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ADP1822 Datasheet(PDF) 15 Page  Analog Devices 

ADP1822 Datasheet(HTML) 15 Page  Analog Devices 
15 / 24 page ADP1822 Rev. B  Page 15 of 24 When fESRZ is approximately the same as the switching frequency, the squareroot sum of the squares of the two ripples applies, or [] ()() 2 2 8 Δ ) ( Δ Δ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + ≅ SW OUT L L OUT f C I ESR I V (6) The lowside MOSFET does not carry the transition losses but does carry the inductor current when the highside MOSFET is off. For high input and low output voltages, the lowside MOSFET carries the current most of the time, and therefore to achieve high efficiency, it is critical to optimize the lowside MOSFET for low on resistance. In some cases, where the power loss exceeds the MOSFET rating, or lower resistance is required than is available in a single MOSFET, connect multiple lowside MOSFETs in parallel. The equation for lowside MOSFET power loss is SELECTING THE MOSFETS The choice of MOSFET directly affects the dctodc converter performance. The MOSFET must have low on resistance to reduce I2R losses and low gate charge to reduce transition losses. In addition, the MOSFET must have low thermal resistance to ensure that the power dissipated in the MOSFET does not result in excessive MOSFET die temperature. () ( ) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − ≅ IN OUT ON LOAD LS V V R I P 1 2 (11) where: P The highside MOSFET carries the load current during on time and carries all the transitions losses of the converter. Typically, the lower the MOSFET on resistance, the higher the gate charge and vice versa. Therefore, it is important to choose a highside MOSFET that balances the two losses. The conduction loss of the highside MOSFET is determined by LS is the lowside MOSFET on resistance. R is the total on resistance of the lowside MOSFET(s). ON If multiple lowside MOSFETs are used in parallel, use the parallel combination of the on resistances for determining RON to solve this equation. () ( ) ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ≅ IN OUT ON LOAD C V V R I P 2 (7) SETTING THE CURRENT LIMIT The internal currentlimit circuit measures the voltage across the lowside MOSFET to determine the load current. When the lowside MOSFET current exceeds the current limit, the high side MOSFET is not allowed to turn on until the current drops below the current limit. where: PC = conduction power loss. R = MOSFET on resistance. ON The gatecharging loss is approximated by ( )( )( ) SW G PVCC T f Q V P ≅ (8) The current limit is set through the currentlimit resistor, RCL. The current sense pin, CSL, sources 50 μA through RCL. This creates an offset voltage of resistance of RCL multiplied by the 50 μA CSL current. When the lowside MOSFET voltage is equal to or greater than the offset voltage, the ADP1822 is in current limit mode and prevents additional ontime cycles. where: PT = gatecharging loss power. VPVCC = gate driver supply voltage. QG = MOSFET total gate charge. fSW = converter switching frequency. Choose the current limit resistor by the equation The highside MOSFET transition loss is approximated by () 2 SW F R LOAD IN SW f t t I V P × + × × = (9) ( ) ( ) μA 42 ONWC LPK CL R I R = (12) where: ILPK is the peak inductor current. R where: P ONWC is the worstcase (maximum) lowside MOSFET on resistance. SW = highside MOSFET switching loss power. tR = MOSFET rise time. tF = MOSFET fall time. The total power dissipation of the highside MOSFET is the sum of all the previous losses, or ( ) ( ) ( ) SW T C HS P P P P + + ≅ (10) The worstcase, lowside MOSFET on resistance can be found in the MOSFET data sheet. Note that MOSFETs typically increase on resistance with increasing die temperature. To determine the worstcase MOSFET on resistance, calculate the worstcase MOSFET temperature (based on the MOSFET power loss) and multiply by the ratio between the typical on resistance at that temperature and the on resistance at 25°C as listed in the MOSFET data sheet. where P is the total highside MOSFET power loss. HS 
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