LT1937 LINER [Linear Technology], LT1937 Datasheet - Page 15

• Manufacturer Part Number: LT1937
• Description: 20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION— OLED DRIVER
a soft transition occurs between the CTRL2 pin and the
internal reference. Figure 10 shows this behavior.
To set the maximum output voltage, select the values of
R
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.
FB2
R
FB
according to the following equation:
2
1.500
1.250
0.250
CAP1
CAP1
1.000
0.750
0.500
LED1
LED1
182
Figure 10. CTRL2 to FB2 Transfer Curve
Figure 11. Feedback Connection Using
the CAP2 Pin or the V
0
0
•
CTRL1
CTRL1
SW1
SW1
V
1 215
0.3
OUT
.
GND1
GND1
V
V
IN
IN
LT3498
LT3498
CTRL2 VOLTAGE (V)
2
0.5
–
GND2
GND2
SW2
SW2
1
0.8
,
OUT2
CAP2
CTRL2
CAP2
CTRL2
k
1.0
Pin
V
V
OUT2
OUT2
FB2
FB2
1.3
C2
3498 F10
1.5
R
R
FB2
FB2
3498 F11
C2
C3
Choosing a Feedback Node
The single feedback resistor may be connected to the V
pin or to the CAP2 pin (see Figure 11). Regulating the
V
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
the resistor divider. Under most conditions, it is advised
that the feedback resistor be tied to the V
Connecting the Load to the CAP2 Node
The effi ciency of the converter can be improved by con-
necting the load to the CAP2 pin instead of the V
The power loss in the PMOS disconnect circuit is then
made negligible. By connecting the feedback resistor to
the V
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
around a diode drop below V
part should only sink current. Never force external power
supplies onto the CAP2 or V
output capacitor should be placed on the node to which
the load is connected.
OUT2
CAP1
LED1
OUT2
pin eliminates the output offset resulting from the
CTRL1
SW1
OUT2
pin, no quiescent current will be consumed
that is slightly lower than the voltage set by
GND1
V
Figure 12. Improved Effi ciency
IN
LT3498
SW2
GND2
CAP2
CTRL2
OUT2
IN
V
. Loads connected to the
OUT2
FB2
3498 F12
pins. The larger value
R
FB2
OUT2
LT3498
C3
pin.
OUT2
15
OUT2
I
pin.
LOAD
3498f