Datasheet

12
LT1676
TYPICAL APPLICATIONS
U
Figure 4a. Minimum Component Count Application
Figure 4b. P
OUT
/P
IN
Efficiency
C1: PANASONIC HFQ
C2: AVX D CASE TPSD107M010R0080
C4, C5: X7R OR COG/NPO
D1: MOTOROLA 100V, 1A, SMD SCHOTTKY
L1: COILCRAFT DO3316P-224
C1
39µF
63V
1676 F04a
V
IN
12V TO
48V
+
C2
100µF
10V
+
D1
MBRS1100
R1
36.5k
1%
V
OUT
5V
0mA to 500mA
R2
12.1k
1%
R3
22k
5%
L1
220µH
C3
2200pF
X7R
C4
100pF
C5
100pF
FOR 3.3V V
OUT
VERSION:
R1: 24.3K, R2: 14.7k
L1: 150µH, DO3316P-154
I
OUT
: 0mA TO 500mA
V
IN
V
CC
V
SW
LT1676
FB
V
C
SHDN
SYNC
2
5
4
3
7
8
1
6
GND
I
LOAD
(mA)
1
60
EFFICIENCY (%)
70
80
90
10 100 1000
1676 F04b
50
40
30
20
V
IN
= 12V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
C1
1676 F05
V
IN
+
C2
+
D1
R1
V
OUT
R2
R3
L1
C3
R4
210k
1%
R5
6.19k
1%
C4
C5
V
IN
V
CC
V
SW
LT1676
FB
V
C
SHDN
SYNC
2
5
4
3
7
8
1
6
GND
Figure 5. User Programmable Undervoltage Lockout
desired V
IN
UVLO voltage minus 1.25V applied across it.
(The 2.5µA factor is an allowance to minimize error due to
SHDN pin input current.)
Behavior is as follows: Normal operation is observed at the
nominal input voltage of 48V. As the input voltage is
decreased to roughly 43V, switching action will stop, V
OUT
will drop to zero, and the LT1676 will draw its V
IN
and V
CC
quiescent currents from the V
IN
supply. At a much lower
input voltage, typically 18V or so at 25°C, the voltage on
the SHDN pin will drop to the shutdown threshold, and the
part will draw its shutdown current only from the V
IN
rail.
The resistive divider of R4 and R5 will continue to draw
power from V
IN
. (The user should be aware that while the
SHDN pin lockout threshold is relatively accurate includ-
ing temperature effects, the SHDN pin shutdown thresh-
old is more coarse, and exhibits considerably more
temperature drift. Nevertheless the shutdown threshold
will always be well below the lockout threshold.)