aoz1038di Alpha & Omega Semiconductor, aoz1038di Datasheet - Page 5

• Manufacturer Part Number: aoz1038di
• Description: Ezbucktm 6 A Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
The AOZ1038 is a current-mode step down regulator
with an integrated high-side PMOS switch and a low-side
NMOS switch. It operates from a 4.5 V to 18 V input
voltage range and supplies up to 6 A of load current.
The duty cycle can be adjusted from 6 % to 100 %
allowing a wide range of output voltages. Features
include enable control, power-on reset, input under
voltage lockout, output over voltage protection, active
high power good state, fixed internal soft-start, and
thermal shut down.
The AOZ1038 is available in a 5x6 DFN-8 package or
an exposed pad SO-8 package.
Enable and Soft Start
The AOZ1038 has an internal soft start feature to limit
in-rush current and ensure the output voltage smoothly
ramps up to regulation voltage. The soft start process
begins when the input voltage rises to 4.1 V and the
voltage on the EN pin is HIGH. In the soft start process,
output voltage is typically ramped to regulation voltage in
4 ms. The 4 ms soft start time is set internally.
The EN pin of the AOZ1038 is active high. Connect the
EN pin to VIN if the enable function is not used. Pulling
EN to ground will disable the AOZ1038. Do not leave the
EN pin open. The voltage on EN must be above 2 V to
enable the AOZ1038. When voltage on the EN pin falls
below 0.6 V, the AOZ1038 is disabled. If an application
circuit requires the AOZ1038 to be disabled, an open
drain or open collector circuit should be used to interface
the EN pin.
Steady-State Operation
Under steady-state conditions, the converter operates
in fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1038 integrates an internal P-MOSFET as the
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to the source of the
high side power MOSFET. Output voltage is divided
down by the external voltage divider at the FB pin. The
difference between the FB pin voltage and reference
voltage is amplified by the internal transconductance
error amplifier. The error voltage, which shows on the
COMP pin, is compared against the current signal. The
current signal is the sum of the inductor current signal
and ramp compensation signal, at the PWM comparator
input. If the current signal is less than the error voltage,
the internal high-side switch is on. When on, the inductor
current flows from the input through the inductor to the
Rev. 1.2 February 2011
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output. When the current signal exceeds the error
voltage, the high-side switch is off. When off, inductor
current is freewheeling through the internal low-side
N-MOSFET switch to output. The internal adaptive FET
driver guarantees no turn on overlap of the high-side and
low-side switches.
Compared to regulators using freewheeling Schottky
diodes, the AOZ1038 uses freewheeling NMOSFET to
realize synchronous rectification. This greatly improves
the converter efficiency and reduces power loss in the
low-side switch.
The AOZ1038 uses a P-Channel MOSFET as the
high-side switch. This eliminates the bootstrap capacitor
normally seen in a circuit using an NMOS switch. It
allows 100 % turn-on of the high-side switch to achieve a
linear regulation mode of operation. The minimum
voltage drop from V
DC resistance of the MOSFET plus DC resistance of
buck inductor. It can be calculated by equation below:
where;
V
V
I
R
between 55 mΩ and 75 mΩ depending on input voltage and
junction temperature.
Switching Frequency
The AOZ1038 switching frequency is fixed and set by an
internal oscillator. The practical switching frequency
could range from 400 kHz to 500 kHz due to device
variation.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin by using a resistor divider network. Refer to
the application circuit shown in Figure 1. The resistor
divider network includes R
started by picking a fixed R
required R
Values of R
listed in Table 1 on the next page.
V
V
O
O_MAX
IN
DS(ON)
O
O_MAX
is the output current from 0 A to 6 A, and
is the input voltage from 4.5 V to 18 V,
=
0.8
is the maximum output voltage,
is the on resistance of internal MOSFET. The value is
1
=
×
1
with equation below.
and R
V
⎛
⎜
⎝
1
IN
+
–
R
------ -
R
2
IN
I
O
1
2
with standard output voltages are
⎞
⎟
⎠
to V
×
R
O
1
DS ON
2
and R
is the load current times
value and calculating the
(
)
2
. Usually, a design is
AOZ1038
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