aoz1240 Alpha & Omega Semiconductor, aoz1240 Datasheet - Page 7

• Manufacturer Part Number: aoz1240
• Description: Ezbucktm 2a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
The AOZ1240 is a current-mode step down regulator with
integrated high side NMOS switch. It operates from a 4.5V
to 32V input voltage range and supplies up to 2A of load
current. The duty cycle can be adjusted from 6% to 85%
allowing a wide range of output voltage. Features include
enable control, Power-On Reset, input under voltage
lockout, fixed internal soft-start and thermal shut down.
The AOZ1240 is available in an SO-8 package.
Enable and Soft Start
The AOZ1240 has internal 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 4.1V and voltage
on EN pin is HIGH. In soft start process, the output
voltage is ramped to regulation voltage in typically 6.8ms.
The 6.8ms soft start time is set internally.
Connect the EN pin to V
Pull it to ground will disable the AOZ1240. Do not leave it
open. The voltage on EN pin must be above 2.5 V to
enable the AOZ1240. When voltage on EN pin falls
below 0.6V, the AOZ1240 is disabled. If an application
circuit requires the AOZ1240 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 and Continuous-Conduction Mode
(CCM).
The AOZ1240 integrates an internal N-MOSFET as the
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to source of the high
side power MOSFET. Since the N-MOSFET requires a
gate voltage higher than the input voltage, a boost
capacitor connected between LX pin and BST pin drives
the gate. The boost capacitor is charged while LX is low.
An internal 10
that LX is pulled to GND even in the light load. 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 high-side switch
is on. The inductor current flows from the input through
the inductor to the output. When the current signal
exceeds the error voltage, the high-side switch is off. The
inductor current is freewheeling through the Schottky
diode to output.
Rev. 1.3 September 2010
Ω
switch from LX to GND is used to insure
IN
if enable function is not used.
www.aosmd.com
Switching Frequency
The AOZ1240 switching frequency is fixed and set by an
internal oscillator. The switching frequency is set 370khz.
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
R2 with equation below.
Some standard values for R
commonly used output voltages 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 AOZ1240 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 AOZ1240 employs
peak current mode control, the COMP pin voltage is
proportional to the peak inductor current. The COMP pin
voltage is limited to be between 0.4V and 2.5V internally.
The peak inductor current is automatically limited
cycle-by-cycle.
The cycle-by-cycle current limit threshold is internally
set. When the load current reaches the current limit
threshold, the cycle by cycle current limit circuit turns
off the high side switch immediately to terminate the
current duty cycle. The inductor current stop rising.
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.
V
O
=
V
0.8
O
0.8
1.2
1.5
1.8
2.5
3.3
5.0
(V)
×
⎛
⎜
⎝
1
+
R
------ -
R
1
2
and R
1
2
value and calculating the required
1
⎞
⎟
⎠
and R
R
1
4.99
12.7
21.5
31.6
52.3
2
1.0
10
. Usually, a design is started
(k
1
2
and R
Ω
should be large enough to
)
2
for the most
AOZ1240
R
Page 7 of 15
Open
2
11.5
10.2
10
10
10
10
(k
Ω
)