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ISL97519A Datasheet(PDF) 6 Page  Intersil Corporation 

ISL97519A Datasheet(HTML) 6 Page  Intersil Corporation 
6 / 9 page 6 FN6683.2 June 30, 2008 During the second cycle, the power FET turns off and the Schottky diode is forward biased, (see Figure 13). The energy stored in the inductor is pumped to the output supplying output current and charging the output capacitor. The Schottky diode side of the inductor is clamped to a Schottky diode above the output voltage. So the voltage drop across the inductor is VIN  VOUT. The change in inductor current during the second cycle is shown in Equation 2: For stable operation, the same amount of energy stored in the inductor must be taken out. The change in inductor current during the two cycles must be the same as shown in Equation 3. Output Voltage An external feedback resistor divider is required to divide the output voltage down to the nominal 1.24V reference voltage. The current drawn by the resistor network should be limited to maintain the overall converter efficiency. The maximum value of the resistor network is limited by the feedback input bias current and the potential for noise being coupled into the feedback pin. A resistor network less than 100k is recommended. The boost converter output voltage is determined by the relationship in Equation 4: The nominal VFB voltage is 1.24V. Inductor Selection The inductor selection determines the output ripple voltage, transient response, output current capability, and efficiency. Its selection depends on the input voltage, output voltage, switching frequency, and maximum output current. For most applications, the inductance should be in the range of 2µH to 33µH. The inductor maximum DC current specification must be greater than the peak inductor current required by the regulator.The peak inductor current can be calculated in Equation 5: Output Capacitor Low ESR capacitors should be used to minimized the output voltage ripple. Multilayer ceramic capacitors (X5R and X7R) are preferred for the output capacitors because of their lower ESR and small packages. Tantalum capacitors with higher ESR can also be used. The output ripple can be calculated as shown in Equation 6: For noise sensitive application, a 0.1µF placed in parallel with the larger output capacitor is recommended to reduce the switching noise coupled from the LX switching node. ΔI L ΔT2 V IN V OUT – L  × = ΔT2 1D – F SW  = (EQ. 2) ΔI1 ΔI2 + 0 = D F SW  V IN L  1D – F SW  V IN V OUT – L  × + × 0 = V OUT V IN  1 1D –  = (EQ. 3) ISL97519A COUT CIN LD VIN VOUT COUT CIN LD VIN VOUT FIGURE 11. BOOST CONVERTER COUT CIN L VIN VOUT ΔT1 ΔV O IL ΔIL1 ISL97519A FIGURE 12. BOOST CONVERTER  CYCLE 1, POWER SWITCH CLOSE ISL97519A COUT CIN LD VIN VOUT ΔT2 ΔVO ΔIL2 IL FIGURE 13. BOOST CONVERTER  CYCLE 2, POWER SWITCH OPEN V OUT V FB 1 R 1 R 2  + ⎝⎠ ⎜⎟ ⎛⎞ × = (EQ. 4) I L PEAK () I OUT V OUT × V IN  12 ⁄ V IN V OUT V IN – () × LV OUT FREQ × ×  × + = (EQ. 5) ΔV O I OUT D × F SW C O ×  I OUT ESR × + = (EQ. 6) ISL97519A 
