Rainbow Electronics MAX1625 User Manual
Page 18
MAX1624/MAX1625
High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
18
______________________________________________________________________________________
A good compromise between size and loss is a 45%
ripple current to load current ratio (LIR = 0.45), which
corresponds to a peak inductor current 1.23 times
higher than the DC load current.
where f is the switching frequency, between 100kHz
and 1MHz; I
OUT
is the maximum DC load current; and
LIR is the ratio of AC to DC inductor current (typically
0.45). The exact inductor value is not critical and can be
adjusted to make trade-offs among size, transient
response, cost, and efficiency. Although lower inductor
values minimize size and cost, they also reduce efficien-
cy due to higher peak currents. In general, higher
inductor values increase efficiency, but at some point
resistive losses due to extra turns of wire exceed the
benefit gained from lower AC current levels. Load-
transient response can be adversely affected by
high inductor values, especially at low (V
IN
- V
OUT
)
differentials.
The peak inductor current at full load is 1.23 x I
OUT
if
the previous equation is used; otherwise, the peak cur-
rent can be calculated using the following equation:
The inductor’s DC resistance is a key parameter for effi-
cient performance, and should be less than the current-
sense resistor value.
Calculating the Current-Sense
Resistor Value
Calculate the current-sense resistor value according to
the worst-case minimum current-limit threshold voltage
(from the
Electrical Characteristics
) and the peak
inductor current required to service the maximum load.
Use I
PEAK
from the equation in the section
Specifying
the Inductor
.
The high inductance of standard wire-wound resistors
can degrade performance. Low-inductance resistors,
such as surface-mount power metal-strip resistors, are
preferred. The current-sense resistor’s power rating
should be higher than the following:
In high-current applications, connect several resistors
in parallel as necessary, to obtain the desired resis-
tance and power rating.
Selecting the Output Filter Capacitor
Output filter capacitor values are generally determined
by effective series resistance (ESR) and voltage-rating
requirements, rather than by the actual capacitance
value required for loop stability. Due to the high switch-
ing currents and demanding regulation requirements in
a typical MAX1624/MAX1625 application, use only spe-
cialized low-ESR capacitors intended for switching-
regulator applications, such as AVX TPS, Sprague
595D, Sanyo OS-CON, or Nichicon PL series. Do not
use standard aluminum-electrolytic capacitors, which
can cause high output ripple and instability due to high
ESR. The output voltage ripple is usually dominated by
the filter capacitor’s ESR, and can be approximated as
I
RIPPLE
x R
ESR
. To ensure stability, the capacitor must
meet
both
minimum capacitance and maximum ESR
values as given in the following equations:
C
V
V
V
V
x R
x f
R
R
OUT
REF
OUT
IN MIN
OUT
SENSE
OSC
ESR
SENSE
(
)
>
+
<
1
R
mV
R
POWER RATING
SENSE
(
)
=
115
2
R
mV
I
SENSE
PEAK
=
85
I
I
V
V
V
f
x L x V
PEAK
OUT
OUT
IN MAX
OUT
OSC
IN MAX
(
)
(
)
=
+
−
(
)
2
L
V
V
V
V
x f
x I
x LIR
OUT
IN MAX
OUT
IN MAX
OSC
OUT
(
)
(
)
=
−
(
)
R3
PLACE VERY CLOSE
TO MAX1625
R2
FB
AGND
V
OUT
LOAD
MAX1625
Figure 7. MAX1625 Adjustable Output Operation