4

LT1945

1945f

BLOCK DIAGRA

Figure 1. LT1945 Block Diagram

OPERATIO

The LT1945 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 NFB1 pin is slightly below

–1.23V, comparator A1 disables most of the internal

circuitry. Output current is then provided by capacitor C2,

which slowly discharges until the voltage at the NFB1 pin

goes above the hysteresis point of A1 (typical hysteresis

at the NFB1 pin is 8mV). A1 then enables the internal

circuitry, turns on power switch Q3, and the current in

inductors L1 and L2 begins ramping up. Once the switch

current reaches 350mA, comparator A2 resets the one-

shot, which turns off Q3 for 400ns. L2 continues to deliver

current to the output while Q3 is off. Q3 turns on again and

the inductor currents ramp back up to 350mA, then A2

again resets the one-shot. This switching action continues

until the output voltage is charged up (until the NFB1 pin

reaches –1.23V), then A1 turns off the internal circuitry

and the cycle repeats.

The second switching regulator is a step-up converter

(which generates a positive output) but the basic opera-

tion is the same.The LT1945 contains additional circuitry

to provide protection during start-up and under short-

circuit conditions. When the FB2 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.

–

+

–

+

8

400ns

ONE-SHOT

DRIVER

RESET

RESET

ENABLE

42mV

0.12

Ω

A2

A1

Q3

9

3

R4

280k

R3

60k

R6

80k

R5

80k

Q2

X10

Q1

1

NFB1

2

SHDN1

10

SW1

PGND

GND

1945 BD

L1

C3

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

L3

L2

C4

D2

R2B

(EXTERNAL)

R1B

(EXTERNAL)

SWITCHER 1

SWITCHER 2

DRIVER