Low-voltage operation, Diode selection, Capacitor selection – Rainbow Electronics MAX13256 User Manual
Page 15: Input bypass capacitor, Output filter capacitor, Suggested external component manufacturers
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MAX13256
36V H-Bridge Transformer
Driver for Isolated Supplies
Transformers for use with the device are typically wound
on a high-permeability magnetic core. To minimize radi-
ated electromagnetic emissions, select a toroid, pot core,
E/I/U core, or equivalent.
Low-Voltage Operation
The MAX13256 can be operated from an +8V supply by
decreasing the turns ratio of the transformer, or by design-
ing a voltage doubler circuit as shown in
.
Optimum performance at +8V is obtained with fewer
turns on the primary winding since the ET product
requirement is lower than for a +24V supply. However,
any of the transformers for use with a +24V supply can
operate properly with a +8V supply. For a given power
level, the transformer currents are higher with a +8V
supply than with a +24V supply. Therefore, the DC resis-
tance of the transformer windings has a larger impact on
the circuit efficiency.
Diode Selection
The high switching speed of the MAX13256 necessitates
high-speed rectifiers. Ordinary silicon signal diodes
such as 1N914 or 1N4148 can be used for low-output
current levels (less than 50mA.) But at higher output cur-
rent levels, their reverse recovery times might degrade
efficiency. At higher output currents, select low forward-
voltage Schottky diodes to improve efficiency. Ensure
that the average forward current rating for the rectifier
diodes exceeds the maximum load current of the circuit.
For surface-mount applications, Schottky diodes such as
the BAT54, MBRS140, and MBRS340 are recommended.
Capacitor Selection
Input Bypass Capacitor
Bypass the supply pin to GND with a 1FF ceramic
capacitor as close as possible to the device. The equiva-
lent series resistance (ESR) of the input capacitors is not
as critical as for the output filter capacitors. Typically
ceramic X7R capacitors are adequate.
Output Filter Capacitor
In most applications, the actual capacitance rating of the
output filter capacitors is less critical than the capacitor’s
ESR. In applications sensitive to output-voltage ripple,
the output filter capacitor must have low ESR. For optimal
performance, the capacitance should meet or exceed
the specified value over the entire operating temperature
range. Capacitor ESR typically rises at low temperatures;
however, OS-CON capacitors can be used at tempera-
tures below 0NC to help reduce output-voltage ripple in
sensitive applications. In applications where low output-
voltage ripple is not critical, standard ceramic 0.1FF
capacitors are sufficient.
Table 1. Component Manufacturers
Suggested External Component Manufacturers
MANUFACTURER
COMPONENT
WEBSITE
Central Semiconductor
Diodes
Halo Electronics
Transformers
Kemet
Capacitors
Sanyo
Capacitors
Taiyo Yuden
Capacitors
TDK
Capacitors