MAX1644 Maxim, MAX1644 Datasheet - Page 9

• Manufacturer Part Number: MAX1644
• Description: Advanced Chemistry-Independent / Level 2 Battery Charger with Input Current Limiting
• Manufacturer: Maxim

Select R
Recommended values for R
470kΩ for off-times of 0.4µs to 4µs.
Three key inductor parameters must be specified:
inductor value (L), peak current (I
tance (R
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
The peak inductor current at full load is 1.125 · I
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.
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
R
Synchronous Rectification and Internal Switches
RIPPLE
LIR = ratio of peak-to-peak AC inductor current
TOFF
OUT
f
TOFF
DC
PWM
I
). The following equation includes a con-
PEAK
= maximum DC load current
according to the formula:
= (t
to DC load current, typically 0.25
= switching frequency in PWM mode
=
(I
L
OFF
=
_______________________________________________________________________________________
I
OUT
LOAD
2A, Low-Voltage, Step-Down Regulator with
=
I
OUT
- 0.07µs) (150kΩ / 1.26µs)
V
> 0.2A)
OUT
I
+
OUT
V
V
OUT
×
OUT
TOFF
×
Capacitor Selection
2
Inductor Selection
t
LIR
OFF
(
×
×
PEAK
V
range from 39kΩ to
IN
V
t
L
IN
OFF
−
), and DC resis-
V
OUT
)
OUT
if
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
(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:
With an AC load regulation setting of 1%, the C
requirement doubles, and the minimum ESR of the out-
put capacitor is halved.
An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C
to V
For stability, choose:
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.
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
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.
CC
compensates the transconductance amplifier.
C
OUT
CC
≥ (t
ESR
or unconnected). A 2% load-regula-
OFF
C
COMP
>
Setting the AC Loop Gain
/ V
1
OUT
%
≥ 470pF
×
Integrator Amplifier
)
✕
t
OFF
L
(64µFV / µs)
RIPPLE
COMP
, from COMP
≥ 2% · V
OUT
OUT
9