LT3498
APPLICATIONS INFORMATION—OLED DRIVER
a soft transition occurs between the CTRL2 pin and the
Choosing a Feedback Node
internal reference. Figure 10 shows this behavior.
ThesinglefeedbackresistormaybeconnectedtotheV
pin or to the CAP2 pin (see Figure 11). Regulating the
OUT2
1.500
1.250
1.000
0.750
0.500
0.250
0
V
pin eliminates the output offset resulting from the
OUT2
voltage drop across the output disconnect PMOS. Regu-
lating the CAP2 pin does not compensate for the voltage
drop across the output disconnect, resulting in an output
voltage V
that is slightly lower than the voltage set by
OUT2
the resistor divider. Under most conditions, it is advised
that the feedback resistor be tied to the V
pin.
OUT2
Connecting the Load to the CAP2 Node
0
0.5
0.8
1.0
1.3
1.5
0.3
The efficiency of the converter can be improved by con-
necting the load to the CAP2 pin instead of the V pin.
CTRL2 VOLTAGE (V)
OUT2
3498 F10
The power loss in the PMOS disconnect circuit is then
Figure 10. CTRL2 to FB2 Transfer Curve
made negligible. By connecting the feedback resistor to
the V
pin, no quiescent current will be consumed
OUT2
To set the maximum output voltage, select the values of
in the feedback resistor string during shutdown since
the PMOS transistor will be open (see Figure 12). The
disadvantage of this method is that the CAP2 node can-
not go to ground during shutdown, but will be limited to
R
FB2
according to the following equation:
V
⎛
⎞
OUT2
RFB2 =182•
–1 , kΩ
⎜
⎝
⎟
⎠
1.215
around a diode drop below V . Loads connected to the
IN
When CTRL2 is used to override the internal reference,
the output voltage can be lowered from the maximum
value down to nearly the input voltage level. If the voltage
source driving the CTRL2 pin is located at a distance to
the LT3498, a small 0.1µF capacitor may be needed to
bypass the pin locally.
part should only sink current. Never force external power
supplies onto the CAP2 or V
pins. The larger value
OUT2
output capacitor should be placed on the node to which
the load is connected.
I
CAP1
SW1
V
SW2
LT3498
LED1 CTRL1 GND1 GND2 CTRL2
CAP2
V
OUT2
CAP1
SW1
V
SW2
LT3498
LED1 CTRL1 GND1 GND2 CTRL2
CAP2
V
LOAD
C3
C2
IN
OUT2
IN
R
FB2
R
FB2
FB2
FB2
3498 F12
C2
Figure 12. Improved Efficiency
CAP1
SW1
V
SW2
LT3498
LED1 CTRL1 GND1 GND2 CTRL2
CAP2
V
C3
IN
OUT2
FB2
R
FB2
3498 F11
Figure 11. Feedback Connection Using
the CAP2 Pin or the V Pin
OUT2
3498f
15
Linear [ Linear ]
