LTC1772B APPLICATIONS INFORMATION size or height requirements in the design. Due to the high Effi ciency Considerations operating frequency of the LTC1772B, ceramic capacitors The effi ciency of a switching regulator is equal to the output can also be used for CIN. Always consult the manufacturer power divided by the input power times 100%. It is often if there is any question. useful to analyze individual losses to determine what is The selection of COUT is driven by the required effective limiting the effi ciency and which change would produce series resistance (ESR). Typically, once the ESR require- the most improvement. Effi ciency can be expressed as: ment is satisfi ed, the capacitance is adequate for fi ltering. Effi ciency = 100% – (η1 + η2 + η3 + ...) The output ripple (ΔVOUT) is approximated by: where η1, η2, etc. are the individual losses as a percent- 1 V age of input power. OUT IRIPPLE ES R + 4fC OUT Although all dissipative elements in the circuit produce where f is the operating frequency, C losses, four main sources usually account for most of OUT is the output capacitance and I the losses in LTC1772B circuits: 1) LTC1772B DC bias RIPPLE is the ripple current in the induc- tor. The output ripple is highest at maximum input voltage since ΔIL increases with input voltage. 105 Manufacturers such as Nichicon, United Chemicon and VREF 100 Sanyo should be considered for high performance through- hole capacitors. The OS-CON semiconductor dielectric 95 VITH capacitor available from Sanyo has the lowest ESR (size) 90 product of any aluminum electrolytic at a somewhat higher price. Once the ESR requirement for C 85 OUT has been met, the RMS current rating generally far exceeds the I NORMALIZED VOLTAGE (%) RIPPLE(P-P) 80 requirement. 752.0 2.2 2.4 2.6 2.8 3.0 Low Supply Operation INPUT VOLTAGE (V) Although the LTC1772B can function down to approxi- 1772 F03 mately 2.0V, the maximum allowable output current is Figure 4. Line Regulation of VREF and VITH reduced when VIN decreases below 3V. Figure 4 shows the amount of change as the supply is reduced down to 2V. Also shown in Figure 4 is the effect of VIN on VREF as VOUT VIN goes below 2.3V. LTC1772B R2 3 VFB Setting Output Voltage R1 The regulated output voltage is determined by: 1772 F04 R2 VOUT = 0.8 1 + Figure 5. Setting Output Voltage R1 For most applications, an 80k resistor is suggested for R1. To prevent stray pickup, locate resistors R1 and R2 close to LTC1772B. 1772bfa 10
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