Diodes AP7311 Manuel d'utilisateur télécharger pdf (Page 10)

AP7311

150mA, Low Quiescent Current, Fast Transient

Low Dropout Linear Regulator

AP7311 Rev. 2 10 of 13 NOVEMBER 2009

www.diodes.com © Diodes Incorporated

Application Note

Input Capacitor

A 1μF ceramic capacitor is recommended to connect between

and GND pins to decouple input power supply glitch and

noise. The amount of the capacitance may be increased

without limit. This input capacitor must be located as close as

possible to the device to assure input stability and less noise.

For PCB layout, a wide copper trace is required for both V

and

GND. A lower ESR capacitor allows the use of less

capacitance, while higher ESR type requires more

capacitance.

Output Capacitor

The output capacitor is required to stabilize and help transient

response for LDO. The AP7311 is stable with very small

ceramic output capacitors. The recommended capacitance is

from 1μF to 4.7μF, Equivalent Series Resistance (ESR) is from

10mΩ to 200mΩ, and temperature characteristic is X7R or X5R.

Higher capacitance values help to improve load/line transient

response. The output capacitance may be increased to keep

low undershoot/overshoot. Place output capacitor as close as

possible to OUT and GND pins, and keep the leads as short as

possible.

Adjustable Operation

The AP7311 provides output voltage from 0.8V to 5.0V through

external resistor divider as shown below.

1uF

GND

OUT

Enable

ADJ

1uF

OUT

AP7331

The output voltage is calculated by:

⎟

⎠

⎞

⎜

⎝

⎛

REFOUT

1VV

Where V

REF

=0.4V (the internal reference voltage)

Rearranging the equation will give the following that is used for

adjusting the output to a particular voltage:

⎟

⎠

⎞

⎜

⎝

⎛

−= 1

REF

OUT

To maintain the stability of the internal reference voltage, R

need to be kept smaller than 125kΩ.

No Load Stability

Other than external resistor divider, no minimum load is

required to keep the device stable. The device will remain

stable and regulated in no load condition.

ON/OFF Input Operation

The AP7311 is turned on by setting the EN pin high, and is

turned off by pulling it low. If this feature is not used, the EN pin

should be tied to IN pin to keep the regulator output on at all

time. To ensure proper operation, the signal source used to

drive the EN pin must be able to swing above and below the

specified turn-on/off voltage thresholds listed in the Electrical

Characteristics section under V

and V

Current Limit Protection

When output current at OUT pin is higher than current limit

threshold, the current limit protection will be triggered and

clamp the output current to approximately 300mA to prevent

over-current and to protect the regulator from damage due to

overheating.

Short Circuit Protection

When OUT pin is short-circuit to GND or OUT pin voltage is

less than 200mV, short circuit protection will be triggered and

clamp the output current to approximately 50mA. This feature

protects the regulator from over-current and damage due to

overheating.

Thermal Shutdown Protection

Thermal protection disables the output when the junction

temperature rises to approximately +140°C, allowing the device

to cool down. When the junction temperature reduces to

approximately +125°C the output circuitry is enabled again.

Depending on power dissipation, thermal resistance, and

ambient temperature, the thermal protection circuit may cycle

on and off. This cycling limits the heat dissipation of the

regulator, protecting it from damage due to overheating.

Ultra Fast Start-up

After enabled, the AP7311 is able to provide full power in as

little as tens of microseconds, typically 80µs, without sacrificing

low ground current. This feature will help load circuitry move in

and out of standby mode in real time, eventually extend battery

life for mobile phones and other portable devices.

Fast Transient Response

Fast transient response LDOs can extend battery life.

TDMA-based cell phone protocols such as Global System for

Mobile Communications (GSM) have a transmit/receive duty

factor of only 12.5 percent, enabling power savings by putting

much of the baseband circuitry into standby mode in between

transmit cycles. In baseband circuits, the load often transitions

virtually instantaneously from 100µA to 100mA. To meet this

load requirement, the LDO must react very quickly without a

large voltage drop or overshoot — a requirement that cannot

be met with conventional, general-purpose LDOs.

The AP7311’s fast transient response from 0 to 150mA

provides stable voltage supply for fast DSP and GSM chipset

with fast changing load.

Small Overshoot and Undershoot

The AP7311 has small and controlled overshoot and

undershoot in load and line transitions. This helps to protect

supplied circuit from damage and operation error caused by

glitches. This feature also permits the usage of small value

output decoupling capacitor with AP7311.

Low Quiescent Current

The AP7311, consuming only around 65µA for all input range

and output loading, provides great power saving in portable

and low power applications.

Wide Output Range

The AP7311, with a wide output range of 0.8V to 5.0V,

provides a versatile LDO solution for many portable

applications.

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Diodes AP7311 Manuel d'utilisateur Page 10

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