LTC1751-5 LINER [Linear Technology], LTC1751-5 Datasheet - Page 10

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LTC1751-5

Manufacturer Part Number
LTC1751-5
Description
Micropower, Regulated Charge Pump DC/DC
Manufacturer
LINER [Linear Technology]
Datasheet
APPLICATIO S I FOR ATIO
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:
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 s up to 1M .
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
output voltage, 2V
From Figure 4 the available current is given by:
10
I
R
R
OUT
2
1
Figure 3. Programming the Adjustable LTC1751
1 205
2
V
.
Figure 4. Equivalent Open-Loop Circuit
OUT
V
IN
R
V
OUT
V
V
GND
1751 F03
OUT
IN(MIN)
OUT
FB
U
1
+
2
1
4
2V
R1
R2
.
R
IN
U
OUT
C
OUT
V
1.205V 1 + R1
OUT
W
V
1751 F04
OUT
+
OUT
R2
, and effective
U
Typical R
shown in Figure 5.
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
V
OUT
OUT
IN
Figure 5. Typical R
10
8
2
6
4
0
2.0
values as a function of input voltage are
T
C
GND
Figure 6. Recommended Layout
A
FLY
= 25 C
= 1 F
2.5
INPUT VOLTAGE (V)
3.0
I
OUT
I
OUT
OUT
= 100mA
= 50mA
3.5
vs Input Voltage
4.0
1751 F05
4.5
SHDN
17515 F03

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