TPS6734I

FIXED 12-V 120-mA BOOST-CONVERTER SUPPLY

SLVS127A – AUGUST 1995 – REVISED JANUARY 1999

12

POST OFFICE BOX 655303

APPLICATION INFORMATION

soft-start capacitor

Soft-start timing is controlled by the value of the SS capacitor. Table 1 lists soft-start time intervals for selected

capacitor values and circuit conditions. If the circuit starts up with no load (e.g. in flash-memory programming

supplies), no soft start is needed. Omitting the soft-start capacitor provides a minimum output-voltage rise time

from the shutdown state, improving the output start-up time.

Table 1. Typical Soft-Start Times

SUPPLY

SOFT-START TIME† (ms) VERSUS CAPACITANCE

(µF)

SUPPLY

VOLTAGE (V)

NO. CAP

0.047

0.1

0.47

1.0

5

0.70

22

42

220

400

7

0.46

15

37

185

225

9

0.38

10

17

88

155

† Soft-start times are

±35%

printed-circuit layout

Printed-circuit-board (PCB) layout is critical to quiet operation. A ground plane is recommended. Special

attention should be given to minimizing the lengths of the switching loops. The first loop is formed by OUT, the

diode, the output capacitor, and GND, the length of which can be minimized by connecting the anode of the diode

close to OUT. The output capacitor should be connected directly between the diode cathode and GND with the

shortest possible path. The second loop is formed by OUT, the inductor, the input capacitor, and GND. This loop

is less critical than the first; however, the connection of OUT, the inductor and the anode of the diode must be

minimized. Bypass capacitors should be located as close to the device as possible to prevent instability and

be bypassed directly with a small ceramic capacitor (e.g., 0.1

µF). The recommended layout, shown in

Figures 14 through 17, can provide guidance for PCB configuration (the ground plane beneath the TPS6734

and the short loops should be noted).

Plastic plug-in-type proto boards, or any construction scheme that allows long leads and the possibility of noise

pickup, should not be used when assembling a breadboard or prototype application circuit implementing the

TPS6734.