Datasheet LT1767, LT1767-1.8, LT1767-2.5, LT1767-3.3, LT1767-5 (Analog Devices) - 9

LT1767/LT1767-1.8/ LT1767-2.5/LT1767-3.3/LT1767-5
applicaTions inForMaTion
input capacitor is reduced so values in the range of 1µF Figure 3 shows a comparison of output ripple for a ceramic to 4.7µF are suitable for most applications. Y5V or similar and tantalum capacitor at 200mA ripple current. type ceramics can be used since the absolute value of ca- pacitance is less important and has no significant effect on loop stability. If operation is required close to the minimum VOUT USING 47µF, 0.1Ω input required by the output of the LT1767, a larger value TANTALUM CAPACITOR (10mV/DIV) may be required. This is to prevent excessive ripple causing dips below the minimum operating voltage, resulting in erratic operation. VOUT USING 2.2µF CERAMIC CAPACITOR If tantalum capacitors are used, values in the 22µF to 470µF (10mV/DIV) range are generally needed to minimize ESR and meet ripple current and surge ratings. Care should be taken to ensure the ripple and surge ratings are not exceeded. The AVX TPS VSW and Kemet T495 series are surge rated. AVX recommends (5V/DIV) derating capacitor operating voltage by 2:1 for high surge applications. 0.2µs/DIV 1767 F03
Figure 3. Output Ripple Voltage Waveform OUTPUT CAPACITOR INDUCTOR CHOICE AND MAXIMUM OUTPUT
Unlike the input capacitor, RMS ripple current in the output
CURRENT
capacitor is normally low enough that ripple current rating is not an issue. The current waveform is triangular, with Maximum output current for a buck converter is equal an RMS value given by: to the maximum switch rating (IP) minus one half peak to peak inductor current. In past designs, the maximum 0.29(V )(V ) I OUT IN − VOUT switch current has been reduced by the introduction of RIPPLE R ( MS) = L ( )(f)(VIN) slope compensation. Slope compensation is required at duty cycles above 50% to prevent an affect called sub- The LT1767 will operate with both ceramic and tantalum harmonic oscillation (see Application Note 19 for details). output capacitors. Ceramic capacitors are generally chosen The LT1767 has a new circuit technique that maintains a for their small size, very low ESR (effective series resistance), constant switch current rating at all duty cycles. and good high frequency operation, reducing output ripple For most applications, the output inductor will be in the voltage. Typical ceramic output capacitors are in the 4.7µF 1µH to 10µH range. Lower values are chosen to reduce to 47µF range. Since the absolute value of capacitance the physical size of the inductor, higher values allow defines the pole frequency of the output stage, an X7R or higher output currents due to reduced peak to peak ripple X5R type ceramic, which have good temperature stability, current, and reduces the current at which discontinuous is recommended. operation occurs. The following formula gives maximum Tantalum capacitors are usually chosen for their bulk capac- output current for continuous mode operation, implying itance properties, useful in high transient load applications. that the peak to peak ripple (2x the term on the right) is ESR rather than capacitive value defines output ripple at less than the maximum switch current. 1.25MHz. Typical LT1767 applications require a tantalum capacitor with less than 0.3Ω ESR at 22µF to 500µF. 1767fb For more information www.linear.com/LT1767 9 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Application