AP7331

300mA, LOW QUIESCENT CURRENT, FAST TRANSIENT

LOW DROPOUT LINEAR REGULATOR

AP7331

Document number: DS31914 Rev. 3 - 2

8 of 13

www.diodes.com

January 2011

© Diodes Incorporated

Application Note

Input Capacitor

A 1

μF ceramic capacitor is recommended to connect

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

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 AP7331 is stable with

very small ceramic output capacitors. The recommended

capacitance is from 1

μF to 4.7μF, Equivalent Series

Resistance

(ESR)

is

from

10m

Ω 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 AP7331 provides output voltage from 0.8V to 5.0V

through external resistor divider as shown below.

1uF

IN

GN D

EN

OU T

E nable

AD J

R2

R1

1uF

AP7331

The output voltage is calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

2

R

1

R

1

REF

V

OUT

V

Rearranging the equation will give the following that is

used for adjusting the output to a particular voltage:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

1

REF

V

OUT

V

2

R

1

R

To maintain the stability of the internal reference voltage,

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 AP7331 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

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

600mA 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

100mA. 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 AP7331 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.