13

LT1676

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

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 50

µA.

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%,

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

C1

39

µF

63V

NC

Q1

PN2484

Q2

2N2369

1676 F06

U2

LTC1440

REF

HYST

OUT

3

7

1

2

4

6

5

8

GND

+

C2

100

µF

10V

+

D1

MBRS1100

R1

36.5

1%

5V

R2

12.1k

1%

R3

22k

R6

22k

R7

2.4M

R4

6.8M

R5

240k

L1

220

µH

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

U1

LT1676

FB

SHDN

SYNC

2

5

4

3

7

8

1

6

GND

Figure 6. Micropower Undervoltage Lockout