Datasheet LT3494, LT3494A (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Micropower Low Noise Boost Converters with Output Disconnect |
| Pages / Page | 12 / 8 — APPLICATIONS INFORMATION. Capacitor Selection. Figure 1. CTRL to FB … |
| File Format / Size | PDF / 273 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Capacitor Selection. Figure 1. CTRL to FB Transfer Curve
Model Line for this Datasheet
Text Version of Document
LT3494/LT3494A
APPLICATIONS INFORMATION Capacitor Selection
1.500 The small size and low ESR of ceramic capacitors makes 1.250 them suitable for most LT3494/LT3494A applications. X5R 1.000 and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges 0.750 than other types such as Y5V or Z5U. A 4.7μF input capaci- FB VOLTAGE (V) tor and a 2.2μF to 10μF output capacitor are suffi cient for 0.500 most LT3494/LT3494A applications. Always use a capacitor 0.250 with a suffi cient voltage rating. Many capacitors rated at 2.2μF to 10μF, particularly 0805 or 0603 case sizes, have 0 0 .25 0.5 .75 1.0 1.25 1.5 greatly reduced capacitance when bias voltages are ap- CTRL VOLTAGE (V) plied. Be sure to check actual capacitance at the desired 3494 F01 output voltage. Generally a 1206 size capacitor will be
Figure 1. CTRL to FB Transfer Curve
adequate. A 0.22μF or 0.47μF capacitor placed on the To set the maximum output voltage, select the values of CAP node is recommended to fi lter the inductor current R1 according to the following equation: while the larger 2.2μF to 10μF placed on the VOUT node will give excellent transient response and stability. Table 2 ⎛ VOUT MA ( X) ⎞ R1= 182 • – 1 kΩ shows a list of several capacitor manufacturers. Consult 1 225 ⎝⎜ . ⎠⎟ the manufacturers for more detailed information and for their entire selection of related parts. When CTRL is used to override the internal reference, the output voltage can be lowered from the maximum
Table 2. Recommended Ceramic Capacitor Manufacturers
value down to nearly the input voltage level. If the voltage
MANUFACTURER PHONE URL
source driving the CTRL pin is located at a distance to the Taiyo Yuden 408-573-4150 www.t-yuden.com LT3494/LT3494A, a small 0.1μF capacitor may be needed AVX 843-448-9411 www.avxcorp.com to bypass the pin locally. Murata 814-237-1431 www.murata.com Kemet 408-986-0424 www.kemet.com
Choosing a Feedback Node Setting Output Voltage and
The single feedback resistor may be connected to the VOUT
the Auxiliary Reference Input
pin or to the CAP pin (see Figure 2). Regulating the VOUT pin eliminates the output offset resulting from the voltage The LT3494/LT3494A are equipped with both an internal drop across the output disconnect PMOS. Regulating the 1.225V reference and an auxiliary reference input. This al- CAP pin does not compensate for the voltage drop across lows the user to select between using the built-in reference the output disconnect, resulting in an output voltage VOUT and supplying an external reference voltage. The voltage that is slightly lower than the voltage set by the resistor at the CTRL pin can be adjusted while the chip is operat- divider. Under most conditions, it is advised that the ing to alter the output voltage of the LT3494/LT3494A for feedback resistor be tied to the VOUT pin. purposes such as display dimming or contrast adjustment. To use the internal 1.225V reference, the CTRL pin must be 1 8 1 8 C1 C1 held higher than 1.5V. When the CTRL pin is held between SW CAP SW CAP 3 7 3 7 0V and 1.5V, the LT3494 will regulate the output such that VCC VOUT VOUT VCC VOUT the FB pin voltage is nearly equal to the CTRL pin voltage. LT3494 R1 C3 LT3494 R1 5 6 5 6 SHDN FB SHDN FB At CTRL voltages close to 1.225V, a soft transition occurs 4 2 4 2 CTRL GND CTRL GND between the CTRL pin and the internal reference. Figure 1 3494 F02 shows this behavior.
Figure 2. Feedback Connection Using the CAP Pin or the VOUT Pin
3494fb 8
APPLICATIONS INFORMATION Capacitor Selection
1.500 The small size and low ESR of ceramic capacitors makes 1.250 them suitable for most LT3494/LT3494A applications. X5R 1.000 and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges 0.750 than other types such as Y5V or Z5U. A 4.7μF input capaci- FB VOLTAGE (V) tor and a 2.2μF to 10μF output capacitor are suffi cient for 0.500 most LT3494/LT3494A applications. Always use a capacitor 0.250 with a suffi cient voltage rating. Many capacitors rated at 2.2μF to 10μF, particularly 0805 or 0603 case sizes, have 0 0 .25 0.5 .75 1.0 1.25 1.5 greatly reduced capacitance when bias voltages are ap- CTRL VOLTAGE (V) plied. Be sure to check actual capacitance at the desired 3494 F01 output voltage. Generally a 1206 size capacitor will be
Figure 1. CTRL to FB Transfer Curve
adequate. A 0.22μF or 0.47μF capacitor placed on the To set the maximum output voltage, select the values of CAP node is recommended to fi lter the inductor current R1 according to the following equation: while the larger 2.2μF to 10μF placed on the VOUT node will give excellent transient response and stability. Table 2 ⎛ VOUT MA ( X) ⎞ R1= 182 • – 1 kΩ shows a list of several capacitor manufacturers. Consult 1 225 ⎝⎜ . ⎠⎟ the manufacturers for more detailed information and for their entire selection of related parts. When CTRL is used to override the internal reference, the output voltage can be lowered from the maximum
Table 2. Recommended Ceramic Capacitor Manufacturers
value down to nearly the input voltage level. If the voltage
MANUFACTURER PHONE URL
source driving the CTRL pin is located at a distance to the Taiyo Yuden 408-573-4150 www.t-yuden.com LT3494/LT3494A, a small 0.1μF capacitor may be needed AVX 843-448-9411 www.avxcorp.com to bypass the pin locally. Murata 814-237-1431 www.murata.com Kemet 408-986-0424 www.kemet.com
Choosing a Feedback Node Setting Output Voltage and
The single feedback resistor may be connected to the VOUT
the Auxiliary Reference Input
pin or to the CAP pin (see Figure 2). Regulating the VOUT pin eliminates the output offset resulting from the voltage The LT3494/LT3494A are equipped with both an internal drop across the output disconnect PMOS. Regulating the 1.225V reference and an auxiliary reference input. This al- CAP pin does not compensate for the voltage drop across lows the user to select between using the built-in reference the output disconnect, resulting in an output voltage VOUT and supplying an external reference voltage. The voltage that is slightly lower than the voltage set by the resistor at the CTRL pin can be adjusted while the chip is operat- divider. Under most conditions, it is advised that the ing to alter the output voltage of the LT3494/LT3494A for feedback resistor be tied to the VOUT pin. purposes such as display dimming or contrast adjustment. To use the internal 1.225V reference, the CTRL pin must be 1 8 1 8 C1 C1 held higher than 1.5V. When the CTRL pin is held between SW CAP SW CAP 3 7 3 7 0V and 1.5V, the LT3494 will regulate the output such that VCC VOUT VOUT VCC VOUT the FB pin voltage is nearly equal to the CTRL pin voltage. LT3494 R1 C3 LT3494 R1 5 6 5 6 SHDN FB SHDN FB At CTRL voltages close to 1.225V, a soft transition occurs 4 2 4 2 CTRL GND CTRL GND between the CTRL pin and the internal reference. Figure 1 3494 F02 shows this behavior.
Figure 2. Feedback Connection Using the CAP Pin or the VOUT Pin
3494fb 8
