AOZ1038DI-1 データシートの表示（PDF） - Alpha and Omega Semiconductor
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AOZ1038DI-1 Datasheet PDF : 13 Pages
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AOZ1038DI-1
Compared with regulators using freewheeling Schottky
diodes, the AOZ1038DI-1 uses a freewheeling
NMOSFET to realize synchronous rectification. This
greatly improves the converter efficiency and reduces
power loss in the low-side switch.
The AOZ1038DI-1 uses a P-Channel MOSFET as the
high-side switch. This saves the bootstrap capacitor
normally seen in a circuit using an NMOS switch.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin by using a resistor divider network as shown
in Figure 1. The resistor divider network includes R1 and
R2. Usually, a design is started by picking a fixed R2
value and calculating the required R1 with equation
below:
VO
=
0.8
×
⎛
⎜1
+
R-----1-⎟⎞
⎝ R2⎠
AOZ1038DI-1 uses asymmetric Rdson of the high-side
PMOS and low-side NMOS to optimize high input and
the low output application.
Protection Features
The AOZ1038DI-1 has multiple protection features to
prevent system circuit damage under abnormal
conditions.
Over Voltage Protection (OVP)
The AOZ1038DI-1 has two over voltage protection
functions. First, once FB voltage is over 960 mV, the
AOZ1038DI-1 turns off both the low-side and the
high-side MOSFETs to prevent either further output
overshoot or excessive negative current.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection. Since the AOZ1038DI-1 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 0.4 V and 2.5 V
internally. The peak inductor current is automatically
limited cycle by cycle.
When the output is shorted to ground under fault
conditions, the inductor current decays very slowly during
a switching cycle because the output voltage is 0 V.
To prevent catastrophic failure, a secondary current limit
is designed inside the AOZ1038DI-1. The measured
inductor current is compared against a preset voltage
which represents the current limit. When the output
current is more than current limit, the high side switch will
be turned off. The converter will initiate a soft start once
the over-current condition is resolved.
Under Voltage Lockout (UVLO)
A power-on reset circuit monitors the input voltage. When
the input voltage exceeds 4.1 V, the converter starts
operation. When input voltage falls below 3.7 V, the
converter will be shut down.
Thermal Protection
An internal temperature sensor monitors the junction
temperature. It shuts down the internal control circuit
and high side PMOS if the junction temperature exceeds
150 ºC. The regulator will restart automatically, under the
control of soft-start circuit, when the junction temperature
decreases to 100 ºC.
Application Information
The basic AOZ1038DI-1 application circuit is show in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the VIN pin
and the PGND pin of the AOZ1038DI-1 to maintain
steady input voltage and filter out the pulsing input
current. The voltage rating of the input capacitor must be
greater than maximum input voltage plus ripple voltage.
The input ripple voltage can be approximated by
equation below:
ΔVIN
=
-------I-O---------
×
⎛
⎜1
–
-V----O---
⎞
⎟
f × CIN ⎝ VIN⎠
× -V----O---
VIN
Since the input current is discontinuous in a buck
converter, the current stress on the input capacitor is
another concern when selecting the capacitor. For a buck
circuit, the RMS value of the input capacitor current can
be calculated by:
ICIN_RMS = IO ×
-V----O---
⎛
⎜1
–
-V----O---
⎞
⎟
VIN⎝ VIN⎠
if we let m equal the conversion ratio:
-V----O--- = m
VIN
Rev. 1.0 September 2011
www.aosmd.com
Page 7 of 13
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