Datasheet

MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
10 ______________________________________________________________________________________
cuit (Figure 1) converting a +12V input to a -72V output
(maximum load 100mA) and a -24V output (maximum
load 400mA).
Maximum Output Power
The maximum output power the MAX1856 can provide
depends on the maximum input power available and
the circuit’s efficiency:
Furthermore, the efficiency and input power are both
functions of component selection. Efficiency losses can
be divided into three categories: 1) resistive losses
across the transformer, MOSFET’s on-resistance, cur-
rent-sense resistor, and the ESR of the input and output
capacitors; 2) switching losses due to the MOSFET’s
transition region, the snubber circuit, which also
increases the transition times, and charging the MOS-
FET’s gate capacitance; and 3) transformer core loss-
es. Typically, 80% efficiency can be assumed for initial
calculations. The input power depends on the current
limit, input voltage, output voltage, inductor value, the
transformer’s turns ratio, and the switching frequency:
where N
P
:N
S
is the transformer’s turns ratio.
Setting the Operating Frequency
(SYNC/SHDN and FREQ)
The SYNC/SHDN pin provides both external-clock syn-
chronization (if desired) and shutdown control. When
SYNC/SHDN is low, all IC functions are shut down. A
logic high at SYNC/SHDN selects operation at a
PVD
V
R
VD
L
D
NV
NV NV
IN MAX IN
CS
CS
IN
OSC
P OUT
P OUT S IN
()
=−
ƒ






=
+
2
P EFFICIENCY P
OUT MAX IN MAX() ()
=×
R3
276kΩ
R2
276kΩ
R1
552kΩ
V
REF
1.25V
I
MAX
I
MIN
100mV
MUX
01
X6
X1
X1
Q
S
R
MUX
0
1
2
7
8
1
10
MAX1856
ANTISAT
SLOPE COMP
REF
9
4
5
6
3
LDO
EXT
PGND
FREQ
FB
CS
F
OSC
1.0V
F
OSC
/3
V
CC
BIAS
SYNC/SHDN
GND
15mV
Figure 2. Functional Diagram