Datasheet LT3495, LT3495B, LT3495-1, LT3495B-1 (Analog Devices) - 10

LT3495/LT3495B/ LT3495-1/LT3495B-1
APPLICATIONS INFORMATION Setting Output Voltage and the Auxiliary Reference Choosing a Feedback Node Input
The single feedback resistor may be connected to the The LT3495 series is equipped with both an internal VOUT pin or to the CAP pin (see Figure 2). Regulating the 1.235V reference and an auxiliary reference input. This VOUT pin eliminates the output offset resulting from the allows the user to select between using the built-in refer- voltage drop across the output disconnect PMOS. Regu- ence and supplying an external reference voltage. The lating the CAP pin does not compensate for the voltage voltage at the CTRL pin can be adjusted while the chip is drop across the output disconnect, resulting in an output operating to alter the output voltage for purposes such voltage VOUT that is slightly lower than the voltage set by as display dimming or contrast adjustment. To use the the resistor divider. Under most conditions, it is advised internal 1.235V reference, the CTRL pin must be held that the feedback resistor be tied to the VOUT pin. higher than 1.5V. When the CTRL pin is held between 0V and 1.235V, the parts will regulate the output such that
Connecting the Load to the CAP Node
the FB pin voltage is nearly equal to the CTRL pin voltage. The efficiency of the converter can be improved by con- At CTRL voltages close to 1.235V, a soft transition occurs necting the load to the CAP pin instead of the V between the CTRL pin and the internal reference. Figure 1 OUT pin. The power loss in the PMOS disconnect circuit is then shows this behavior. made negligible. By connecting the feedback resistor to To set the maximum output voltage, select the values of the VOUT pin, no quiescent current will be consumed in the R1 according to the following equation: feedback resistor string during shutdown since the PMOS transistor will be open (see Figure 3). The disadvantage ⎛ V R1 OUT ⎞ = 76 • – 1 of this method is that the CAP node cannot go to ground ⎝⎜ 1.235 ⎠⎟ kΩ during shutdown, but will be limited to around a diode drop below V When CTRL is used to override the internal reference, CC. Loads connected to the part should only sink current. Never force external power supplies onto the output voltage can be lowered from the maximum the CAP or V value down to nearly the input voltage level. If the volt- OUT pins. age source driving the CTRL pin is located at a distance C1 C1 to the LT3495, a small 0.1µF capacitor may be needed to SW CAP SW CAP bypass the pin locally. VCC VOUT VOUT VCC VOUT LT3495 R1 C3 LT3495 R1 SHDN FB SHDN FB 1.5 CTRL GND CTRL GND 3495 F02 1.2
Figure 2. Feedback Connection Using the CAP Pin or the VOUT Pin
0.9 TAGE (V) 0.6 FB VOL C1 ILOAD SW CAP 0.3 VCC VOUT LT3495 0 SHDN FB 0 0.3 0.6 0.9 1.2 1.5 CTRL VOLTAGE (V) CTRL GND 3495 F01 3495 F03
Figure 1. CTRL to FB Transfer Curve Figure 3. Improved Efficiency Connection
3495b1b1fa 10 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Infomation Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts