7

LT1312

D1

MBRS140

D3

MBRS130T3

D2

MBRS140

Q3

VN7002

6.5V TO 18V

C4

1000pF

C2

1000pF

C3

220µF

C1

68µF

C5

22µF

1µF

5V

OUTPUT

Q1

10

9

20

15

14

EN0

EN1

VALID

*LPE-6562-A026 DALE (605) 665-9301

14V AUXILIARY SUPPLY

T1

1.8T

30µH*

Q2

R4

22

Ω

R1

100

Ω

R3

18k

R2

100

Ω

R5

0.033

Ω

LT1312 F1

+

+

+

EN0

TO CARD VPP PIN

0V, 3.3V, 5V, 12V OR HI-Z

FROM

LT1312

EN1

VALID

SENSE

GND

+

PDRIVE

NDRIVE

1/2 LTC1142HV

5V REG

loaded without regard to the loading on the 5V output of

the LTC1142HV.

Continuous mode operation is only invoked when the

LT1312 is programmed to 12V. If the LT1312 is pro-

grammed to 0V, 3.3V or 5V, power is obtained directly

from the main power source (battery pack) through diode

D1. Again, the LT1312 output can be loaded without

regard to the loading of the main 5V output.

R4 and C4 absorb transient voltage spikes associated with

the leakage inductance inherent in T1's secondary winding

and ensure that the auxiliary supply does not exceed 20V.

Figure 2 is a graph of output voltage versus output current

for the auxiliary 14V supply shown in Figure 1. Note that

the auxiliary supply voltage is slightly higher when the 5V

output is heavily loaded. This is due to the increased

energy flowing through the main 5V inductor.

LTC1142 Auxiliary Power from the 3.3V Output

The circuit of Figure 1 can be modified for operation with

low-battery count applications (6 cell). As the input volt-

age falls, the 5V duty cycle increases to the point where

there is simply not enough time to transfer energy from the

5V primary to the auxiliary winding. For applications

where heavy 12V load currents exist in conjunction with

low input voltages (<6.5V), the auxiliary winding can be

derived from the 3.3V section instead of the 5V section of

the LTC1142. In this case, a transformer with a turns ratio

of 1:3.4 to 1:3.6 should be used in place of the 3.3V section

AUXILIARY OUTPUT CURRENT (mA)

0.1

11

15

16

17

10

1

100

1000

LT1312 F2

14

13

12

EN0 = HI

Figure 2. LTC1142 Auxiliary Supply Voltage

Figure 1. Deriving 14V Power from an Auxiliary Winding on the LTC1142HV 5V Regulator