Datasheet LT3663 (Analog Devices) - 9

LT3663
applicaTions inForMaTion Catch Diode
Use X5R or X7R types and keep in mind that a ceramic The catch diode conducts current only during switch-off capacitor biased with VOUT has less than its specified time. Average forward current in normal operation is nominal capacitance. calculated from: High performance electrolytic capacitors can be used for V – V the output capacitor. Low ESR is important, so choose I = I IN OUT D(AVG) OUT one that is intended for use in switching regulators. Keep VIN the ESR less than 0.1Ω. where IOUT is the maximum output load current pro-
I
grammed by the I
LIM Resistor
LIM resistor. Peak reverse voltage is equal to the regulator input voltage. Use a Schottky diode with a The LT3663 output current limit controls the maximum reverse voltage rating greater than the input voltage. The current delivered from the LT3663 regulator. This al ows overvoltage protection feature in the LT3663 keeps the tighter control of the system power dissipation and also switch off when V protects the inductor and diode from overheating during IN > 39V (typical) al owing the use of a 45V rated Schottky even when V an overload or short circuit condition. A resistor con- IN ranges up to 60V. nected from ILIM pin to GND programs the output current.
Input Capacitor
Table 1 details the ILIM resistor values for specific output current limits. Bypass the input of the LT3663 circuit with a 1µF or higher value ceramic capacitor of X7R or X5R type. Y5V types
Table 1. Output Current Limit vs RLIM Value
have poor performance over temperature and applied volt-
OUTPUT CURRENT LIMIT (A) RILIM VALUE (kΩ)
age and are not recommended. If the input power source 0.4 140 has high impedance, or there is significant inductance 0.6 75 due to long wires or cables, additional bulk capacitance 0.8 48.7 may be necessary. This can be provided with a low per- 1.0 36.5 formance electrolytic capacitor. Step-down regulators 1.2 28.7 draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3663 and There may be a reason in some applications to move the to force this very high frequency switching current into output current limit. The fol owing simple circuit demon- a tight local loop, minimizing EMI. Place the capacitor in strates how the output current limit can be adjusted: close proximity to the LT3663 and the catch diode; (see the PCB Layout section).
Output Capacitor
ILIM LT3663 RA A 10µF or greater ceramic capacitor is recommended to provide low output ripple and good transient response. ON, R 3663 F03 ILIM = RA RB Ceramic capacitors have very low equivalent series re- OFF, RILIM = RA + RB sistance (ESR) and provide the best ripple performance.
Figure 3. Circuit for Multiple Values of Output Current Limit
3663fc For more information For more information www.linear.com/LT3663 9 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Operation Applications Information Package Description