AAT1153 Datasheet, PDF (13/19 Pages) Advanced Analogic Technologies – 2A Step-Down Converter

A laboratory test set-up typically consists of two long
wires running from the bench power supply to the evalu-
ation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
In applications where the input power source lead induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum electrolytic should be placed in parallel with the
low ESR, ESL bypass ceramic. This dampens the high Q
network and stabilizes the system.
Output Capacitor Selection
The function of output capacitance is to store energy to
attempt to maintain a constant voltage. The energy is
stored in the capacitor’s electric field due to the voltage
applied.
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 induc-
tor is usually determined by L, V_OUT and V_IN, the series
impedance of the capacitor primarily determines the out-
put 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 output voltage droop due to a load transient is
dominated by the capacitance of the ceramic output
capacitor. During a step increase in load current, the
ceramic output capacitor 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_S
DATA SHEET
AAT1153
2A Step-Down Converter
In many practical designs, to get the required ESR, a
capacitor with much more capacitance than is needed
must be selected.
For both continuous or discontinuous inductor current
mode operation, the ESR of the C_OUT needed to limit the
ripple to ∆V_O, V peak-to-peak is:
ESR ≤
ΔV_O
ΔI_L
Ripple current flowing through a capacitor’s ESR causes
power dissipation in the capacitor. This power dissipation
causes a temperature increase internal to the capacitor.
Excessive temperature can seriously shorten the expect-
ed life of a capacitor. Capacitors have ripple current rat-
ings that are dependent on ambient temperature and
should not be exceeded. The output capacitor ripple cur-
rent is the inductor current, I_L, minus the output current,
I_O. The RMS value of the ripple current flowing in the
output capacitance (continuous inductor current mode
operation) is given by:
I_RMS = ΔI_L ·
3
6
= ΔI_L · 0.289
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 out-
put voltage ripple small and regulation loop stability,
ceramic capacitors with X5R or X7R dielectrics are rec-
ommended due to their low ESR and high ripple current
ratings. The output ripple V_OUT is determined by:
ΔV_OUT ≤
V_OUT · (V_IN - V_OUT)
V_IN · f_OSC · L
·
⎛⎝_{ESR +}
1
⎞
8 · f_OSC · C_OUT ^⎠
A 22μF ceramic capacitor can satisfy most applications.
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