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MAX845

Isolated Transformer Driver for PCMCIA Applications

Maxim Integrated 

Isolated Transformer Driver

for PCMCIA Applications

RS

SIMPLE SHUNT ZENER

RS

22k

TL431

22k

5V OUTPUT

PROGRAMMABLE-IC SHUNT REGULATOR (STAND ALONE)

RS

1k

22k

2N2907

TL431

5V OUTPUT

22k

PROGRAMMABLE-IC SHUNT REGULATOR WITH DISCRETE PNP

Figure 12. Shunt-Regulator Circuits

Output Regulator

Since the output voltage is not regulated against

changes in the input voltage or load current, an output

voltage regulator may be needed. A series linear regu-

lator gives good performance and reasonably good

efficiency at low cost. A shunt regulator costs less,

occupies less space, and gives adequate performance

for some applications.

Series regulators such as the MAX666, MAX667,

MAX882/MAX883/MAX884, or MAX603/MAX604 simpli-

fy designs. Just select one with the desired output volt-

age and current capability, and connect it.

The simplest voltage regulator is the shunt zener shown

in Figure 12. The series resistor (RS) value should be as

high as possible to still deliver the maximum expected

load current with minimum input voltage. Be sure that no

ratings are exceeded at maximum input voltage and

minimum load current conditions; under such conditions,

the zener diode may have to dissipate much more power

than the load. Alternatively, start with the maximum allow-

able zener dissipation and select the series resistor

under light-load, high-line conditions. Then verify that

there is sufficient output current available with worst-

case low input voltage.

For better regulation than the simple shunt zener, con-

sider a shunt regulator IC such as the TL431. This

device behaves like a zener diode whose voltage can

be programmed by a resistor ratio. It can be used as a

stand-alone device or can be boosted above its 150mA

maximum rating without compromising its accuracy by

adding a discrete PNP transistor, as shown in Figure 12.

The input power of a shunt regulator is nearly indepen-

dent of load, so efficiency at light loads tends to be

worse than it would be with a series regulator.

Output Filter Capacitor

Ceramic capacitors can be used as output capacitors

because of the lower level of output ripple current. In

applications where output ripple is not critical, a 0.33µF

chip or ceramic capacitor is normally sufficient. Refer to

Table 3 for suggested capacitor suppliers.

In applications sensitive to output-ripple noise, the out-

put filter capacitor (C2) should have a low equivalent

series resistance (ESR) and a low equivalent series

inductance (ESL), and its capacitance should remain

fairly constant over temperature.

Sprague 595D surface-mount solid tantalum capacitors

and Sanyo OS-CON through-hole capacitors are recom-

mended, if space allows, due to their extremely low ESR.

Capacitor ESR usually rises at low temperatures, but OS-

CON capacitors provide very low ESR below 0°C.

Input Bypass Capacitor

The input bypass capacitor (C1) is not critical. Unlike

switching regulators, the MAX845’s supply current is

fairly constant, and is therefore less dependent on the

input bypass capacitor. A low-cost 0.33µF chip or

ceramic capacitor is normally sufficient for input

bypassing.

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