Datasheet LT3464 (Analog Devices) - 9

LT3464
U U W U APPLICATIO S I FOR ATIO Capacitor Selection
user to select between using the built-in reference, and The small size and low ESR of ceramic capacitors makes supplying an external reference voltage. The voltage at the them suitable for LT3464 applications. X5R and X7R types CTRL pin can be adjusted while the chip is operating to are recommended because they retain their capacitance alter the output voltage of the LT3464 for purposes such over wider voltage and temperature ranges than other as display dimming or contrast adjustment. To use the types such as Y5V or Z5U. A 1 µF input capacitor and a internal 1.25V reference, the CTRL pin must be held higher 0.22µF to 0.47µF output capacitor are sufficient for most than 1.5V, which can be done by tying it to VIN. When the LT3464 applications. Always use a capacitor with a suffi- CTRL pin is held between 0V and 1.2V the LT3464 will cient voltage rating. Table 2 shows a list of several capaci- regulate the output such that the FB pin voltage is equal to tor manufacturers. Consult the manufacturers for more the CTRL pin voltage. detailed information and for their entire selection of related To set the output voltage, select the values of R1 and R2 parts. according to the following equation (see Figure 4).
Table 2. Recommended Ceramic Capacitor Manufacturers
 V 
MANUFACTURER PHONE URL
R2 = R OUT 1 – 1 Taiyo Yuden 408-573-4150 www.t-yuden.com  VREF  AVX 843-448-9411 www.avxcorp.com Where V Murata 814-237-1431 www.murata.com REF =1.25V if the internal reference is used, or V Kemet 408-986-0424 www.kemet.com REF = VCTRL if VCTRL is between 0V and 1.2V.
Choosing a Feedback Node Output Voltage Ripple
The top of the feedback divider may be connected to the Using low ESR capacitors will help minimize the output OUT pin or to the CAP pin (see Figure 4). Regulating the ripple voltage, but proper selection of the inductor and the OUT pin eliminates the output offset resulting from the output capacitor also plays a big role. The LT3464 pro- voltage drop across the output disconnect. However, in vides energy to the load in bursts by ramping up the the case of a short-circuit fault at the OUT pin, the LT3464 inductor current, then delivering that current to the load. will switch continuously because the FB pin is low. While If too large an inductor value or too small a capacitor value operating in this open-loop condition, the rising voltage at is used, the output ripple voltage will increase because the the CAP pin is limited only by the current limit of the output capacitor will be slightly overcharged each burst cycle. To disconnect. Given worst-case parameters this voltage reduce this effect, a larger output capacitor may be used. may reach 25V. When the short-circuit is removed, the The LT3464 also includes an on-chip phase-lead capacitor OUT pin will bounce up to the voltage on the CAP pin, between the CAP pin and the FB pin to greatly reduce potentially exceeding the set output voltage until the ripple; however, certain applications can benefit from capacitor voltages fall back into regulation. While this is additional capacitance in parallel with the integrated ca- harmless to the LT3464, this should be considered in the pacitor, which may be added externally between the CAP context of the external circuitry if short-circuit events are and FB pins. Typical effective values range from 4.7pF to expected. 20pF. Since the FB pin sits at a low voltage, be sure the Regulating the CAP pin ensures that the voltage on the chosen capacitor has a sufficient voltage rating. OUT pin never exceeds the set output voltage after a short- circuit event. However, this setup does not compensate
Setting Output Voltage and the
for the voltage drop across the output disconnect, result-
Auxiliary Reference Input
ing in an output voltage that is slightly lower than the The LT3464 is equipped with both an internal 1.25V voltage set by the resistor divider. The next section dis- reference and an auxiliary reference input. This allows the cusses how to compensate for this drop. 3464f 9