MAX8664
Low-Cost, Dual-Output, Step-Down
Controller with Fast Transient Response
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V
RIPPLE
= V
RIPPLE(ESR)
+ V
RIPPLE(C)
+ V
RIPPLE(ESL)
The output voltage ripple as a consequence of the
ESR, ESL, and output capacitance is:
where I
P-P
is the peak-to-peak inductor current:
These equations are suitable for initial capacitor selec-
tion, but final values should be chosen based on a pro-
totype or evaluation circuit. As a general rule, a smaller
ripple current results in less output-voltage ripple. Since
the inductor ripple current is a factor of the inductor
value and input voltage, the output-voltage ripple
decreases with larger inductance, and increases with
higher input voltages. Ceramic, tantalum, or aluminum
polymer electrolytic capacitors are recommended. The
aluminum electrolytic capacitor is the least expensive;
however, it has higher ESR and ESL. To compensate for
this, use a ceramic capacitor in parallel to reduce the
switching ripple and noise. For reliable and safe opera-
tion, ensure that the capacitor’s voltage and ripple-cur-
rent ratings exceed the calculated values.
The response to a load transient depends on the
selected output capacitors. After a load transient, the
output voltage instantly changes by ESR x ΔI
LOAD
.
Before the controller can respond, the output voltage
deviates further depending on the inductor and output
capacitor values. After a short period of time (see the
Typical Operating Characteristics
), the controller
responds by regulating the output voltage back to its
nominal state. The controller response time depends on
its closed-loop bandwidth. With a higher bandwidth,
the response time is faster, thus preventing the output
voltage from further deviation from its regulating value.
Setting the Output Voltages and Voltage
Positioning
Figure 7 shows the feedback network used on the
MAX8664. With this configuration, a portion of the feed-
back signal is sensed on the switched side of the
inductor (LX), and the output voltage droops slightly as
the load current is increased due to the DC resistance
of the inductor (DCR). This allows the load regulation to
be set to match the voltage droop during a load tran-
sient (voltage positioning), reducing the peak-to-peak
output voltage deviation during a load transient, and
reducing the output capacitance requirements.
To set the magnitude of the voltage positioning, select
a value for R2 in the 8kΩ to 24kΩ range, then calculate
the value of R1 as follows:
where I
OUT(MAX)
is the maximum output current and
Δ V
OUT(MAX)
is the maximum allowable droop in the
output voltage at full load.
