2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
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.
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:
ESR
>
1%
×
L
t
OFF
MAX1644
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:
L
=
V
OUT
×
t
OFF
I
OUT
×
LIR
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.
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:
I
PEAK
V
×
t
OFF
=
I
OUT
+
OUT
2
×
L
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.
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:
I
RIPPLE
=
I
LOAD
V
OUT
V
IN
−
V
OUT
V
IN
(
)
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9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
