10

LTC1751/LTC1751-3.3/LTC1751-5

Programming the LTC1751 Output Voltage (FB Pin)

While the LTC1751-3.3/LTC1751-5 versions have internal

resistive dividers to program the output voltage, the

programmable LTC1751 may be set to an arbitrary voltage

via an external resistive divider. Since it employs a voltage

doubling charge pump, it is not possible to achieve output

voltages greater than twice the available input voltage.

Figure 3 shows the required voltage divider connection.

The voltage divider ratio is given by the expression:

R

R

V

V

OUT

1

2

1 205

1

=

.

–

APPLICATIO S I FOR ATIO

2

1

4

FB

GND

R1

1751 F03

R2

1.205V 1 + R1

R2

()

Figure 3. Programming the Adjustable LTC1751

+

+

–

1751 F04

shown in Figure 5.

Figure 4. Equivalent Open-Loop Circuit

The sum of the voltage divider resistors can be made large

to keep the quiescent current to a minimum. Any standing

current in the output divider (given by 1.205V/R2) will be

reflected by a factor of 2 in the input current. Typical values

for total voltage divider resistance can range from several

k

Ωs up to 1MΩ.

Maximum Available Output Current

For the adjustable LTC1751, the maximum available out-

put current and voltage can be calculated from the effec-

From Figure 4 the available current is given by:

I

VV

R

OUT

IN

OUT

OUT

=

2–

Figure 6. Recommended Layout

Layout Considerations

Due to high switching frequency and high transient cur-

rents produced by the LTC1751 product family, careful

board layout is necessary. A true ground plane and short

connections to all capacitors will improve performance and

ensure proper regulation under all conditions. Figure 6

shows the recommended layout configuration.

Thermal Management

For higher input voltages and maximum output current,

there can be substantial power dissipation in the

LTC1751. If the junction temperature increases above

approximately 160

°C, the thermal shutdown circuitry

will automatically deactivate the output. To reduce the

maximum junction temperature, a good thermal connec-

tion to the PC board is recommended. Connecting the

GND pin (Pin 4) to a ground plane, and maintaining a

solid ground plane under the device on two layers of the

PC board, will reduce the thermal resistance of the

package and PC board system considerably.

GND

SHDN

17515 F03

INPUT VOLTAGE (V)

2.0

6

8

10

4.0

1751 F05

4

2

0

2.5

3.0

3.5

4.5