Rainbow Electronics MAX1865 - Manual (Page 23)

5) Ensure all feedback connections are short and
direct. Place the feedback resistors as close to the
MAX1864/MAX1865 as possible.
6) When trade-offs in trace lengths must be made, it’s
preferable to allow the inductor charging path to be
made longer than the discharge path. For example,
it is better to allow some extra distance between the
input capacitors and the high-side MOSFET than to
allow distance between the inductor and low-side
MOSFET or between the inductor and output filter
capacitor.
7) Route high-speed switching nodes away from sensi-
tive analog areas (B_, FB_, COMP, ILIM).
Regulating High Voltage
The linear regulator controllers can be configured to
regulate high output voltages by adding a cascode
transistor to buffer the base-drive output. For example,
to generate an output voltage between 30V and 60V,
add a 2N5550 high-voltage NPN transistor as shown in
Figure 8a where V
BIAS is a DC voltage between 3V and
20V that can source at least 1mA. R DROP protects the
cascode transistor by decreasing the voltage across
the transistor when the pass transistor saturates.
Similarly, to regulate a negative output voltage between
-20V and -120V, add a 2N5401 high-voltage PNP tran-
sistor as shown in Figure 8b. Chip Information
TRANSISTOR COUNT: 1617
PROCESS: BiCMOS
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________ 23
20
19
18
17
16
15
14
13
1
2
3
4
5
6
7
8
IN
VL
BST
DH FB
OUT
COMP
POK
LX
DL
GND
ILIM FB3
B3
FB2
B2
12
11
9
10
FB5
B5 FB4
B4
MAX1865
20 QSOP
TOP VIEW
Pin Configurations (continued)