Ap7331 – Diodes AP7331 User Manual
Page 8
AP7331
300mA, LOW QUIESCENT CURRENT, FAST TRANSIENT
LOW DROPOUT LINEAR REGULATOR
AP7331
Document number: DS31914 Rev. 3 - 2
8 of 13
January 2011
© Diodes Incorporated
Application Note
Input Capacitor
A 1
μF ceramic capacitor is recommended to connect
between V
IN
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
IN
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 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
G N D
E N
O U T
E nable
A D J
R 2
R 1
1uF
V
IN
V
O U T
A P 7331
The output voltage is calculated by:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
=
2
R
1
R
1
REF
V
OUT
V
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
V
OUT
V
2
R
1
R
To maintain the stability of the internal reference voltage,
R
2
need to be kept smaller than 125k
Ω.
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
Characteristics section under V
IL
and V
IH
.
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.