minimum required
amount of capacitance is provided over the full operating
temperature range. In general, a good tantalum capacitor
will show very little capacitance variation with temperature,
but a ceramic may not be as good (depending on dielectric
type).
Aluminum electrolytics also typically have large
temperature variation of capacitance value.
Equally important to consider is a capacitor's ESR change
with temperature: this is not an issue with ceramics, as
their ESR is extremely low. However, it is very important in
Tantalum and aluminum electrolytic capacitors. Both show
increasing ESR at colder temperatures, but the increase
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.
www.richtek.com
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DS9166/A-18 June 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
