Datasheet

MAX1858
Connect ILIM_ to V
L
for the default 100mV (typ) current-
limit threshold. For an adjustable threshold, connect a
resistor (R
ILIM
_) from ILIM_ to GND. The relationship
between the current-limit threshold (V
ITH
_) and R
ILIM
_ is:
where R
ILIM
_ is in Ω and V
ITH
_ is in V. An R
ILIM
resis-
tance range of 100kΩ to 600kΩ corresponds to a current-
limit threshold of 50mV to 300mV. When adjusting the
current limit, 1% tolerance resistors minimizes error in the
current-limit threshold. For foldback current limit, a resis-
tor (R
FBI
) is added from ILIM pin to output. The value of
R
ILIM
and R
FBI
can then be calculated as follows:
First select the percentage of foldback (P
FB
) from 15%
to 30%, then:
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents
as defined by the following equation:
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
IN
= 2V
OUT
), so
I
RMS(MAX)
= I
LOAD
/ 2. For most applications, nontanta-
lum capacitors (ceramic, aluminum, polymer, or
OSCON) are preferred at the input due to their robust-
ness with high inrush currents typical of systems that can
be powered from very low impedance sources.
Additionally, two (or more) smaller-value low-ESR capaci-
tors can be connected in parallel for lower cost. Choose
an input capacitor that exhibits less than +10°C tem-
perature rise at the RMS input current for optimal long-
term reliability.
Output Capacitor
The key selection parameters for the output capacitor
are capacitance value, ESR, and voltage rating. These
parameters affect the overall stability, output ripple volt-
age, and transient response. The output ripple has two
components: variations in the charge stored in the out-
put capacitor, and the voltage drop across the capaci-
tor’s ESR caused by the current flowing in to and out of
the capacitor.
The output voltage ripple as a consequence of the ESR
and output capacitance is:
where I
P-P
is the peak-to-peak inductor current (see the
Inductor Selection section). These equations are suitable
for initial capacitor selection, but final values should be
verified by testing in a prototype or evaluation circuit.
As a general rule, a smaller inductor ripple current
results in less output ripple voltage. Since inductor rip-
ple current depends on the inductor value and input
voltage, the output ripple voltage decreases with larger
inductance and increases with higher input voltages.
However, the inductor ripple current also impacts tran-
sient-response performance, especially at low
V
IN
- V
OUT
differentials. Low inductor values allow the
inductor current to slew faster, replenishing charge
VIR
V
I
Cf
I
VV
fL
V
V
RIPPLE ESR P P ESR
RIPPLE C
PP
OUT SW
PP
IN OUT
SW
OUT
IN
()
()
=
=
=












-
-
-
-
8
VV V
RIPPLE RIPPLE ESR RIPPLE C
≅+
() ()
II
VVV
V
RMS LOAD
OUT IN OUT
IN
=
()-
( )
( )
[ ( )]
R
PV
P
and R
VPR
VVP
FBI
FB OUT
FB
ILIM
ITH FB FBI
OUT ITH FB
=
×
×
=
××
×
5101
10 1
10 1
6-
-
-
--
R
V
A
ILIM
ITH
_
_
.
=
µ05
Dual 180° Out-of-Phase PWM Step-Down
Controller with Power Sequencing and POR
14 ______________________________________________________________________________________
MAX1858
OUT_
R_A
R_B
FB_
V
OUT_
> 1V
MAX1858
OUT_
R_C
R_A
FB_
REF
V
OUT_
< 1V
Figure 6. Adjustable Output Voltage