aoz1605 Alpha & Omega Semiconductor, aoz1605 Datasheet - Page 10

• Manufacturer Part Number: aoz1605
• Description: 500 Ma / 3 Mhz Ezbuck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be
considered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck
converter circuit, output ripple voltage is determined by
inductor value, switching frequency, output capacitor
value and ESR. It can be calculated by the equation
below:
where,
C
ESR
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the switching
frequency dominates. Output ripple is mainly caused by
capacitor value and inductor ripple current. The output
ripple voltage calculation can be simplified to:
If the impedance of ESR at switching frequency
dominates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum are recommended
to be used as output capacitors.
In a buck converter, output capacitor current is
continuous. The RMS current of output capacitor is
decided by the peak to peak inductor ripple current. It can
be calculated by:
I
ΔV
ΔV
CO_RMS
O
ΔV
Rev. 1.0 March 2011
is output capacitor value and
CO
O
O
O
is the Equivalent Series Resistor of output capacitor.
=
=
=
ΔI
ΔI
ΔI
=
L
L
L
×
×
----------
ΔI
×
ESR
⎛
⎝
12
-------------------------
8
ESR
L
×
f C
CO
1
×
CO
O
+
-------------------------
8 f
×
1
×
C
O
⎞
⎠
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Usually, the ripple current rating of the output capacitor
is a smaller issue because of the low current stress.
When the buck inductor is selected to be very small
and inductor ripple current is high, output capacitor could
be overstressed.
Thermal Shutdown
In most applications the AOZ1605 does not dissipate
much heat due to its high efficiency. But in an application
where the AOZ1605 is running at high ambient
temperature with low supply voltage and high duty cycle,
the heat dissipated may exceed the maximum junction
temperature. If the junction temperature reaches
approximately 140 °C (typical), the internal High Side
and Low Side MOSFET switching is disabled until the
temperature on the die has sufficiently fallen below
105°C. The device remains in thermal shutdown until the
junction temperature falls below the thermal shutdown
hysteresis.
Undervoltage Lockout
The undervoltage lockout circuit prevents the device from
malfunctioning at low input voltages and from excessive
discharge of the battery by disabling the output stage of
the converter. The AOZ1605 will resume normal
operation when the input supply voltage rises high
enough to properly function. The undervoltage lockout
threshold is typically 2.3 Volts.
Soft Start
The AOZ1605 has a soft-start circuit that limits the
inrush current during startup. Soft start is activated when
EN goes from logic low to logic high after Vin reaches 2.3
Volts.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection. Since the AOZ1605 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 slow during
a switching cycle because of V
catastrophic failure, a secondary current limit is designed
inside the AOZ1605. The measured inductor current is
compared against a preset voltage which represents the
current limit, approximately 1 A. 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 disappears.
O
= 0 V. To prevent
AOZ1605
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