Datasheet

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DIODE SELECTION

CAPACITOR SELECTION

LAYOUT CONSIDERATIONS

TPS65120, TPS65121, TPS65123, TPS65124

SLVS531A – JUNE 2004 – REVISED MARCH 2005

To achieve high efficiency, use a Schottky diode. The voltage rating must be higher than the input voltage plus

the absolute value of the negative output. The current rating of the diode must meet the converter peak

inductor-current rating when servicing the VGL output. The main parameter affecting the efficiency of the

converter is the forward voltage and the reverse leakage current of the diode, both should be as low as possible.

The following diodes from different suppliers listed in Table 2 have been used with the TPS6512x converter.

Table 2. List of Diodes

MANUFACTURER REFERENCE REVERSE VOLTAGE

ROHM RB521G-30 30 V

VISHAY BAT54-HT3 30 V

ZETEX ZUMD54 30 V

The TPS65120 converter requires six capacitors. The input capacitor is primarily a function of the board layout.

In designs with long traces, for good input filtering, we recommend a ceramic input capacitor (X5R/X7R type) of

at least 1 µF placed as close as possible to the converter.

To operate properly, the TPS6512x requires a bootstrap capacitor of 1 µF (or larger) on the BOOT output.

Additionally the minimum BOOT capacitance must be larger than two times the capacitor value connected to the

MAIN and AUXILIARY LDO outputs (in case LDO AUX is connected to the BOOT output).

The TPS6512x peak-current control scheme is inherently stable. The filtering capacitors on VGH and VGL

outputs are basically determined as a function of the required current and permissible ripple voltage. For small

form-factor TFT-LCD applications, typical values in the range of 100 nF to 1 µF are usually required. A good

starting point is 220 nF. For high output power on VGH and VGL outputs, the capacitance may need to approach

2 µF.

For stable operation, TPS6512x requires a 220-nF ceramic capacitor on the MAIN and AUXILIARY LDO outputs.

Larger capacitor values can be used to achieve lower output-voltage noise without sacrificing stability.

In general, ceramic X5R types are strongly recommended for their low ESR and ESL and capaci-

tance-versus-bias-voltage stability. Be certain that the capacitors used are rated for the maximum voltage with

adequate safety margin.

As for all switching power supplies, the layout is an important step in the design. If the layout is not carefully

done, the regulator could become unstable, displaying double or missing pulses as well as EMI problems.

Therefore, use wide, short traces for the main current paths. Route these traces first.

Place the input capacitor as close as possible to the IC pins as well as the inductor and output capacitors. Place

the inductor and diode as close as possible to the switch pins to minimize noise coupling into other circuits.

Use a common ground node for power ground and a different one for control ground (AGND) to minimize the

effects of ground noise. Connect these ground nodes together (star point) at any place close to one of the

ground pins of the IC and make sure that small-signal components returning to the AGND pin do not share the

switching-current paths.

Feedback pins and divider networks are high-impedance nodes and should therefore be routed away from the

inductor and shielded with a ground plane or trace to minimize noise coupling into the control loop.