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DS9166B-01 April 2011

www.richtek.com

RT9166B

Application Information

Like any linear regulator, the RT9166B requires input and

output decoupling capacitors. These capacitors must be

correctly selected for good performance Please note that

linear regulators have high internal loop gains which

require care in guarding against oscillation caused by

insufficient decoupling capacitance.

Input Capacitor

An input capacitance of 1

μF is required between the device

input pin and ground directly (the capacitance may be

increased without limit). The input capacitor must be

located less than 1cm from the device to assure input

stability A lower ESR capacitor allows the use of less

capacitance, while higher ESR type (like aluminum

electrolytic) requires 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 for 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 1

μF

capacitance over the entire operating temperature range.

Output Capacitor

The RT9166B is designed specifically to work with very

small ceramic output capacitors. The recommended

minimum capacitance (temperature characteristics X7R

or X5R) is 3.3

μF with 10mΩ to 50mΩ range ceramic

capacitor between LDO output and GND for transient

stability.

Higher capacitance helps to improve the transient

response. The output capacitor's ESR is critical because

it forms a zero to provide phase lead which is required for

loop stability.

Input-Output (Dropout) Voltage

A regulator's minimum input-to-output differential voltage

(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-

current :

Current Limit

The RT9166B monitors and controls the PMOS gate

voltage, with a minimum limit of the output current at

600mA. 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 in the event of a peak

over-current condition, such that the short-circuit control

loop rapidly drives 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 by cycling to a lower frequency. Please refer to

the section on thermal information for power dissipation

calculations.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

is the ambient temperature, and

ambient thermal resistance.

For recommended operating condition specifications of

RT9166B, the maximum junction temperature is 125

°C

thermal resistance,

packages, the thermal resistance,

standard JEDEC 51-7 four-layer thermal test board. The

by the following formula :