4

LT1944

BLOCK DIAGRA

Figure 1. LT1944 Block Diagram

OPERATIO

The LT1944 uses a constant off-time control scheme to

provide high efficiencies over a wide range of output

current. Operation can be best understood by referring to

the block diagram in Figure 1. Q1 and Q2 along with R3 and

R4 form a bandgap reference used to regulate the output

voltage. When the voltage at the FB1 pin is slightly above

1.23V, comparator A1 disables most of the internal cir-

cuitry. Output current is then provided by capacitor C2,

which slowly discharges until the voltage at the FB1 pin

drops below the lower hysteresis point of A1 (typical

hysteresis at the FB pin is 8mV). A1 then enables the

internal circuitry, turns on power switch Q3, and the

current in inductor L1 begins ramping up. Once the switch

current reaches 350mA, comparator A2 resets the one-

shot, which turns off Q3 for 400ns. L1 then delivers

current to the output through diode D1 as the inductor

current ramps down. Q3 turns on again and the inductor

current ramps back up to 350mA, then A2 resets the one-

shot, again allowing L1 to deliver current to the output.

This switching action continues until the output voltage is

charged up (until the FB1 pin reaches 1.23V), then A1

turns off the internal circuitry and the cycle repeats. The

LT1944 contains additional circuitry to provide protection

during start-up and under short-circuit conditions. When

the FB1 pin voltage is less than approximately 600mV, the

switch off-time is increased to 1.5

µs and the current limit

is reduced to around 250mA (70% of its normal value).

This reduces the average inductor current and helps

minimize the power dissipation in the power switch and in

the external inductor and diode. The second switching

regulator operates in the same manner.

–

+

–

+

8

400ns

ONE-SHOT

DRIVER

RESET

DRIVER

RESET

ENABLE

42mV

0.12

Ω

A2

A1

Q3

9

3

R4

140k

R3

30k

R6

40k

R5

40k

Q2

X10

Q1

1

FB1

2

SHDN1

10

SW1

PGND

GND

1944 BD

L1

C2

D1

R2

(EXTERNAL)

R1

(EXTERNAL)

C1

–

+

–

+

400ns

ONE-SHOT

ENABLE

42mV

0.12

Ω

A2B

A1B

Q3B

7

R4B

140k

R3B

30k

R6B

40k

R5B

40k

Q2B

X10

Q1B

5

FB2

4

SHDN2

6

SW2

PGND

L2

C3

D2

R2B

(EXTERNAL)

R1B

(EXTERNAL)