Datasheet

MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
______________________________________________________________________________________ 15
This gives a value of about 22Ω for R7. However, this
snubber adds capacity to the MOSFET output, and this
in turn increases the dissipation in the MOSFET during
turn-on.
The selection of the input and output snubbers is an
interactive process. The design procedures above pro-
vide initial component recommendations, but the actual
values depend on the layout and transformer winding
practices used in the actual application.
Applications Information
Voice-over-IP CPE systems have +5V or +12V available
from which the talk battery voltage and the ringer volt-
age must be generated. The examples given below are
circuits using these supply voltages to generate the
negative power supplies needed in such applications.
Low Input Voltage
IP phones and routers require -48V. For cost-sensitive
applications, this needs to be used from an available
+5V supply. The circuit in Figure 4 is an example of
such a circuit using an off-the-shelf transformer from
Coiltronics and ICE components.
SLIC Power Supply with Split Feedback
Telephones in broadband systems use low-power-con-
suming SLICs that reduce the power drain by providing
the option of using two voltages for loop supervision.
The load on each output is dependent on the number
of lines on- or off-hook. The higher voltage is used to
generate ring battery voltage when the subscriber is on-
hook, while a second lower voltage is used to generate
talk battery voltage when off-hook is detected. The actu-
al value of these two voltages can be adjusted based on
system requirements and the specific SLIC used. The
design given here specifically addresses the supply
requirements for the AMD79R79 SLIC device with on-
chip ringing. The input voltage is 12V nominal, and the
output voltages are -24V at 400mA and -72V at 100mA.
The transformer turns ratio is 1:2:2:2, where 24V appears
across each secondary winding. The -72V output is
derived from the -24V output by stacking the secondary
windings in series as shown in Figure 1. A split feedback
is used, using resistors R1, R2, and R3. This allows for
accurate regulation of both outputs (see Typical
Operating Characteristics).
R
t
C
7
6
=
ƒ
CS
EXT
FB
LDO
FREQ
INPUT
3V TO 28V
R
OSC
C4
C
LDO
R
CS
C
IN
GND
REF
V
CC
PGND
SYNC/SHDN
MAX1856
OUTPUT
T1
D1
C5
C6
8
6
5
4
R6
R7
R5
9
10
1
2
3
7
R4
R1
R3
C
REF
C1
C3
C
FB
M1
Figure 3. Feedback Compensation and Snubber Circuits