LT1676 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1676' PDF : 16 Pages View PDF
LT1676
TYPICAL APPLICATIONS
to maintain proper short-circuit protection. Transistors
Q1, Q2 and resistor R7 form a high VIN, 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 VIN 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 VIN 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.
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 ILIM value, but
this will overheat the inductor. Momentary short circuits
of a few seconds or less can still be tolerated.
VIN
+
5
C1
6
VIN
2
R7
SYNC VCC
10M
3
VSW
U1
Q1
LT1676
PN2484
1 SHDN
7
FB
8
VC
Q2
GND
2N2369
4
NC
7
8
V+
OUT
IN +
3
C1: PANASONIC HFQ 39µF AT 63V
IN – 4
C2: AVX D CASE 100µF 10V
U2
TPSD107M010R0080
D1: MOTOROLA 100V, 1A,
SMD SCHOTTKY
LTC1440 6
REF
5
HYST
MBRS1100 (T3)
L1: COILCRAFT DO3316-224
V – GND
2
1
L1
D1
+
R1
C2 39k
5%
R2
10k
C3
5%
100pF
R5
22k
R6
2.4M
(a)
VOUT
5V
R3
323k
1%
R4
100k
1%
1676 F07a
90
80
VIN = 12V
70
VIN = 48V
60
VIN = 36V
VIN = 24V
50
40
30
20
1
10
100
ILOAD (mA)
(b)
1000
1676 F07b
Figure 7. Burst Mode Operation Configuration for High Efficiency at Light Load
14
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