AOZ1017DI Datasheet, PDF (8/16 Pages) Alpha & Omega Semiconductors – EZBuck™ 3A Simple Regulator

AOZ1017D
The combination of R₁ and R₂ should be large enough to
avoid drawing excessive current from the output, which
will cause power loss.
Since the switch duty cycle can be as high as 100%, the
maximum output voltage can be set as high as the input
voltage minus the voltage drop on upper PMOS and
inductor.
Protection Features
The AOZ1017D has multiple protection features to pre-
vent system circuit damage under abnormal conditions.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection. Since the AOZ1017D employs peak
current mode control, the COMP pin voltage is propor-
tional to the peak inductor current. The COMP pin volt-
age 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 set between
4A and 5A. 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.
When cycle by cycle current limit circuit is triggered, the
output voltage drops as the duty cycle is decreasing.
The AOZ1017D has internal short circuit protection to
protect itself from catastrophic failure under output short
circuit conditions. The FB pin voltage is proportional to
the output voltage. Whenever FB pin voltage is below
0.2V, the short circuit protection circuit is triggered.
As a result, the converter is shut down and hiccups at a
frequency equals to 1/8 of normal switching frequency.
The converter will start up via a soft start once the short
circuit condition is resolved. In short circuit protection
mode, the inductor average current is greatly reduced
because of the low hiccup frequency.
Power-On Reset (POR)
A power-on reset circuit monitors the input voltage. When
the input voltage exceeds 4V, the converter starts opera-
tion. When input voltage falls below 3.7V, the converter
will be shut down.
Output Over Voltage Protection (OVP)
The AOZ1017D monitors the feedback voltage: when the
feedback voltage is higher than 960mV, it immediately
turns off the PMOS to protect the output voltage
overshoot at fault condition. When feedback voltage is
lower than 840mV, the PMOS is allowed to turn on in
the next cycle.
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°C.
Application Information
The basic AOZ1017D application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the V_IN pin
and PGND pin of the AOZ1017D to maintain steady input
voltage and filter out the pulsing input current. The
voltage rating of the input capacitor must be greater than
the maximum input voltage plus the ripple voltage.
The input ripple voltage can be approximated by the
following equation:
ΔV_IN
=
-_f---_×---^I-^O_C-----_I--_N--
×
⎛
⎜
1
⎝
–
V^-V----_IO^-_N^--_{⎠⎟⎞}
× _V-^V----_IO-_N--
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 input capacitor current can be
calculated by:
I_{CIN_RMS} = I_O × _V-^V----_IO-_N-- _{⎝⎜⎛}1 – _V-^V----_IO-_N--_{⎠⎟⎞}
if let m equal the conversion ratio:
V^-V----_IO^-_N^{-- = m}
The relation between the input capacitor RMS current
and voltage conversion ratio is calculated and shown in
Figure 2 on the next page. It can be seen that when V_O is
half of V_IN, C_IN is under the worst current stress. The
worst current stress on C_IN is 0.5 x I_O.
Rev. 1.1 April 2009
www.aosmd.com
Page 8 of 16