Datasheet

TPS6734I
FIXED 12-V 120-mA BOOST-CONVERTER SUPPLY
SLVS127A – AUGUST 1995 – REVISED JANUARY 1999
12
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
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
noise pickup. If a large V
CC
-to-GND bypass capacitor cannot be placed adjacent to the IC pins, the pins should
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.