Datasheet

TPS6734I
FIXED 12-V 120-mA BOOST-CONVERTER SUPPLY
SLVS127A – AUGUST 1995 – REVISED JANUARY 1999
11
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
efficiency
Typical efficiency for the converter circuit shown in Figure 13 is plotted in Figure 10. The efficiency falls off rapidly
at very light currents because the supply current is a significant percentage of the load.
82
80
76
74
10 20 30 40 50 60 70
% – Efficiency
84
86
EFFICIENCY
vs
OUTPUT CURRENT
88
80 90 100 140
78
110120 130
I
O
– Output Current – mA
V
CC
= 5.5 V
V
CC
= 5 V
V
CC
= 4.5 V
Figure 10
inductor selection
Inductance value is directly proportional to the input voltage and inversely proportional to the output power. The
18 µH shown in the typical circuit is the proper value for operation from 5-V sources up to 2-W loads. A lower
inductance value should be used when operating from 3-V sources. Operation from 7 V and higher sources may
require inductance values greater than 18 µH. The inductor’s saturation current rating should be greater than
three times the dc load current for 5-V inputs and five times the dc load for 3-V inputs.
output filter capacitor selection
The output filter capacitor should be selected for minimum ESR (equivalent series resistance). Capacitor
ESR × ∆I
L
(change in inductor current) determines the amplitude of the high-frequency ripple on the output
voltage. The ESR of the capacitor should be less than 0.25 Ω to keep the output ripple less than 50 mV
peak-to-peak over the entire current range (using 18-µH inductor).
diode
A Schottky diode or a high-speed silicon rectifier should be used. The continuous current rating of the diode
should be at least 300 mA for full load (120 mA) operation.