AAT1155(2004) Ver la hoja de datos (PDF) - Analog Technology Inc
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AAT1155 Datasheet PDF : 16 Pages
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Current Limit and Over Temp
Protection
The AAT1155 over temp and current limit circuitry
protects the AAT1155 as well as the external
Schottky diode during overload, short circuit and
excessive ambient temperature conditions. The
junction over temp threshold is 140°C nominal and
has 15°C of hysteresis. Typical graphs of the over
temp load current vs. input voltage and ambient tem-
perature are shown in the Typical Characteristics
section.
Inductor
The output inductor is selected to limit the ripple
current to 20-40% of the full load current at the
maximum input voltage. Manufacturer's specifica-
tions list both the inductor DC current rating, which
is a thermal limitation, and the peak current rating,
which is determined by the inductor saturation
characteristics. The inductor should not show any
appreciable saturation under all normal load condi-
tions. During overload and short circuit conditions
the inductor can exceed its peak current rating
without affecting the converter performance. Some
inductors may have sufficient peak and average
current ratings yet result in excessive losses due to
a high DCR. The losses associated with the DCR
and its affect on the total converter efficiency must
be considered.
For a 2.5 Amp load and the ripple current set to
40% at the maximum input voltage, the maximum
peak to peak ripple current is 1 Amp. Assuming a
5V ± 5% input voltage and 40% ripple the output
inductance required is
L
=
VOUT
IOUT · k ·
FSW
·
1
-
VOUT
VIN(MAX)
=
2.5A
3.3V
· 0.4 ·
1MHz
·
1
-
3.3V
5.25V
= 1.23µH
AAT1155
1MHz 2.5A Buck DC/DC Converter
The factor "k" is the fraction of the full load (40%)
selected for the ripple current at the maximum input
voltage.
The corresponding inductor RMS current is:
IRMS =
IO2
+
∆I 2
12
≈
I O = 2.5A
∆I is the peak to peak ripple current which is fixed by
the inductor selection above. For a peak to peak cur-
rent of 40% of the full load current the peak current
at full load will be 120% of the full load. The 1.5µH
inductor selected from the Sumida CDRH6D38
series has a 11mΩ DCR and a 4.0 Amp DC current
rating with a height of 4 mm. At full load the inductor
DC loss is 70 mW for a 0.84% loss in efficiency.
Schottky Freewheeling Diode
The Schottky average current is the load current
times one minus the duty cycle.
1-
VO
VIN
For VIN at 5 Volts and Vout at 3.3 Volts the average
diode current is
I
AVG
=
IO
·
1
-
VO = 2.5A · 1-
VIN
3.3V
5.0V
=
0.85A
With a 125°C maximum junction temperature and a
120°C/W thermal resistance the maximum average
current is
IAVG =
TJ(MAX)- TAMB
θJ-A · VFWD
=
125°C - 70°C
120°C/W · 0.4V
=
1.14A
For overload, short circuit, and excessive ambient
conditions the AAT1155 enters the over-tempera-
ture shutdown mode protecting the AAT1155 as well
as the output Schottky. In this mode the output cur-
rent is limited internally until the junction tempera-
ture reaches the temperature limit (see over temp
characteristics graphs). The diode reverse voltage
must be rated to withstand the input voltage.
10
1155.2004.08.1.2
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