MAX1856 Maxim, MAX1856 Datasheet - Page 10

• Manufacturer Part Number: MAX1856
• Description: Wide Input Range / Synchronizable / PWM SLIC Power Supply
• Manufacturer: Maxim

Wide Input Range, Synchronizable,
PWM SLIC Power Supply
Figure 2. Functional Diagram
cuit (Figure 1) converting a +12V input to a -72V output
(maximum load 100mA) and a -24V output (maximum
load 400mA).
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-
10
______________________________________________________________________________________
P
OUT MAX
9
4
5
6
3
(
V
GND
REF
FB
CS
CC
)
=
SLOPE COMP
100mV
EFFICIENCY
15mV
Maximum Output Power
276kΩ
R3
×
ANTISAT
276kΩ
P
V
1.25V
REF
IN MAX
R2
(
I
I
MAX
MIN
)
X6
X1
X1
MAX1856
R1
552kΩ
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
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
P
:N
1.0V
S
P
D
R
S
IN MAX
Setting the Operating Frequency
is the transformer’s turns ratio.
=
(
N V
Q
P OUT
)
N V
0
=
P OUT
MUX
V D
1
IN
+
(SYNC/SHDN and FREQ)
MUX
N V
1
S IN



R
V
0
F
CS
CS
F
BIAS
OSC
OSC
/3
−
SYNC/SHDN
2
ƒ
V D
OSC
IN
PGND
FREQ
LDO
EXT
L



10
1
8
7
2