Datasheet LT1765, LT1765-1.8, LT1765-2.5, LT1765-3.3, LT1765-5 (Analog Devices) - 8

LT1765/LT1765-1.8/LT1765-2.5/ LT1765-3.3/LT1765-5
APPLICATIONS INFORMATION FB RESISTOR NETWORK
rating and turn-on surge problems. Y5V or similar type ceramics can be used since the absolute value of capaci- If an output voltage of 1.8V, 2.5V, 3.3V or 5V is required, tance is less important and has no signifi cant effect on the respective fi xed option part, -1.8, -2.5, -3.3 or -5, loop stability. If operation is required close to the minimum should be used. The FB pin is tied directly to the output; input required by the output or the LT1765, a larger value the necessary resistive divider is already included on may be required. This is to prevent excessive ripple caus- the part. For other voltage outputs, the adjustable part ing dips below the minimum operating voltage resulting should be used and an external resistor divider added. in erratic operation. The suggested resistor (R2) from FB to ground is 10k. This reduces the contribution of FB input bias current to If tantalum capacitors are used, values in the 22μF to 470μF output voltage to less than 0.25%. The formula for the range are generally needed to minimize ESR and meet resistor (R1) from VOUT to FB is: ripple current and surge ratings. Care should be taken to ensure the ripple and surge ratings are not exceeded. The R2(V – 1 2 . ) R OUT 1= AVX TPS and Kemet T495 series tantalum capacitors are . – ( . μ 1 2 R2 0 25 A) surge rated. AVX recommends derating capacitor operating voltage by 2:1 for high surge applications. LT1765 (ADJ) VSW OUTPUT
OUTPUT CAPACITOR
ERROR AMPLIFIER + 1.2V Unlike the input capacitor, RMS ripple current in the output R1 capacitor is normally low enough that ripple current rating FB + – is not an issue. The current waveform is triangular, with an RMS value given by: R2 10k 0 2 . 9 V V V ( )( − ) 1765 F02 I OUT IN OUT V ( ) = C GND RIPPLE RMS L f V ( )( )( ) IN
Figure 2. Feedback Network
The LT1765 will operate with both ceramic and tantalum
INPUT CAPACITOR
output capacitors. Ceramic capacitors are generally cho- sen for their small size, very low ESR (effective series Step-down regulators draw current from the input supply resistance), and good high frequency operation. Ceramic in pulses. The rise and fall times of these pulses are very output capacitors in the 1μF to 10μF range, X7R or X5R fast. The input capacitor is required to reduce the voltage type are recommended. ripple at the input of LT1765 and to force the switching current into a tight local loop, thereby minimizing EMI. Tantalum capacitors are usually chosen for their bulk The RMS ripple current can be calculated from: capacitance properties, useful in high transient load ap- plications. ESR rather than absolute value defi nes output I I V V V V ( )/ 2 ripple at 1.25MHz. Typical LT1765 applications require a RIPPLE RMS OUT OUT IN OUT IN ( ) = − tantalum capacitor with less than 0.3Ω ESR at 22μF to Ceramic capacitors are ideal for input bypassing. At higher 500μF, see Table 2. This ESR provides a useful zero in the switching frequency, the energy storage requirement of frequency response. Ceramic output capacitors with low the input capacitor is reduced so values in the range of ESR usually require a larger VC capacitor or an additional 1μF to 4.7μF are suitable for most applications. Their high series R to compensate for this. frequency capacitive nature removes most ripple current 1765fd 8