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ADP5052ACPZR7 Datasheet(HTML) 25 Page  Analog Devices 

ADP5052ACPZR7 Datasheet(HTML) 25 Page  Analog Devices 
Data Sheet ADP5052 Rev. 0  Page 25 of 40 The output voltage ripple is determined by the ESR of the output capacitor and its capacitance value. Use the following equations to select a capacitor that can meet the output ripple requirements: RIPPLE OUT SW L RIPPLE OUT V f I C _ _ 8 âˆ† Ã— Ã— âˆ† = L RIPPLE OUT ESR I V R âˆ† âˆ† = _ where: Î”IL is the inductor ripple current. fSW is the switching frequency. Î”VOUT_RIPPLE is the allowable output voltage ripple. RESR is the equivalent series resistance of the output capacitor. Select the largest output capacitance given by COUT_UV, COUT_OV, and COUT_RIPPLE to meet both load transient and output ripple requirements. The voltage rating of the selected output capacitor must be greater than the output voltage. The minimum rms current rating of the output capacitor is determined by the following equation: 12 _ L rms C I I OUT âˆ† = INPUT CAPACITOR SELECTION The input decoupling capacitor attenuates high frequency noise on the input and acts as an energy reservoir. Use a ceramic capac itor and place it close to the PVINx pin. The loop composed of the input capacitor, the highside NFET, and the lowside NFET must be kept as small as possible. The voltage rating of the input capacitor must be greater than the maximum input voltage. Make sure that the rms current rating of the input capacitor is larger than the following equation: ( ) D D I I OUT rms CIN âˆ’ Ã— Ã— = 1 _ where D is the duty cycle (D = VOUT/VIN). LOWSIDE POWER DEVICE SELECTION Channel 1 and Channel 2 include integrated lowside MOSFET drivers, which can drive lowside Nchannel MOSFETs (NFETs). The selection of the lowside Nchannel MOSFET affects the performance of the buck regulator. The selected MOSFET must meet the following requirements: â€¢ Draintosource voltage (VDS) must be higher than 1.2 Ã— VIN. â€¢ Drain current (ID) must be greater than 1.2 Ã— ILIMIT_MAX, where ILIMIT_MAX is the selected maximum currentlimit threshold. â€¢ The selected MOSFET can be fully turned on at VGS = 4.5 V. â€¢ Total gate charge (Qg at VGS = 4.5 V) must be less than 20 nC. Lower Qg characteristics provide higher efficiency. When the highside MOSFET is turned off, the lowside MOSFET supplies the inductor current. For low duty cycle applications, the lowside MOSFET supplies the current for most of the period. To achieve higher efficiency, it is important to select a MOSFET with low on resistance. The power conduction loss for the low side MOSFET can be calculated using the following equation: PFET_LOW = IOUT2 Ã— RDSON Ã— (1 âˆ’ D) where: RDSON is the on resistance of the lowside MOSFET. D is the duty cycle (D = VOUT/VIN). Table 11 lists recommended dual MOSFETs for various current limit settings. Ensure that the MOSFET can handle thermal dissipation due to power loss. Table 11. Recommended Dual MOSFETs Vendor Part No. VDS (V) ID (A) RDSON (mâ„¦) Qg (nC) Size (mm) IR IRFHM8363 30 10 20.4 6.7 3 Ã— 3 IRLHS6276 20 3.4 45 3.1 2 Ã— 2 Fairchild FDMA1024 20 5.0 54 5.2 2 Ã— 2 FDMB3900 25 7.0 33 11 3 Ã— 2 FDMB3800 30 4.8 51 4 3 Ã— 2 FDC6401 20 3.0 70 3.3 3 Ã— 3 Vishay Si7228DN 30 23 25 4.1 3 Ã— 3 Si7232DN 20 25 16.4 12 3 Ã— 3 Si7904BDN 20 6 30 9 3 Ã— 3 Si5906DU 30 6 40 8 3 Ã— 2 Si5908DC 20 5.9 40 5 3 Ã— 2 SiA906EDJ 20 4.5 46 3.5 2 Ã— 2 AOS AON7804 30 22 26 7.5 3 Ã— 3 AON7826 20 22 26 6 3 Ã— 3 AO6800 30 3.4 70 4.7 3 Ã— 3 AON2800 20 4.5 47 4.1 2 Ã— 2 PROGRAMMING THE UVLO INPUT The precision enable input can be used to program the UVLO threshold of the input voltage, as shown in Figure 38. To limit the degradation of the input voltage accuracy due to the internal 1 Mâ„¦ pulldown resistor tolerance, ensure that the bottom resistor in the divider is not too largeâ€”a value of less than 50 kâ„¦ is recommended. The precision turnon threshold is 0.8 V. The resistive voltage divider for the programmable VIN startup voltage is calculated as follows: VIN_STARTUP = (0.8 nA + (0.8 V/RBOT_EN)) Ã— (RTOP_EN + RBOT_EN) where: RTOP_EN is the resistor from VIN to EN. RBOT_EN is the resistor from EN to ground. 
