f
PWM
= switching frequency in PWM mode
(I
OUT
> 0.2A)
Select R
TOFF
according to the formula:
R
TOFF
= (t
OFF
- 0.07µs) (150kΩ / 1.26µs)
Recommended values for R
TOFF
range from 39kΩ to
470kΩ for off-times of 0.4µs to 4µs.
Inductor Selection
Three key inductor parameters must be specified:
inductor value (L), peak current (I
PEAK
), and DC resis-
tance (R
DC
). The following equation includes a con-
stant, denoted as LIR, which is the ratio of peak-
to-peak inductor AC current (ripple current) to maxi-
mum DC load current. A higher value of LIR allows
smaller inductance but results in higher losses and rip-
ple. A good compromise between size and losses is
found at approximately a 25% ripple-current to load-
current ratio (LIR = 0.25), which corresponds to a peak
inductor current 1.125 times higher than the DC load
current:
where: I
OUT
= maximum DC load current
LIR = ratio of peak-to-peak AC inductor current
to DC load current, typically 0.25
The peak inductor current at full load is 1.125 · I
OUT
if
the above equation is used; otherwise, the peak current
is calculated by:
Choose an inductor with a saturation current at least as
high as the peak inductor current. To minimize loss,
choose an inductor with a low DC resistance.
Capacitor Selection
The input filter capacitor reduces peak currents and
noise at the voltage source. Use a low-ESR and low-
ESL capacitor located no further than 5mm from IN.
Select the input capacitor according to the RMS input
ripple-current requirements and voltage rating:
The output filter capacitor affects the output voltage rip-
ple, output load-transient response, and feedback loop
stability. For stable operation, the MAX1644 requires a
minimum output ripple voltage of V
RIPPLE
≥ 2% · V
OUT
(with 2% load regulation setting).
The minimum ESR of the output capacitor should be:
Stable operation requires the correct output filter
capacitor. When choosing the output capacitor, ensure
that:
C
OUT
≥ (t
OFF
/ V
OUT
)
✕
(64µFV / µs)
With an AC load regulation setting of 1%, the C
OUT
requirement doubles, and the minimum ESR of the out-
put capacitor is halved.
Integrator Amplifier
An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C
COMP
, from COMP
to V
CC
compensates the transconductance amplifier.
For stability, choose:
C
COMP
≥ 470pF
A large capacitor value maintains a constant average
output voltage but slows the loop response to changes
in output voltage. A small capacitor value speeds up
the loop response to changes in output voltage but
decreases stability. Choose the capacitor values that
result in optimal performance.
Setting the AC Loop Gain
The MAX1644 allows selection of a 1% or 2% AC load-
regulation error when the adjustable output voltage
mode is selected (Table 2). A 2% setting is automati-
cally selected in preset output voltage mode (FBSEL
connected to V
CC
or unconnected). A 2% load-regula-
tion error setting reduces output filter capacitor require-
ments, allowing the use of smaller and less expensive
capacitors. Selecting a 1% load-regulation error
reduces transient load errors, but requires larger capac-
itors.
