Rainbow Electronics MAX1865 User Manual
Page 15
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________
15
where f
SW
is 200kHz for MAX186_T and 100kHz for
MAX186_U. The exact inductor value is not critical and
can be adjusted to make trade-offs among size, cost,
and efficiency. Lower inductor values minimize size
and cost, but they also increase the output ripple and
reduce the efficiency due to higher peak currents. On
the other hand, higher inductor values increase effi-
ciency, but at some point resistive losses due to extra
turns of wire will exceed the benefit gained from lower
AC current levels.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
cores are often the best choice, though powdered iron
is inexpensive and can work well at 200kHz. The cho-
sen inductor’s saturation rating must exceed the peak
inductor current:
Setting the Current Limit
The minimum current-limit threshold must be high
enough to support the maximum load current at the
minimum tolerance level of the current-limit circuit. The
valley of the inductor current occurs at I
LOAD(MAX)
minus half of the ripple current:
where R
DS(ON)
is the on-resistance of the low-side
MOSFET (N
L
). For the MAX1864/MAX1865, the mini-
mum current-limit threshold is 190mV (for the typical
250mV default setting). Use the worst-case maximum
value for R
DS(ON
) from the MOSFET N
L
data sheet, and
add some margin for the rise in R
DS(ON)
over tempera-
ture. A good general rule is to allow 0.5% additional
resistance for each °C of the MOSFET junction temper-
ature rise.
Connect ILIM to VL for the default 250mV (typ) current-
limit threshold. For an adjustable threshold, connect a
resistive-divider from VL to ILIM to GND. The 500mV to
2.5V external adjustment range corresponds to a
106mV to 530mV current-limit threshold. When adjust-
ing the current limit, use 1% tolerance resistors and a
10µA divider current to prevent a significant increase in
the current-limit tolerance.
V
R
I
LIR
I
VALLEY LOW
DS ON
LOAD MAX
LOAD MAX
(
)
(
)
(
)
(
)
>
-
2
I
I
LIR
I
PEAK
LOAD MAX
LOAD MAX
=
+
(
)
(
)
2
BST
C1
D1
C2
R2
R1
N
H
N
L
C
BST
R
POK
R
COMP
C
COMP
C
OUT
C
IN
DH
LX
L
OUTPUT
1.25V TO 5V*
INPUT
4.5V TO 28V
DL
OUT
GND
FB
IN
VL
ILIM
POK
COMP
MAX1864
MAX1865
* FOR OUTPUT VOLTAGES > 5V, SEE "OUTPUT VOLTAGE SELECTION."
Figure 4. Adjustable Output Voltage