4

LT3463/LT3463A

3463f

BLOCK DIAGRA

Figure 1. Block Diagram

OPERATIO

The LT3463 uses a constant off-time control scheme to

provide high efficiency over a wide range of output cur-

rent. Operation can be best understood by referring to the

block diagram in Figure 1. When the voltage at the FB1 pin

is slightly above 1.25V, comparator A1 disables most of

the internal circuitry. Output current is then provided by

capacitor C2, which slowly discharges until the voltage at

the FB1 pin goes below the hysteresis point of A1 (typical

hysteresis at the FB1 pin is 8mV). A1 then enables the

internal circuitry, turns on power switch Q1, and the

current in inductor L1 begins ramping up. Once the switch

current reaches 250mA, comparator A2 resets the one-

shot, which turns off Q1 for 300ns. Q1 turns on again and

the inductor currents ramp back up to 250mA, then A2

again resets the one-shot. This switching action continues

until the output voltage is charged up (until the FB1 pin

reaches 1.25V), then A1 turns off the internal circuitry and

the cycle repeats. The second switching regulator is an

inverting converter (which generates a negative output)

but the basic operation is the same.

SHDN1 (Pin 9): Shutdown Pin for Switcher 1. Pull this pin

above 1.5V to enable Switcher 1. Pull below 0.3V to turn

it off. Do not leave this pin floating.

FB1 (Pin 10): Feedback Pin for Switcher 1. Set the output

voltage by selecting values for R1 and R2.

GND (Pin 11): Exposed Pad. Solder this exposed pad

directly to the local ground plane. This pad must be

electrically connected for proper operation.

PI FU CTIO S

–

+

–

+

–

+

3

300ns

ONE-SHOT

25mV

25mV

1.25V

1.25V

A2

A1

Q1

10

FB1

FB2

1

SW1

3463 F01

L1

R1

R2

R4

R3

C1

C2

C3

–

+

300ns

ONE-SHOT

A4

A3

Q2

11

8

4

7

6

5

SW2

GND

D2

D2

D1

D3

SWITCHER 1

SWITCHER 2

L2

2

C4

SHDN2

SHDN2

9

SHDN1

SHDN1