6

LTC1261

MODES OF OPERATION

The LTC1261 uses a charge pump to generate a negative

output voltage that can be regulated to a value either

higher or lower than the original input voltage. It has two

modes of operation: a “doubler” inverting mode, which

can provide a negative output equal to or less than the

positive power supply and a “tripler” inverting mode,

which can provide negative output voltages either larger or

smaller in magnitude than the original positive supply. The

tripler offers greater versatility and wider input range but

requires four external capacitors and a 14-pin package.

The doubler offers the SO-8 package and requires only

three external capacitors.

Doubler Mode

Doubler mode allows the LTC1261 to generate negative

output voltage magnitudes up to that of the supply voltage,

capacitor to invert the input supply voltage, and the output

voltage is stored on the output bypass capacitor between

switch cycles. The LTC1261CS8 is always configured in

doubler mode and has only one pair of flying capacitor

pins (Figure 1a). The LTC1261CS can be configured in

doubler mode by connecting a single flying capacitor

floating (Figure 1b).

Tripler Mode

The LTC1261CS can be used in a tripler mode which can

generate negative output voltages up to twice the supply

be used to generate – 5V from a single positive 3.3V

supply. Tripler mode requires two external flying capaci-

relatively high voltages that can be generated in this mode,

care must be taken to ensure that the total input-to-output

voltage never exceeds 12V or the LTC1261 may be dam-

aged. In most applications the output voltage will be kept

in check by the regulation loop. Damage is possible

however, with supply voltages above 4V in tripler mode

and above 6V in doubler mode. As the input supply voltage

rises the allowable output voltage drops, finally reaching

– 4V with an 8.5V supply. To avoid this problem use

doubler mode whenever possible with high input supply

voltages.

Figure 1. Flying Capacitor Connections

1

2

3

4

5

6

7

14

13

12

11

10

9

8

C1

C2

LTC1261 • F01

1

2

3

4

5

6

7

14

13

12

11

10

9

8

C1

1

2

3

4

8

7

6

5

C1

LTC1261

a.) LTC1261CS8

DOUBLER MODE

b.) LTC1261CS

DOUBLER MODE

c.) LTC1261CS

TRIPLER MODE

THEORY OF OPERATION

A block diagram of the LTC1261 is shown in Figure 2. The

heart of the LTC1261 is the charge pump core shown in

the dashed box. It generates a negative output voltage by

ground. It then stacks the flying capacitors on top of each

other and connects the top of the stack to ground forcing

the bottom of the stack to a negative voltage. The charge

on the flying capacitors is transferred to the output bypass

capacitor, leaving it charged to the negative output volt-

age. This process is driven by the internal clock.

Figure 2 shows the charge pump configured in tripler

by S1, S3, S5 and S7. At the next rising clock edge, S1, S3,

S5 and S7 open and S2, S4 and S6 close, stacking C1 and

setting it to a negative voltage. Doubler mode works the

same way except that the single flying capacitor (C1) is

anything useful in doubler mode. C1 is charged initially by

S1 and S7 and connected to the output by S2 and S6.