Diodes AP133 - Manual (Page 11)

AP133
300mA, LOW QUIESCENT CURRENT, FAST TRANSIENT
LOW DROPOUT LINEAR REGULATOR
AP133 Rev. 6 11 of 15 FEBRUARY 2009
www.diodes.com © Diodes Incorporated
Application Note
Input Capacitor
A1 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 require more capacitance.
Output Capacitor
The output capacitor is required to stabilize and help transient
response for LDO. The AP133 is stable with very small ceramic
output capacitors. The recommended capacitance is from 1
μF
to 4.7 μF, Equivalent Series Resistance (ESR) is from 10mΩ to
100m
Ω, and temperature characteristics 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 AP133 provides output voltage from 1.0V to 5.0V through
external resistor divider as shown below.
C
IN
IN
G N
D
EN
OUT
Enable
AD J
R2
R1
V
IN V
OUT
AP133
C O U T
The output voltage is calculated by:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+ =
2
1 1
R
R
V V
REF OUT
Where V
REF
=0.8V (the internal reference voltage)
Rearranging the equation will give the following that is used for
adjusting the output to a particular voltage:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
− =
1
2 1
REF
OUT
V
V
R R
To maintain the stability of the internal reference voltage, R
2
needs to be kept smaller than 250k 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 AP133 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 120mA. 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 +145°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 AP133 is able to provide full power in as little
as tens of microseconds, typically 25µs, 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 also 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 AP133’s fast transient response from 0 to 300mA provides
stable voltage supply for fast DSP and GSM chipset with fast
changing load.
Small Overshoot and Undershoot
The AP133 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 AP133.