RT9001
6
DS9001-06 April 2011www.richtek.com
Application Information
Like any low-dropout regulator, the RT9001 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.
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. 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 RT9001 is designed specifically to work with very small
ceramic output capacitors. The recommended minimum
capacitance (temperature characteristics X7R or X5R) is
from 1μF to 4.7μF 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.)
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.
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, R
DS(ON)
, multiplied by the load current :
V
DROPOUT
= V
IN
− V
OUT
= R
DS(ON)
x I
OUT
Current Limit
The RT9001 monitors output current and controls the
PMOS' gate voltage to limit the output current to 600mA
(MIN). 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 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).
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.
Aluminum electrolytics also typically have large
temperature variation of capacitance value.
