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C1608X7R0J225K Datasheet(PDF) 11 Page - Intersil Corporation |
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C1608X7R0J225K Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 20 page  ISL78419 11 FN8292.2 January 24, 2014 Applications Information Enable Control With VIN > UVLO, only the Logic output channel is activated. All other functions in ISL78419 are shut down when the enable pin is pulled down. When the voltage at the enable pin reaches high threshold, the whole chip turns on. Frequency Selection The ISL78419 switching frequency can be user selected to operate at either constant 600kHz or 1.2MHz. Lower switching frequency can save power dissipation at very light load conditions. Also, low switching frequency more easily leads to discontinuous conduction mode, while higher switching frequency allows for smaller external components, such as inductor and output capacitors, etc. Higher switching frequency will get higher efficiency within some loading ranges depending on VIN, VOUT, and external components, as shown in Figure 1. Connecting the FREQ pin to GND sets the PWM switching frequency to 600kHz, or connecting FREQ pin to VIN for 1.2MHz. Soft-Start The soft-start is provided by an internal current source to charge the external soft-start capacitor. The ISL78419 ramps up the current limit from 0A up to the full value, as the voltage at the SS pin ramps from 0V to 0.8V. Hence, the soft-start time is 3.2ms when the soft-start capacitor is 22nF, 6.8ms for 47nF and 14.5ms for 100nF. Operation The boost converter is a current mode PWM converter operating at either 600kHz or 1.2MHz. It can operate in both discontinuous conduction mode (DCM) at light load and continuous conduction mode (CCM). In continuous conduction mode, current flows continuously in the inductor during the entire switching cycle in steady state operation. The voltage conversion ratio in continuous current mode is given by Equation 1: Where D is the duty cycle of the switching MOSFET. The boost regulator uses a summing amplifier architecture consisting of gm stages for voltage feedback, current feedback and slope compensation. A comparator looks at the peak inductor current cycle-by-cycle and terminates the PWM cycle if the current limit is reached. An external resistor divider is required to divide the output voltage down to the nominal reference voltage. 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 in the order of 60kΩ is recommended. The boost converter output voltage is determined by Equation 2: The current through the MOSFET is limited to 1.5APEAK. This restricts the maximum output current (average) based on Equation 3: Where ΔIL is the peak-to-peak inductor ripple current, and is set by Equation 4: Where fS is the switching frequency (600kHz or 1.2MHz). Capacitor An input capacitor is used to suppress the voltage ripple injected into the boost converter. The ceramic capacitor with a capacitance larger than 10µF is recommended. The voltage rating of the input capacitor should be larger than the maximum input voltage. Some input capacitors are recommended in Table 1. Inductor The boost inductor is a critical part that influences the output voltage ripple, transient response, and efficiency. Values of 3.3µH to 10µH are used to match the internal slope compensation. The inductor must be able to handle the following average and peak currents shown in Equation 5: Some inductors are recommended in Table 2 for different design considerations. Rectifier Diode A high-speed diode is necessary due to the high switching frequency. Schottky diodes are recommended because of their fast recovery time and low forward voltage. The reverse voltage rating of this diode should be higher than the maximum output voltage. The rectifier diode must meet the output current and peak inductor current requirements. Table 3 shows some recommendations for boost converter diode. V Boost V IN ------------------- 1 1D – ------------- = (EQ. 1) V Boost R 1 R 2 + R 2 --------------------- V FB × = (EQ. 2) TABLE 1. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION CAPACITOR SIZE MFG PART NUMBER 10µF/6.3V 0603 TDK C1608X5R0J106M 10µF/16V 1206 TDK C3216X7R1C106M 10µF/10V 0805 Murata GRM21BR61A106K 22µF/10V 1210 Murata GRB32ER61A226K TABLE 2. BOOST CONVERTER INDUCTOR RECOMMENDATION INDUCTOR DIMENSIONS (mm) MFG PART NUMBER NOTE 10µH/ 4Apeak 8.3x8.3x4.5 Sumida CDR8D43-100NC Efficiency optimization 6.8µH/ 1.8Apeak 5.0x5.0x2.0 TDK PLF5020T-6R8M1R8 10µH/ 2.2Apeak 6.6x7.3x1.2 Cyntec PCME061B-100MS PCB space/profile optimization I OMAX I LMT ΔI L 2 -------- – ⎝⎠ ⎛⎞ VIN V O --------- × = (EQ. 3) ΔI L V IN L --------- D f s ---- × = (EQ. 4) I LAVG I O 1D – ------------- = I LPK I LAVG ΔI L 2 -------- + = (EQ. 5) |
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