RT9185 RICHTEK [Richtek Technology Corporation], RT9185 Datasheet - Page 8

• Manufacturer Part Number: RT9185
• Description: Triple, Ultra-Fast CMOS LDO Regulator
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9185
Application Information
Like any low-dropout regulator, the RT9185 requires input
and output decoupling capacitors. The device is specifically
designed for portable applications requiring minimum board
space and smallest components. 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 ≅2.2μF 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 ≅2.2μF over the entire operating
temperature range.
Output Capacitor
The RT9185 is designed specifically to work with very small
ceramic output capacitors. The recommended minimum
capacitance (temperature characteristics X7R, X5R, Z5U,
or Y5V) are 2.2μF to 4.7μF range with 10mΩ to 50mΩ
range ceramic capacitors between each 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.
www.richtek.com
8
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
V
Current Limit
The RT9185 monitors and controls the PMOS’ gate
voltage, limiting the output current to 1.9A, 1.3A and 0.5A
(typ.) respectively. The outputs 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). Aluminum electrolytics also typically have large
temperature variation of capacitance value.
DROPOUT
= V
DD
DS(ON)
−
V
OUT
, multiplied by the load current:
= R
DS(ON)
DS9185-11 March 2007
x I
OUT