AAT2514
Dual Channel 600mA Step-Down Converter
affect converter performance. This problem often
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 COUT needed to limit the ripple to ∆VO, V
peak-to-peak is:
becomes apparent in the form of excessive ringing in
the output voltage during 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 applica-
tions where the input power source lead inductance
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.
∆VO
∆IL
ESR ≤
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 expected life of a capacitor.
Capacitors have ripple current ratings that are
dependent on ambient temperature and should not
be exceeded. The output capacitor ripple current is
the inductor current, IL, minus the output current, IO.
The RMS value of the ripple current flowing in the
output capacitance (continuous inductor current
mode operation) is given by:
Output Capacitor Selection
The function of output capacitance is to store ener-
gy 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 select-
ed 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, VOUT
,
3
and VIN, the series impedance of the capacitor pri-
marily determines the output voltage ripple. The
three elements of the capacitor that contribute to its
impedance (and output voltage ripple) are equiva-
lent 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 two or three switch-
ing cycles, the loop responds and the inductor cur-
rent increases to match the load current demand.
The relationship of the output voltage droop during
the three switching cycles to the output capaci-
tance can be estimated by:
I
RMS = ∆IL ·
∆IL · 0.289
6
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 sev-
eral smaller ones connected in parallel.
In conclusion, in order to meet the requirement of
output voltage ripple small and regulation loop stabil-
ity, ceramic capacitors with X5R or X7R dielectrics
are recommended due to their low ESR and high rip-
ple current ratings. The output ripple VOUT is deter-
mined by:
V
PP · (VIN - VOUT
VIN · fOSC · L
)
1
∆VOUT
≤
· ESR +
8 · fOSC · COUT
3 · ∆ILOAD
COUT
=
V
DROOP · FS
A 10µF ceramic capacitor can satisfy most applica-
tions.
12
2514.2007.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
