aoz1905 Alpha & Omega Semiconductor, aoz1905 Datasheet - Page 8

• Manufacturer Part Number: aoz1905
• Description: Ezboost 2a General Purpose Regulator
• Manufacturer: Alpha & Omega Semiconductor

The cycle-by-cycle current limit protection directly limits
inductor peak current. The average inductor current is
also limited due to the limitation on peak inductor current.
When cycle by cycle current limit circuit is triggered, the
output voltage drops as the duty cycle decreasing.
Power-On Reset (POR)
A power-on reset circuit monitors the input voltage. When
the input voltage exceeds 2.6V, the converter starts
operation. When input voltage falls below 2.2V, the
converter will stop switching.
Thermal Protection
An internal temperature sensor monitors the junction
temperature. It shuts down the internal control circuit and
NMOS switch if the junction temperature exceeds 145°C.
Application Information
The basic AOZ1905 application circuit is shown in
Figure 1. Component selection is explained below.
Input capacitor
The input capacitor (C
the V
steady input voltage. The voltage rating of input capacitor
must be greater than maximum input voltage + ripple
voltage. The RMS current rating should be greater than
the inductor ripple current:
The input capacitor value should be greater than 4.7µF
for normal operation. The capacitor can be electrolytic,
tantalum or ceramic. The input capacitor should be
placed as close as possible to the IC; if not possible, put
a 0.1µF decoupling ceramics capacitor between IN pin
and GND.
Inductor
The inductor is used to supply higher output voltage
when the NMOS switch is off. For given input and output
voltage, inductance and switching frequency together
decide the inductor ripple current, which is,
The peak inductor current is:
I
Lpeak
I
I
Rev. 1.4 May 2008
L
L
IN
=
=
pin and GND pin of the AOZ1905 to maintain
----------- -
f
----------- -
f
V
V
=
IN
IN
I
L
L
IN
+
1
1
-------- -
2
–
–
I
L
V
--------- -
V
--------- -
V
V
1
IN
IN
O
O
in Figure 1) must be connected to
www.aosmd.com
High inductance gives low inductor ripple current but
requires larger size inductor to avoid saturation. Low
ripple current reduces inductor core losses. It also
reduces RMS current through inductor, switch and
freewheeling diode, which results in less conduction loss.
Usually, peak to peak ripple current on the inductor is
designed to be 30% to 50% of input current.
When selecting the inductor, make sure it is able to
handle the peak current without saturation even at the
highest operating temperature.
The inductor takes the highest current in a boost circuit.
The conduction loss on inductor needs to be checked for
thermal and efficiency requirements.
Surface mount inductors in different shape and styles are
available from Coilcraft, Elytone and Murata. Shielded
inductors are small and radiate less EMI noise. But they
cost more than unshielded inductors. The choice
depends on EMI requirement, price and size.
Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be consid-
ered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a boost
converter circuit, output ripple voltage is determined by
load current, input voltage, output voltage, switching
frequency, output capacitor value and ESR. It can be
calculated by the equation below:
where;
I
C
ESR
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the switching
frequency dominates. Output ripple is mainly caused by
capacitor value and load current with the fixed frequency,
LOAD
O
V
is the output capacitor value, and
O
CO
is the load current,
=
is the Equivalent Series Resistor of output capacitor.
I
LOAD
--------- -
V
V
IN
O
ES R
CO
+
------------------------------ -
1
f
–
AOZ1905
--------------- -
V
Page 8 of 16
V
C
OUT
IN
O