AAT2550_08 Datasheet, PDF (22/34 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2550178
Total Power Solution for Portable Applications
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 140°C with 15°C of hys-
teresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) guarantees sufficient V_IN
bias and proper operation of all internal circuitry prior to
activation.
Step-Down Converter
Applications Information
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 AAT2550 is
0.24A/μs. This equates to a slope compensation that is
75% of the inductor current down slope for a 1.5V out-
put and 4.7μH inductor.
m=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.5V
4.7μH
=
0.24
A
μsec
This is the internal slope compensation for the step-
down converter. When externally programming the 0.6V
version to 2.5V, the calculated inductance is 7.5μH.
L=
0.75 ⋅ V_O
m
=
0.75 ⋅ V_O
A
≈
3
μsec
A
⋅
V_O
0.24A _μsec
μsec
=3 _A
⋅ 2.5V = 7.5μH
In this case, a standard 6.8μH value is selected.
For high-voltage output (≥2.5V), m = 0.48A/μs. Table 5
displays inductor values for the AAT2550 step-down con-
verters.
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 Sumida 4.7μH CDRH2D14 series inductor has a
135mΩ DCR and a 1A DC current rating. At full load, the
inductor DC loss is 48.6mW, which gives a 4% loss in
efficiency for a 600mA, 1.5V output.
Input Capacitor
Select a 4.7μF to 10μF 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 ⎠
The input capacitor RMS ripple current varies with the
input and output voltage and will always be less than or
equal to half of the total DC load current.
V_{O ·} ⎛_{1 -} V_O ⎞ _{= D · (1 - D) = 0.52 =} 1
V_IN ⎝ V_IN ⎠
2
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2550.2008.02.1.3