AOZ1232-01
Once the product of V x T is constant, the switching
frequency keeps constant and is independent with input
voltage.
function of the inductor value as well as input and output
voltages. The current limit will keep the low-side
MOSFET ON and will not allow another high-side on-
time, until the current in the low-side MOSFET reduces
below the current limit. Figure 2 shows the inductor
current during the current limit.
IN
ON
An external resistor between the IN and TON pin sets the
switching frequency according to the following equation:
12
V
10
OUT
(3)
---------------------------------
=
F
SW
26.3 R
TON
Inductor
Current
A further simplified equation will be:
Ilim
38000 V
V
OUT
(4)
----------------------------------------------
F
kHz =
SW
Time
R
k
TON
is 137k, the switching frequency
TON
Figure 2. Inductor Current
If V
is 1.8V, R
OUT
After 128s (typical), the AOZ1232-01 considers this is a
true failed condition and therefore, turns-off both high-
side and low-side MOSFETs and latches off. When
triggered, only the enable can restart the AOZ1232-01
again.
will be 500kHz.
This algorithm results in a nearly constant switching
frequency despite the lack of a fixed-frequency clock
generator.
Output Voltage Under-Voltage Protection
True Current Mode Control
If the output voltage is lower than 25% by over-current or
short circuit, the AOZ1232-01 will wait for 128s (typical)
and turns-off both high-side and low-side MOSFETs and
latches off. When triggered, only the enable can restart
the AOZ1232-01 again.
The constant-on-time control scheme is intrinsically
unstable if output capacitor’s ESR is not large enough as
an effective current-sense resistor. Ceramic capacitors
usually cannot be used as output capacitor.
The AOZ1232-01 senses the low-side MOSFET current
and processes it into DC and AC current information
using AOS proprietary technique. The AC current
information is decoded and added on the FB pin on
phase. With AC current information, the stability of
constant-on-time control is significantly improved even
without the help of output capacitor’s ESR, and thus the
pure ceramic capacitor solution can be applicable. The
pure ceramic capacitor solution can significantly reduce
the output ripple (no ESR caused overshoot and
undershoot) and less board area design.
Output Voltage Over-Voltage Protection
The threshold of OVP is set 15% higher than 800mV.
When the V voltage exceeds the OVP threshold, high-
FB
side MOSFET is turned-off and low-side MOSFETs is
turned-on until V voltage is lower than 800mV.
FB
Power Good Output
The power good (PGOOD) output, which is an open
drain output, requires the pull-up resistor. When the
output voltage is 10% below than the nominal regulation
voltage for 50s (typical), the PGOOD is pulled low.
When the output voltage is 15% higher than the nominal
regulation voltage, the PGOOD is also pulled low.
Valley Current-Limit Protection
The AOZ1232-01 uses the valley current-limit protection
by using R
of the lower MOSFET current sensing.
DSON
When combined with the under-voltage-protection circuit,
this current limit method is effective in almost every
circumstance. In forced-PWM mode, the AOZ1232-01
also implements a negative current limit to prevent
excessive reverse inductor currents when VOUT is
sinking current.
To detect real current information, a minimum constant-
off (150ns typical) is implemented after a constant-on
time. If the current exceeds the valley current-limit
threshold, the PWM controller is not allowed to initiate a
new cycle. The actual peak current is greater than the
valley current-limit threshold by an amount equal to the
inductor ripple current. Therefore, the exact current-limit
characteristic and maximum load capability are a
Rev. 1.3 October 2012
www.aosmd.com
Page 10 of 18
AOS [ ALPHA & OMEGA SEMICONDUCTORS ]
