Datasheet LTC1874 (Analog Devices) - 8

LTC1874
U U W U APPLICATIO S I FOR ATIO
IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE
Power MOSFET Selection
occurs at the maximum input voltage. The main selection criteria for the power MOSFET are the In Burst Mode operation on an LTC1874 controller, the threshold voltage VGS(TH), the “on” resistance RDS(ON), ripple current is normally set such that the inductor reverse transfer capacitance CRSS and total gate charge. current is continuous during the burst periods. Therefore, Since the controller is designed for operation down to low the peak-to-peak ripple current must not exceed: input voltages, a logic level threshold MOSFET (RDS(ON) guaranteed at V . V GS = 2.5V) is required for applications that IRIPPLE ≤ 0 03 work close to this voltage. When these MOSFETs are used, RSENSE make sure that the input supply to the controller is less than the absolute maximum V This implies a minimum inductance of: GS rating, typically 8V. The required minimum RDS(ON) of the MOSFET is gov- V V  V + V  erned by its allowable power dissipation. For applications L IN OUT OUT D MIN = −  that may operate the controller in dropout, i.e., 100% duty .   V + V  0 03 f IN D cycle, at its worst case the required R  DS(ON) is given by: RSENSE P R P = (Use V DS O ( N) IN(MAX) = VIN) DC= % 100 2 I (1+ p ( ) δ ) OUT MAX ( ) A smaller value than LMIN could be used in the circuit; however, the inductor current will not be continuous where PP is the allowable power dissipation and δp is the during burst periods. temperature dependency of RDS(ON). (1 + δp) is generally given for a MOSFET in the form of a normalized RDS(ON) vs
Inductor Core Selection
temperature curve, but δp = 0.005/°C can be used as an Once the value of inductor is known, an off the shelf approximation for low voltage MOSFETs. inductor can be selected. The inductor should be rated for In applications where the maximum duty cycle is less than the calculated peak current. Some manufacturers specify 100% and the controller is in continuous mode, the both peak saturation current and peak RMS current. Make RDS(ON) is governed by: sure that the RMS current meets your continuous load requirements. Also, you may want to compare the DC P R P resistance of different inductors in order to optimize the DS ON ( ) ≅ ( ) 2(1+δ ) efficiency. DC I p OUT Inductor core losses are usually not specified and you will where DC is the maximum operating duty cycle of the need to evaluate them yourself. Usually, the core losses controller. are not a problem because the inductors operate with relatively low magnetic flux swings. The best way to
Output Diode Selection
evaluate the core losses is by measuring the converters The catch diode carries load current during the off-time. efficiency. Converter efficiency will reveal the difference in The average diode current is therefore dependent on the both DC current losses and core losses. MOSFET duty cycle. At high input voltages the diode Off the shelf inductors are available from numerous manu- conducts most of the time. As VIN approaches VOUT the facturers. Some of the most common manufacturers are diode conducts only a small fraction of the time. The most Coilcraft, Coiltronics, Panasonic, Toko, Tokin, Murata and stressful condition for the diode is when the output is Sumida. 8