RT9183
Application Information
No Load Stability
Like any low-dropout regulator, the RT9183 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.
The device will remain stable and in regulation with no
external load. This is specially important in CMOS RAM
keep-alive applications.
Input-Output (Dropout) Voltage
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:
Input Capacitor
An input capacitance of ≅10μF 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.
VDROPOUT = VIN - VOUT = RDS(ON) × IOUT
Current Limit
The RT9183 monitors and controls the PMOS' gate
voltage, minimum limiting the output current to 2A . The
output can be shorted to ground for an indefinite period of
time without damaging the part.
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.
Short-Circuit Protection
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≅10μF over the entire operating
temperature range.
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.
Output Capacitor
The RT9183 is designed specifically to work with very
small ceramic output capacitors. The recommended
minimum capacitance (temperature characteristics X7R or
X5R) are 10μF to 47μF range with 10mΩ to 25mΩ range
ceramic capacitors between each LDO output andGNDfor
transient stability, but it may be increased without limit.
Higher capacitance values help to improve transient.
Capaacitor Characteristics
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.
The output capacitor's ESR is critical because it forms a
zero to provide phase lead which is required for loop
stability.
DS9183-12 March 2007
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