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ISL97656IRTZ-TK Datasheet(PDF) 5 Page - Intersil Corporation |
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ISL97656IRTZ-TK Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 8 page 5 FN6439.4 April 2, 2010 Applications Information The ISL97656 is a high frequency, high efficiency boost regulator operated at constant frequency PWM mode. The boost converter stores energy from an input voltage source and deliver it to a higher output voltage. The input voltage range is 2.3V to 6.0V and output voltage range is 5V to 25V. The switching frequency is selectable between 640kHz and 1.2MHz allowing smaller inductors and faster transient response. An external compensation pin gives the user greater flexibility in setting output transient response and tighter load regulation. The converter soft-start characteristic can also be controlled by external CSS capacitor. The EN pin allows the user to completely shutdown the device. Boost Converter Operations Figure 8 shows a boost converter with all the key components. In steady state operating and continuous conduction mode where the inductor current is continuous, the boost converter operates in two cycles. During the first cycle, as shown in Figure 9, the internal power FET turns on and the Schottky diode is reverse biased and cuts off the current flow to the output. The output current is supplied from the output capacitor. The voltage across the inductor is VIN and the inductor current ramps up in a rate of VIN/L, L is the inductance. The inductance is magnetized and energy is stored in the inductor. The change in inductor current is shown in Equation 1: During the second cycle, the power FET turns off and the Schottky diode is forward biased, (see Figure 10). 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 clamp 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. FIGURE 7. TRANSIENT RESPONSE Typical Performance Curves (Continued) VO = 12V fs = 1.2MHz VIN = 3.3V IO = 50mA to 300mA ΔI L1 ΔT1 VIN L --------- × = ΔT1 D fSW ---------- = D Duty Cycle = ΔV O IOUT COUT ---------------- ΔT 1 × = (EQ. 1) ΔI L ΔT2 VIN VOUT – L -------------------------------- × = ΔT2 1D – fSW ------------- = (EQ. 2) ΔI1 ΔI2 + 0 = D fSW ---------- VIN L --------- 1D – fSW ------------- VIN VOUT – L -------------------------------- × + × 0 = VOUT VIN ---------------- 1 1D – ------------- = (EQ. 3) ISL97656 COUT CIN LD VIN VOUT FIGURE 8. BOOST CONVERTER ISL97656 COUT CIN L VIN VOUT ΔT1 ΔVO IL ΔIL1 FIGURE 9. BOOST CONVERTER - CYCLE 1, POWER SWITCH CLOSED ISL97656 |
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