Datasheet

19
LT1777
TYPICAL APPLICATIONS
U
Basic 3.3V Output Application
Figure 10 shows a circuit similar to the previous example,
but modified for a 3.3V output. Once again, Efficiency and
Internal Power Dissipation graphs are shown in Figure 11
for input voltages of 12V, 24V and 36V, and for sense
inductor values of 0µH, 1µH and 2.2µH. It is interesting to
note that internal LT1777 dissipation is very close to the
5V example. This confirms the fact that internal LT1777
dissipation is largely determined by input voltage, load
current and sense inductor, and is only a weak function of
output voltage.
The data as shown were performed using an off-the-shelf
Coilcraft DO3316-154 as the main inductor. This is a cost-
effective inductor using an open style of construction. For
a toroidal style inductor, the Coiltronics CTX150-4 or
similar may be substituted.
+
V
IN
V
CC
V
SW
LT1777
V
D
SHDN
SYNC
4
10
7
6
D1
L1
0µH TO 2.2µH
(SEE BELOW)
L2
150µH
C2
100µF
10V
R1
20k
1%
1777 F10
V
OUT
3.3V
R2
12.1k
1%
5
3
V
IN
10V TO 40V
12
14
13
V
C
FB
SGND
+
C1
39µF
63V
R3
12k
C3
2200pF
C4
100pF
C1: PANASONIC HFQ ELECTROLYTIC
C2: AVX D CASE TPSD107M010R0080
C3, C4, C5: NPO OR X7R
C6, C7: Z5U
D1: MOTOROLA 100V, 1A SMD SCHOTTKY
MBRS1100
L1: SENSE INDUCTOR CAN VARY FROM 0µH TO 2.2µH
AS PER APPLICATION. GRAPHICAL DATA TAKEN WITH:
1µH = D01608C-102, COILCRAFT OR SIMILAR
2.2µH = D01608C-222, COILCRAFT OR SIMILAR (SEE TEXT)
L2: COILCRAFT D03316-154 OR SIMILAR (SEE TEXT)
C5
100pF
C6
0.1µF
C7
0.1µF
Figure 10. Basic 3.3V Output Application