AAT1232
24V 100mA Step-Up Converter
diodes are recommended for outputs less than 15V,
while 30V rated Schottky diodes are recommended
for outputs greater than 15V.
1
= TON + TOFF
FS
The average diode current is equal to the output
current.
The ON time is the period which the N-channel
power MOSFET is conducting and storing energy
in the boost inductor. Duty cycle is defined as the
ON time divided by the total switching interval.
IAVG = IOUT
TON
D =
The average output current multiplied by the for-
ward diode voltage determines the loss of the out-
put diode.
T
ON + TOFF
= TON ⋅ FS
PLOSS_DIODE = IAVG
= IOUT
·
·
VF
VF
The maximum duty cycle can be estimated from
the relationship for a continuous mode boost con-
verter. Maximum duty cycle (DMAX) is the duty
cycle at minimum input voltage (VIN(MIN)).
Diode junction temperature can be estimated.
(VOUT + VF - VIN(MIN)
(VOUT + VF)
)
DMAX
=
TJ = TAMB + ΘJA · PLOSS_DIODE
The junction temperature should be maintained
below 110ºC, but may vary depending on applica-
tion and/or system guidelines. The diode θJA can
be minimized with additional PCB area on the
cathode. PCB heatsinking the anode may degrade
EMI performance.
Where VF is the Schottky diode forward voltage
and can be estimated at 0.5V. Manufacturer’s
specifications list both the inductor DC current rat-
ing, which is a thermal limitation, and peak inductor
current rating, which is determined by the satura-
tion characteristics. Measurements at full load and
high ambient temperature should be completed to
ensure that the inductor does not saturate or exhib-
it excessive temperature rise.
The reverse leakage current of the rectifier must be
considered to maintain low quiescent (input) cur-
rent and high efficiency under light load. The recti-
fier reverse current increases dramatically at high
temperatures.
The output inductor (L) is selected to avoid satura-
tion at minimum input voltage, maximum output load
conditions. Peak current may be calculated from the
following equation, again assuming continuous con-
duction mode. Worst-case peak current occurs at
minimum input voltage (maximum duty cycle) and
maximum load. Switching frequency can be estimat-
ed at 500kHz with a 2.2µH inductor.
Selecting the Boost Inductor
The AAT1232 controller utilizes hysteretic control
and the switching frequency varies with output load
and input voltage. The value of the inductor deter-
mines the maximum switching frequency of the
AAT1232 boost converter. Increased output induc-
tance decreases the switching frequency, resulting
in higher peak currents and increased output volt-
age ripple. To maintain 2MHz maximum switching
frequency, an output inductor sized from 1.5µH to
2.7µH is recommended.
IOUT
(1 - DMAX
DMAX
·
· VIN(MIN)
FS · L)
IPEAK
=
+
)
(2
The RMS current flowing through the boost induc-
tor is equal to the DC plus AC ripple components.
Under worst-case RMS conditions, the current
waveform is critically continuous. The resulting
The switching period is divided between ON and
OFF time intervals.
1232.2006.12.1.3
11
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