aoz1905 Alpha & Omega Semiconductor, aoz1905 Datasheet - Page 7

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

Applications Information
The AOZ1905 is a current-mode step up regulator (Boost
Converter) with integrated NMOS switch. It operates from
a 2.7V to 5.5V input voltage range and supplies up to
24V output voltage. The duty cycle can be adjusted to
obtain a wide range of output voltage up to 24V. Features
include enable control, cycle by cycle current limit, input
under voltage lockout, adjustable soft-start and thermal
shut down.
The AOZ1905 is available in MSOP-8 and DFN-10 3x3
packages.
Enable and Soft Start
The AOZ1905 has the adjustable soft start feature to limit
in-rush current and ensure the output voltage ramps up
smoothly to regulation voltage. A soft start process
begins when the input voltage rises to 2.6V and voltage
on EN pin is HIGH. In soft start process, a 10µA internal
current source charges the external capacitor at SS. As
the SS capacitor is charged, the voltage at SS rises. The
SS voltage clamps the reference voltage of the error
amplifier, therefore output voltage rising time follows the
SS pin voltage. With the slow ramping up output voltage,
the inrush current can be prevented.
The EN pin of the AOZ1905 is active high. Connect the
EN pin to V
ground will disable the AOZ1905. Do not leave it open.
The voltage on EN pin must be above 1.5 V to enable the
AOZ1905. When voltage on EN pin falls below 0.4V, the
AOZ1905 is disabled. If an application circuit requires the
AOZ1905 to be disabled, an open drain or open collector
circuit should be used to interface to EN pin.
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency.
The AOZ1905 integrates an internal N-MOSFET as the
control switch. Inductor current is sensed by amplifying
the voltage drop across the drain to source of the control
power MOSFET. Output voltage is divided down by the
external voltage divider at the FB pin. The difference of
the FB pin voltage and reference is amplified by the
internal transconductance error amplifier. The error
voltage, which shows on the COMP pin, is compared
against the current signal, which is sum of inductor
current signal and ramp compensation signal, at PWM
comparator input. If the current signal is less than the
error voltage, the internal NMOS switch is on. The induc-
tor current ramps up. When the current signal exceeds
the error voltage, the switch is off. The inductor current is
freewheeling through the Schottky diode to output.
Rev. 1.4 May 2008
IN
if enable function is not used. Pulling EN to
www.aosmd.com
Switching Frequency
The AOZ1905 switching frequency is fixed and set by
an internal oscillator and FSEL. When the voltage of
FSEL is high (connected to V
is 1.2MHz; when the voltage of FSEL is low (connected
to GND), the switching frequency is 600kHz.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin with a resistor divider network. In the
application circuit shown in Figure 1. The resistor divider
network includes R
by picking a fixed R
R
Some standard value of R
output voltage values are listed in Table 1.
Table 1.
The combination of R
avoid drawing excessive current from the output, which
will cause power loss.
Protection Features
The AOZ1905 has multiple protection features to prevent
system circuit damage under abnormal conditions.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection. Since the AOZ1905 employs peak
current mode control, the COMP pin voltage is propor-
tional to the peak inductor current. The peak inductor
current is automatically limited cycle by cycle.
The cycle by cycle current limit threshold is set between
2A and 3A. When the current of control NMOS reaches
the current limit threshold, the cycle by cycle current limit
circuit turns off the NMOS immediately to terminate the
current duty cycle. The inductor current stop rising.
V
2
O
with equation below.
=
V
O
1.2
12
16
18
25
8
(V)
1
+
R
------ -
R
1
1
and R
value and calculating the required
2
1
1
and R
R
1
2
2
170
270
370
420
595
, R
. Usually, a design is started
(k )
2
IN
2
should be large enough to
) The switching frequency
for most commonly used
AOZ1905
R
Page 7 of 16
1
30
30
30
30
30
(k )