Datasheet LTC1265, LTC1265-3.3, LTC1265-5 (Analog Devices) - 8

LTC1265/LTC1265-3.3/LTC1265-5
U U W U APPLICATIO S I FOR ATIO
P-channel switch duty cycle. At high input voltages, the worst-case RMS ripple current in the output capacitor is diode conducts most of the time. As VIN approaches VOUT, given by: the diode conducts only a small fraction of the time. The 150mV most stressful condition for the diode is when the output IRMS ≈ (ARMS) 2(R is short circuited. Under this condition, the diode must SENSE) safely handle ISC(PK) at close to 100% duty cycle. Most Generally, once the ESR requirement for COUT has been LTC1265 circuits will be well served by either a 1N5818 or met, the RMS current rating far exceeds the IRIPPLE(P-P) a MBRS130LT3 Schottky diode. An MBRS0520 is a good requirement. choice for IOUT(MAX) ≤ 500mA. ESR is a direct function of the volume of the capacitor.
C
Manufacturers such as Nichicon, AVX and Sprague should
IN
be considered for high performance capacitors. The In continuous mode, the input current of the converter is OS-CON semiconductor dielectric capacitor available a square wave of duty cycle VOUT/ VIN. To prevent large from Sanyo has the lowest ESR for its size at a somewhat voltage transients, a low ESR input capacitor must be higher price. used. In addition, the capacitor must handle a high RMS current. The C In surface mount applications, multiple capacitors may IN RMS current is given by: have to be paralleled to meet the capacitance, ESR or RMS 1 I /2 current handling requirement of the application. Alumi- I OUT [VOUT (VIN – VOUT)] RMS ≈ (A V RMS) num electrolyte and dry tantalum capacitors are both IN available in surface mount configurations. In the case of This formula has a maximum at VIN = 2VOUT, where IRMS tantalum, it is critical that the capacitors are both available = IOUT/2. This simple worst case is commonly used for in surface mount configuration and are surge tested for design because even significant deviations do not offer use in switching power supplies. An excellent choice is the much relief. Note that capacitor manufacturer’s ripple AVX TPS series of surface mount tantalums, available in current ratings are often based on only 2000 hours life- case heights ranging from 2mm to 4mm. Consult the time. This makes it advisable to further derate the capaci- manufacturer for other specific recommendations. tor, or to choose a capacitor rated at a higher temperature than required.
Do not underspecify this component
. An When the capacitance of COUT is made too small, the additional 0.1µF ceramic capacitor is also required on output ripple at low frequencies will be large enough to trip PWR V the voltage comparator. This causes Burst Mode opera- IN for high frequency decoupling. tion to be activated when the LTC1265 would normally be
COUT
in continuous operation. The effect will be most pro- The selection of C nounced with low value of RSENSE and can be improved at OUT is based upon the effective series higher frequencies with lower values of L. resistance (ESR) for proper operation of the LTC1265. The required ESR of COUT is:
Low-Battery Detection
ESRCOUT < 50mV/IRIPPLE The low-battery comparator senses the input voltage where IRIPPLE is the ripple current of the inductor. For the through an external resistive divider. This divided voltage case where the IRIPPLE is 25mV/RSENSE, the required ESR connects to the (–) input of a voltage comparator (Pin 4) of COUT is: which is compared with a 1.25V reference voltage. Ne- glecting Pin 4 bias current, the following expression is ESRCOUT < 2(RSENSE) used for setting the trip limit: To avoid overheating, the output capacitor must be sized to handle the ripple current generated by the inductor. The ) V ) LB_TRIP = 1.25 1 + R4 R3 8