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
WUUU
2
1
4
V
OUT
FB
GND
R1
1751 F03
C
OUT
R2
V
OUT
1.205V 1 +
R1
R2
()
Figure 3. Programming the Adjustable LTC1751
+
–
2V
IN
+
–
V
OUT
1751 F04
R
OUT
Typical R
OUT
values as a function of input voltage are
shown in Figure 5.
Figure 4. Equivalent Open-Loop Circuit
Figure 5. Typical R
OUT
vs Input Voltage
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
kΩs 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-
tive open-loop output resistance, R
OUT
, and effective
output voltage, 2V
IN(MIN)
.
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.
V
OUT
V
IN
GND
SHDN
17515 F03
INPUT VOLTAGE (V)
2.0
OUTPUT RESISTANCE (Ω)
6
8
10
4.0
1751 F05
4
2
0
2.5
3.0
3.5
4.5
T
A
= 25°C
C
FLY
= 1µF
I
OUT
= 100mA
I
OUT
= 50mA
