AAT1138 Datasheet, PDF (16/21 Pages) Skyworks Solutions Inc. – 2A Step-Down Converter with Adjustable Input Current Limit

DATA SHEET
AAT1138
2A Step-Down Converter with Adjustable Input Current Limit
Considering the step-down converter at 2A 3.8V V_OUT is
90%.
I_BUCKIN
=
(5
-
3.8 · 2
0.15) · 90%
=
1.74A
I_CSYSOUT = 1.74 - 0.5 = 1.24A
T_ON is 576μs for a 217Hz 12.5% duty cycle load pulse.
1.24A · 576μs
C_{SYSOUT_Min} =
0.65V
= 1099μF
Considering 20% capacitance tolerance, the minimum
capacitance should be 1319μF. So select 4x330μF tanta-
lum capacitor as C_SYSOUT, as well as an additional 22μF
ceramic capacitor to closely filter the input voltage
V_SYSOUT of the step-down converter on the PCB board.
C_OUTB Selection
The value of output capacitance is generally selected to
limit output voltage ripple to the level required by the
specification. Since the ripple current in the output
inductor is usually determined by L, V_OUT and V_IN, the
series impedance of the capacitor primarily determines
the output voltage ripple. The three elements of the
capacitor that contribute to its impedance (and output
voltage ripple) are equivalent series resistance (ESR),
equivalent series inductance (ESL), and capacitance (C).
The formula below gives the general output voltage rip-
ple calculation:
∆V_OUT
≤
V_OUT ·
V_IN
(V_IN - V_OUT)
· f_OSC · L
·
1
ESR + _{8 · fOSC · COUT}
The output voltage droop due to a load transient is
dominated by the capacitance of the output capacitor.
During a step increase in load current, the output capac-
itor alone supplies the load current until the loop
responds. Within three switching cycles, the loop
responds and the inductor current increases to match
the load current demand. The relationship of the output
voltage droop during the three switching cycles to the
output capacitance can be estimated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_OSC
In many practical designs, to get the required ESR, a
capacitor with much more capacitance than is needed
must be selected.
For both continuous and discontinuous inductor current
mode operation, the ESR of the C_OUT needed to limit the
ripple to ΔV_O, V peak-to-peak is:
ESR ≤
ΔV_O
ΔI_L
ESL can be a problem by causing ringing in the low
megahertz region but can be controlled by choosing low
ESL capacitors, limiting lead length (PCB and capacitor),
and replacing one large device with several smaller ones
connected in parallel.
In conclusion, in order to meet the requirement of low
output voltage ripple and regulation loop stability, ceram-
ic capacitors with X5R or X7R dielectrics are recommend-
ed due to their low ESR and high ripple current ratings. A
22μF ceramic capacitor can satisfy most applications.
Inductor Selection
For most designs, the AAT1138 operates with inductor
values of 2.0μH to 6.8μH. Inductors with low inductance
values are physically smaller but generate higher induc-
tor current ripple leading to higher output voltage ripple.
Refer to the “Capacitor Selection” section of this datasheet
for the output ripple calculation. The inductor ripple cur-
rent can be derived from the following equation:
∆I_L
≤
V_OUT ·
V_IN
(V_IN -
· f_OSC
V_OUT)
·L
Large value inductors lower ripple current and small
value inductors result in high ripple currents. Choose
inductor ripple current approximately 30% of the maxi-
mum load current 2A, or
∆I_L = 600mA
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. The
DC current rating of the inductor should be at least equal
to the maximum load current plus half the inductor rip-
ple current to prevent core saturation (2A + 300mA).
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
16
201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013