Datasheet LTC3035 (Analog Devices) - 7

LTC3035
U U W U APPLICATIO S I FOR ATIO
V R2 OUT VOUT = 0.4V 1 + ( ) R1 R2 VOUT ADJ COUT 20mV/DIV AC R1 GND 3035 F04 300mA IOUT
Figure 4. Programming the LTC3035
0mA V 200 3035 F02 IN = 3.6V µs/DIV current change of 1mA to 300mA produces a – 0.2mV VOUT = 3.3V drop at the ADJ input. To calculate the change refered to COUT = 1µF the output simply multiply by the gain of the feedback
Figure 2. Output Step Response
network (i.e., 1 + R2/R1). For example, to program the output for 3.3V choose R2/R1 = 7.25. In this example an output current change of 1mA to 300mA produces ON SHDN –0.2mV • (1 + 7.25) = 1.65mV drop at the output. OFF V
Output Capacitance and Transient Response
BIAS 2V/DIV The LTC3035 is designed to be stable with a wide range of 0V ceramic output capacitors. The ESR of the output capaci- VOUT 2V/DIV tor affects stability, most notably with small capacitors. An 0V output capacitor of 1µF or greater with an ESR of 0.05Ω or 3035 F03 less is recommended to ensure stability. The LTC3035 is V 500 IN = 3.6V µs/DIV VOUT = 3.3V a micropower device and output transient response will be COUT = 1µF CBIAS = 1µF a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide
Figure 3. Bias and Output Start-Up Waveforms
improved transient response for larger load current changes. Note that bypass capacitors used to decouple
Adjustable Output Voltage
individual components powered by the LTC3035 will The output voltage is set by the ratio of two external increase the effective output capacitor value. High ESR resistors as shown in Figure 4. The device servos the tantalum and electrolytic capacitors may be used, but a output to maintain the ADJ pin voltage at 0.4V (referenced low ESR ceramic capacitor must be in parallel at the to ground). Thus the current in R1 is equal to 0.4V/R1. For output. There is no minimum ESR or maximum capacitor good transient response, stability and accuracy the size requirements. current in R1 should be at least 8µA, thus the value of R1 Extra consideration must be given to the use of ceramic should be no greater than 50k. The current in R2 is the capacitors. Ceramic capacitors are manufactured with a current in R1 plus the ADJ pin bias current. Since the ADJ variety of dielectrics, each with different behavior across pin bias current is typically <10nA it can be ignored in the temperature and applied voltage. The most common di- output voltage calculation. The output voltage can be electrics used are Z5U, Y5V, X5R and X7R. The Z5U and calculated using the formula in Figure 4. Note that in Y5V dielectrics are good for providing high capacitances shutdown the output is turned off and the divider current in a small package, but exhibit strong voltage and tem- will be zero once COUT is discharged. perature coefficients as shown in Figures 5 and 6. When The LTC3035 operates at a relatively high gain of used with a 3.3V regulator, a 1µF Y5V capacitor can lose –0.7µV/mA referred to the ADJ input. Thus a load as much as 80% of its rated capacitance over the operating 3035f 7