Datasheet LT3653 (Analog Devices) - 8

LT3653
APPLICATIONS INFORMATION Inductor Selection
The input capacitor is required to reduce the resulting volt- age ripple at the LT3653 and to force this very high frequency A 4.7μH inductor is recommended for most LT3653 ap- switching current into a tight local loop, minimizing EMI. plications. This value provides a good tradeoff between Place the capacitor in close proximity to the LT3653 and size and ripple current. The inductor’s RMS current rating the catch diode (see the PCB Layout section). must be greater than the maximum load current and its saturation current should be about 30% higher. The output
Output Capacitor
current limit circuit tightly controls the maximum average inductor current therefore the inductor RMS current rating A 10μF or greater ceramic capacitor is required for low does not have to be overrated to handle short-circuit or output ripple and good transient response. Ceramic ca- overload conditions. For high effi ciency, keep the series pacitors have very low equivalent series resistance (ESR) resistance (DCR) less than 0.1Ω. Output voltage ripple and provide the best ripple performance. Use X5R or X7R can be reduced by using a higher value inductor. The cost types and keep in mind that a ceramic capacitor biased is a larger physical size and poorer transient response. with VOUT has less than its nominal capacitance. A lower value inductor has higher ripple currents but is High performance electrolytic capacitors can be used for physically smaller or, for the same size, has lower DCR the output capacitor. Low ESR is important, so choose usually resulting in higher effi ciency. one that is intended for use in switching regulators. Keep the ESR less than 0.1Ω.
Catch Diode I
The catch diode conducts current only during switch off
LIM Resistor
time. Average forward current in normal operation is The LT3653 output current limit controls the maximum calculated from: current delivered from the LT3653 regulator. This allows tighter control of the system power dissipation and also V V I I IN OUT protects the inductor and diode from overheating during D(AVG) = − OUT V IN an overload or short-circuit condition. A resistor connected from the ILIM pin to GND programs the output current where IOUT is the maximum output load current pro- limit. Table 1 details the ILIM resistor values for specifi c grammed by the ILIM resistor. Peak reverse voltage is desired output current limits equal to the regulator input voltage. Use a Schottky diode
Table 1. Output Current Limit vs R
with a reverse voltage rating greater than the maximum
ILIM Value
input voltage. The overvoltage protection feature in the
OUTPUT CURRENT LIMIT (A) RILIM VALUE (kΩ)
LT3653 keeps the switch off when V 0.4 121 IN > 33V (typical), allowing the use of a 40V rated Schottky, even when V 0.6 66.5 IN ranges up to 60V. 0.8 45.3 1 33.2
Input Capacitor
1.2 27.4 Bypass the input of the LT3653 circuit with a 4.7μF or higher value ceramic capacitor of X7R or X5R type. Y5V types have
Boost Capacitor Selection
poor performance over temperature and applied voltage and The boost capacitor is calculated with the following for- should not be used. If the input power source has high im- mula: pedance, or there is signifi cant inductance due to long wires or cables, additional bulk capacitance may be necessary. I ( ) ( ) • V C OUT MAX OUT = This can be provided with a low performance electrolytic BOOST . 0 V 1 •30•V • . 1 M 5 Hz IN capacitor. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. Typically, a 0.1μF capacitor is used. 3653f 8