13
LT1676
TYPICAL APPLICATIONS
U
Micropower Undervoltage Lockout
Certain applications may require very low current drain
when in undervoltage lockout mode. This can be accom-
plished with the addition of a few more external compo-
nents. Figure 6 shows an LTC
®
1440 micropower
comparator/reference added to control the LT1676 via its
SHDN pin. The extremely low input bias current of the
CMOS comparator allows the impedance of the resistor
divider R4/R5 to be increased, thereby minimizing power
drain. Hysteresis is externally programmable via resistor
divider R6/R7. The LTC1440 output directly controls the
LT1676 via its shutdown pin, driving it to either 5V (ON) or
0V (Full Shutdown). A simple linear voltage regulator to
power the LTC1440 is provided by Q1, Q2 and R7. Just
below the UVLO threshold, nominally 43V, total current
drain is typically 50µA.
Burst Mode Operation Configuration
Figure 4b demonstrates that power supply efficiency de-
grades with lower output load current. This is not surpris-
ing, as the LT1676 itself represents a fixed power overhead.
A possible way to improve light load efficiency is in Burst
Mode operation.
Figure 7 shows the LT1676 configured for Burst Mode
operation. Output voltage regulation is now provided in a
“bang-bang” digital manner, via comparator U2, an
LTC1440. Resistor divider R3/R4 provides a scaled ver-
sion of the output voltage, which is compared against U2’s
internal reference. Intentional hysteresis is set by the R5/
R6 divider. As the output voltage falls below the regulation
range, the LT1676 is turned on. The output voltage rises,
and as it climbs above the regulation range, the LT1676 is
turned off. Efficiency is maximized, as the LT1676 is only
powered up while it is providing heavy output current.
Figure 7b shows that efficiency is typically maintained at
75% or better down to a load current of 10mA. Even at a
load of 1mA, efficiency is still a respectable 59% to 68%,
depending on V
IN
.
Resistor divider R1/R2 is still present, but does not
directly influence output voltage. It is chosen to ensure
that the LT1676 delivers high output current throughout
the voltage regulation range. Its presence is also required
V
IN
V
IN
C1
39µF
63V
NC
Q1
PN2484
Q2
2N2369
1676 F06
V
+
V
–
IN
+
IN
–
U2
LTC1440
REF
HYST
OUT
3
7
12
4
6
5
8
GND
+
C2
100µF
10V
+
D1
MBRS1100
R1
36.5
1%
V
OUT
5V
R2
12.1k
1%
R3
22k
R6
22k
R7
2.4M
R4
6.8M
R5
240k
L1
220µH
C3
2200pF
C4
100pF
R8
10M
C1: PANASONIC HFQ
C2: AVX D CASE TPSD107M010R0080
C4, C5: X7R OR COG/NPO
D1: MOTOROLA 100V, 1A, SMD SCHOTTKY
L1: COILCRAFT DO3316P-224
V
IN
V
CC
V
SW
U1
LT1676
FB
V
C
SHDN
SYNC
2
5
4
3
7
8
1
6
GND
Figure 6. Micropower Undervoltage Lockout
