AAT2786 Datasheet, PDF (17/27 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter and 150mA LDO

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PRODUCT DATASHEET
AAT2786
1.5A Step-Down Converter and 150mA LDO
In applications where there is a possibility of V_OUT
exceeding V_IN for brief amounts of time during normal
operation, the use of a larger value C_IN capacitor is
highly recommended. A larger value of C_IN with respect
to C_OUT will effect a slower C_IN decay rate during shut-
down, thus preventing V_OUT from exceeding V_IN. In appli-
cations where there is a greater danger of V_OUT exceed-
ing V_IN for extended periods of time, it is recommended
to place a Schottky diode across V_IN to V_OUT (connecting
the cathode to V_IN and anode to V_OUT). The Schottky
diode forward voltage should be less than 0.45V.
This LDO regulator has complete short-circuit and ther-
mal protection. The integral combination of these two
internal protection circuits gives the AAT2786 LDO regu-
lator a comprehensive safety system to guard against
extreme adverse operating conditions. Device power
dissipation is limited to the package type and thermal
dissipation properties. Refer to the Thermal Considerations
section of this datasheet for details on device operation
at maximum output current loads.
Component Selection For
Step-Down Converter
Inductor Selection
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
The internal slope compensation for the adjustable and
low voltage fixed versions is 0.75A/μs. This equates to a
slope compensation that is 75% of the inductor current
down slope for a 1.8V output and 1.8μH inductor.
m
=
0.75 ·
L
V_O
=
0.75 · 1.8V
1.8µH
=
0.75
A
µs
L=
0.75 · V_O
m
=
0.75
·
3.3V
A
=
3.3µH
0.75 _µs
The inductor should be set equal to the output voltage
numeric value in micro henries (μH). This guarantees
that there is sufficient internal slope compensation.
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the satura-
tion characteristics. The inductor should not show any
appreciable saturation under normal load conditions.
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
The 3.3μH CDRH4D28 series Sumida inductor has a
49.2mΩ worst case DCR and a 1.57A DC current rating.
At full 1.5A load, the inductor DC loss is 97mW which
gives less than 1.5% loss in efficiency for a 1.5A, 3.3V
output.
Input Capacitor
Select a 10μF to 22μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (V_PP) and
solve for C. The calculated value varies with input volt-
age and is a maximum when V_IN is double the output
voltage.
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
C_IN =
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
Always examine the ceramic capacitor DC voltage coeffi-
cient characteristics when selecting the proper value. For
example, the capacitance of a 10μF, 6.3V, X5R ceramic
capacitor with 5.0V DC applied is actually about 6μF.
The maximum input capacitor RMS current is:
I_RMS = I_O ·
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
2786.2008.04.1.0
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