Datasheet
14
LT1676
TYPICAL APPLICATIONS
U
Minimum Size Inductor Application
Figure 4a employs power path parts that are capable of
delivering the full rated output capability of the LT1676.
Potential users with low output current requirements may
be interested in substituting a physically smaller and less
costly power inductor. The circuit shown on the last page
of this data sheet is topologically identical to the basic
application, but specifies a much smaller inductor, and, a
somewhat smaller input electrolytic capacitor. This circuit
is capable of delivering up to 150mA at 5V, or, up to 200mA
at 3.3V. The only disadvantage is that due to the increased
resistance in the inductor, the circuit is no longer capable
of withstanding indefinite short circuits to ground. The
LT1676 will still current limit at its nominal I
LIM
value, but
this will overheat the inductor. Momentary short circuits
of a few seconds or less can still be tolerated.
(a)
(b)
I
LOAD
(mA)
1
60
EFFICIENCY (%)
70
80
90
10 100 1000
1676 F07b
50
40
30
20
V
IN
= 12V
V
IN
= 36V
V
IN
= 48V
V
IN
= 24V
V
IN
C1
NC
Q1
PN2484
Q2
2N2369
1676 F07a
V
+
V
–
IN
+
IN
–
U2
LTC1440
REF
HYST
OUT
3
7
12
4
6
5
8
GND
+
C2
+
D1
R1
39k
5%
V
OUT
5V
R2
10k
5%
R5
22k
R6
2.4M
L1
R3
323k
1%
R4
100k
1%
C3
100pF
R7
10M
C1: PANASONIC HFQ 39µF AT 63V
C2: AVX D CASE 100µF 10V
TPSD107M010R0080
D1: MOTOROLA 100V, 1A,
SMD SCHOTTKY
MBRS1100 (T3)
L1: COILCRAFT DO3316-224
V
IN
V
CC
V
SW
U1
LT1676
FB
V
C
SHDN
SYNC
2
5
4
3
7
8
6
1
GND
Figure 7. Burst Mode Operation Configuration for High Efficiency at Light Load
to maintain proper short-circuit protection. Transistors
Q1, Q2 and resistor R7 form a high V
IN
, low quiescent
current voltage regulator to power U2.
Burst Mode Operation Configuration with UVLO
Figure 7a uses an external comparator to control the
LT1676 via its SHDN pin. As such, the user’s ability to set
an undervoltage lockout (UVLO) threshold with a resistor
divider from V
IN
to SHDN pin to ground is lost. This ability
is regained in the slightly more complicated circuit shown
in Figure 8.
A dual comparator, the LTC1442, replaces the previous
single comparator. The second comparator monitors a
resistive divider between V
IN
and ground to provide the
(user-adjustable) UVLO function. The two comparator
outputs are logically combined in a CMOS NOR gate (U3)
to drive the LT1676 SHDN pin.