Datasheet LT3430, LT3430-1 (Analog Devices) - 9

LT3430/LT3430-1
APPLICATIONS INFORMATION
LT3430 VSW TO FREQUENCY L1 SHIFTING OUTPUT 5V 1.4V Q1 ERROR AMPLIFIER + 1.2V R1 R3 R4 1k 2k FB + – C1 BUFFER Q2 R2 TO SYNC CIRCUIT VC GND 3430 F02
Figure 2. Frequency and Current Limit Foldback
tion should be used if resistors are increased beyond the suggested values and short-circuit conditions occur with VOUT USING 100µF CERAMIC 20mV/DIV high input voltage. High frequency pickup will increase OUTPUT CAPACITOR and the protection accorded by frequency and current foldback will decrease. VOUT USING 100µF 0.08Ω
Choosing the Inductor
20mV/DIV TANTALUM OUTPUT CAPACITOR For most applications, the output inductor will fall into the range of 5µH to 47µH (10µH to 100µH for LT3430-1). Lower values are chosen to reduce physical size of the V 3430 F03 IN = 40V 2µs/DIV VOUT = 5V inductor. Higher values allow more output current because L = 22µH they reduce peak current seen by the LT3430/LT3430-1 switch, which has a 3A limit. Higher values also reduce
Figure 3. LT3430 Output Ripple Voltage Waveforms. Ceramic vs Tantalum Output Capacitors
output ripple voltage. ceramic output capacitor; the signifi cant decrease in out- When choosing an inductor you will need to consider put ripple voltage is due to the very low ESR of ceramic output ripple voltage, maximum load current, peak induc- capacitors. tor current and fault current in the inductor. In addition, other factors such as core and copper losses, allowable Output ripple voltage is determined by ripple current (ILP-P) component height, EMI, saturation and cost should also through the inductor and the high frequency impedance of be considered. The following procedure is suggested the output capacitor. At high frequencies, the impedance as a way of handling these somewhat complicated and of the tantalum capacitor is dominated by its effective confl icting requirements. series resistance (ESR).
Output Ripple Voltage Tantalum Output Capacitor
Figure 3 shows a comparison of output ripple voltage for The typical method for reducing output ripple voltage the LT3430/LT3430-1 using either a tantalum or ceramic when using a tantalum output capacitor is to increase the output capacitor. It can be seen from Figure 3 that output inductor value (to reduce the ripple current in the inductor). ripple voltage can be signifi cantly reduced by using the The following equations will help in choosing the required 34301fa 9