xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
INPUT
4.5V TO 28V
D1
C
IN
MAX1864
MAX1865
IN
BST
DH
LX
C1
C2
N
N
H
L
C
BST
L
OUTPUT
1.25V TO 5V*
VL
C
OUT
ILIM
DL
R
POK
OUT
GND
POK
R1
R2
R
COMP
COMP
FB
C
COMP
* FOR OUTPUT VOLTAGES > 5V, SEE "OUTPUT VOLTAGE SELECTION."
Figure 4. Adjustable Output Voltage
where f
is 200kHz for MAX186_T and 100kHz for
valley of the inductor current occurs at I
minus half of the ripple current:
SW
LOAD(MAX)
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.
V
LIR
2
VALLEY(LOW)
> I
-
I
LOAD(MAX)
LOAD(MAX)
R
DS(ON)
where R
is the on-resistance of the low-side
DS(ON)
MOSFET (N ). For the MAX1864/MAX1865, the mini-
L
mum current-limit threshold is 190mV (for the typical
250mV default setting). Use the worst-case maximum
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:
value for R
) from the MOSFET N data sheet, and
DS(ON L
add some margin for the rise in R
over tempera-
DS(ON)
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.
LIR
2
I
= I
+
I
LOAD(MAX)
PEAK
LOAD(MAX)
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
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
