CS5171, CS5172, CS5173, CS5174

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17

TJ + TA)(PDqJA)

where:

°C);

°C);

For the CS517x,

whether the CS517x can be used in an application is settled.

°C, the absolute maximum allowable

junction temperature, the CS517x is not suitable for that

application.

°C, the designer should consider

possible means of reducing the junction temperature.

Perhaps another converter topology could be selected to

reduce the switch current. Increasing the airflow across the

Circuit Layout Guidelines

In any switching power supply, circuit layout is very

important for proper operation. Rapidly switching currents

combined

with

trace

inductance

generates

voltage

transitions that can cause problems. Therefore the following

guidelines should be followed in the layout.

1.

In boost circuits, high AC current circulates within the

loop composed of the diode, output capacitor, and

on−chip power transistor. The length of associated

traces and leads should be kept as short as possible. In

the flyback circuit, high AC current loops exist on both

sides of the transformer. On the primary side, the loop

consists of the input capacitor, transformer, and

on−chip power transistor, while the transformer,

rectifier diodes, and output capacitors form another

loop on the secondary side. Just as in the boost circuit,

all traces and leads containing large AC currents

should be kept short.

2.

Separate the low current signal grounds from the

power grounds. Use single point grounding or ground

plane construction for the best results.

3.

Locate the voltage feedback resistors as near the IC as

possible to keep the sensitive feedback wiring short.

Connect feedback resistors to the low current analog

ground.

+

+

+

C2

D2

C3

D1

C4

R2

1

2

3

4

5

6

7

8

22

mF

22

mF

−12 V

Test

NFB

SS

0.01

mF

C1

AGND

PGND

SS

5.0 V

5.0 k

R1

R3

1.27 k

4.87 k

MBRS120T3

MBRS120T3

22

mF

22

mH

L1

Figure 41. Additional Application Diagram, 5.0 V to −12 V/ 75 mA Inverting Converter