AAT1142 Datasheet, PDF (17/22 Pages) Advanced Analogic Technologies – 800mA Voltage-Scaling Step-Down Converter

AAT1142
800mA Voltage-Scaling Step-Down Converter
Component Selection
Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stability
for duty cycles greater than 50%. The output induc-
tor value must be selected so the inductor current
down slope meets the internal slope compensation
requirements. The internal slope compensation for
the programmable AAT1142 is 0.61A/µsec. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and
2.2µH inductor.
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.8V
2.2µH
=
0.61
A
µsec
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 saturation 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 con-
sider the losses associated with the DCR and its
effect on the total converter efficiency when
selecting an inductor.
The 2.2µH CDRH2D14 series Sumida inductor has
a 94mΩ DCR and a 1.5A DC current rating. At full
800mA load, the inductor DC loss is 60mW which
gives a 4.8% loss in efficiency for an 800mA, 1.0V
output.
Input Capacitor
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (V_PP) and solve for C. The calculated
value varies with input voltage and is a maximum
when V_IN is double the output voltage.
1142.2006.07.1.0
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
coefficient characteristics when selecting the prop-
er 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
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
D · (1 - D) =
0.5² = ¹
2
for V_IN = 2 · V_O
I = _RMS(MAX)
I_O
2
The term
V_O
V_IN
·
⎛⎝₁
-
V_O ⎞
V_IN ⎠
appears in both the input
voltage ripple and input capacitor RMS current
equations and is a maximum when V_O is twice V_IN.
This is why the input voltage ripple and the input
capacitor RMS current ripple are a maximum at
50% duty cycle.
The input capacitor provides a low impedance loop
for the edges of pulsed current drawn by the
AAT1142. Low ESR/ESL X7R and X5R ceramic
17