AN017
3
Efficiency
Typical efficiency vs. load current curves for 5.0V, 7.0V, and 12.0V input voltages are
shown in Figure 1.
40
45
50
55
60
65
70
75
80
0
0.5
1
1.5
2
E
ff
ici
en
cy
(%
)
Load Current (A)
Efficiency
V
OUT
=
1.5V
V
IN
=
7V
V
IN
=
5V
V
IN
=
12V
40
45
50
55
60
65
70
75
80
85
90
0
0.5
1
1.5
2
Ef
fi
ci
en
cy
(%)
Load Current (A)
Efficiency
V
IN
=
7V
V
IN
=
5V
V
IN
=
12V
V
OUT
=
3.3V
Figure
1
Boost Pin and Boost Drive (BD)
The Boost pin provides drive voltage greater than V
IN
to the base of the power transistor.
Using a voltage greater than V
IN
ensures hard saturation of the power switch significantly
improving overall efficiency. Connect a capacitor between Boost and SW to store a charge.
A boost pin voltage of at least 2.3V, relative to the SW pin, is required
throughout the on-
time of the switch to guarantee that it remains saturated. Capacitor C2 and an internal boost
Schottky diode are used to generate a boost voltage that is higher than the input voltage. In
most cases a 0.22
μ
F capacitor will work well. Figure 2 shows two ways to arrange the boost
circuit.
(2a)
V
OUT
< 3.0V
(2b) V
OUT
≥
3.0V
Figure 2. Boost/BD Pin Configurations
For outputs less than 3.0V, Figure 2a is best—BD tied to the input voltage. For outputs
greater than 3.0V figure 2b is best—BD tied to the output. Tying BD to VIN reduces the
maximum allowable input voltage to 30V. The circuit in Figure 2b is more efficient because
