Datasheet LT1936 (Analog Devices) - 9

LT1936
APPLICATIONS INFORMATION
inductance may result in discontinuous mode operation, combined with trace or cable inductance forms a high which is okay but further reduces maximum load current. quality (under damped) tank circuit. If the LT1936 circuit For details of maximum output current and discontinuous is plugged into a live supply, the input voltage can ring to mode operation, see Linear Technology Application Note twice its nominal value, possibly exceeding the LT1936’s 44. Finally, for duty cycles greater than 50% (VOUT/VIN voltage rating. This situation is easily avoided; see the Hot > 0.5), there is a minimum inductance required to avoid Plugging Safety section. subharmonic oscillations. Choosing L greater than 1.6 For space sensitive applications, a 2.2μF ceramic capaci- (VOUT + VD) μH prevents subharmonic oscillations at all tor can be used for local bypassing of the LT1936 input. duty cycles. However, the lower input capacitance will result in in- creased input current ripple and input voltage ripple, and
Catch Diode
may couple noise into other circuitry. Also, the increased A 1A Schottky diode is recommended for the catch diode, voltage ripple will raise the minimum operating voltage D1. The diode must have a reverse voltage rating equal of the LT1936 to ~3.7V. to or greater than the maximum input voltage. The ON Semiconductor MBRM140 is a good choice. It is rated
Output Capacitor
for 1A DC at a case temperature of 110°C and 1.5A at a The output capacitor has two essential functions. Along case temperature of 95°C. Diode Incorporated’s DFLS140L with the inductor, it fi lters the square wave generated is rated for 1.1A average current; the DFLS240L is rated by the LT1936 to produce the DC output. In this role it for 2A average current. The average diode current in an determines the output ripple, and low impedance at the LT1936 application is approximately IOUT (1 – DC). switching frequency is important. The second function is to store energy in order to satisfy transient loads and
Input Capacitor
stabilize the LT1936’s control loop. Bypass the input of the LT1936 circuit with a 4.7μF or Ceramic capacitors have very low equivalent series re- higher value ceramic capacitor of X7R or X5R type. Y5V sistance (ESR) and provide the best ripple performance. types have poor performance over temperature and ap- A good value is: plied voltage, and should not be used. A 4.7μF ceramic is adequate to bypass the LT1936 and will easily handle 150 C the ripple current. However, if the input power source has OUT = VOUT high impedance, or there is signifi cant inductance due to long wires or cables, additional bulk capacitance may be where COUT is in μF. Use X5R or X7R types. This choice necessary. This can be provided with a low performance will provide low output ripple and good transient response. electrolytic capacitor. Transient performance can be improved with a high value Step-down regulators draw current from the input sup- capacitor if the compensation network is also adjusted to ply in pulses with very fast rise and fall times. The input maintain the loop bandwidth. capacitor is required to reduce the resulting voltage A lower value of output capacitor can be used, but transient ripple at the LT1936 and to force this very high frequency performance will suffer. With an external compensation switching current into a tight local loop, minimizing EMI. network, the loop gain can be lowered to compensate for the A 4.7μF capacitor is capable of this task, but only if it is lower capacitor value. When using the internal compensa- placed close to the LT1936 and the catch diode; see the tion network, the lowest value for stable operation is: PCB Layout section. A second precaution regarding the 66 ceramic input capacitor concerns the maximum input COUT > voltage rating of the LT1936. A ceramic input capacitor V OUT 1936fd 9