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ADP1147AN-5 Fiches technique(PDF) 10 Page - Analog Devices |
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ADP1147AN-5 Fiches technique(HTML) 10 Page - Analog Devices |
10 / 12 page ADP1147-3.3/ADP1147-5 –10– REV. 0 Losses are encountered in all elements of the circuit, but the four major sources for the circuit shown in Figure 1 are: 1. The ADP1147 dc bias current. 2. The MOSFET gate charge current. 3. The I 2 × R losses. 4. The voltage drop of the Schottky diode. 1. The ADP1147’s dc bias current is the amount of current that flows into VIN of the device minus the gate charge current. With VIN = 10 volts, the dc supply current to the device is typically 160 µA for a no load condition, and increases pro- portionally with load to a constant of 1.6 mA in the continu- ous mode of operation. Losses due to dc bias currents increase as the input voltage VIN is increased. At VIN = 10 volts the dc bias losses are usually less than 1% with a load current greater than 30 mA. When very low load currents are encountered the dc bias current becomes the primary point of loss. 2. The MOSFET gate charge current is due to the switching of the power MOSFET’s gate capacitance. As the MOSFET’s gate is switched from a low to a high and back to a low again, charge impulses dQ travel from VIN to ground. The current out of VIN is equal to dQ/dt and is usually much greater than the dc supply current. When the device is operating in the continuous mode the I gate charge is = f (QP). Typically a P-channel power MOSFET with an RDS on of 135 mΩ will have a gate charge of 40 nC. With a 100 kHz, switching frequency in the continuous mode, the I gate charge would equate to 4 mA or about a 2%–3% loss with a VIN of 10 volts. It should be noted that gate charge losses increase with switching frequency or input voltage. A design requiring the highest efficiency can be obtained by using more moderate switching frequencies. 3. I 2 × R loss is a result of the combined dc circuit resistance and the output load current. The primary contributors to circuit dc resistance are the MOSFET, the Inductor and RSENSE. In the continuous mode of operation the average output current is switched between the MOSFET and the Schottky diode and a continuous current flows through the inductor and RSENSE. Therefore the RDS(ON) of the MOSFET is multiplied by the on portion of the duty cycle. The result is then combined with the resistance of the Inductor and RSENSE. The following equations and example show how to approximate the I 2 × R losses of a circuit. RDS(ON) × (Duty Cycle) + R INDUCTOR + RSENSE = R ILOAD 2 × R = P LOSS VOUT × I LOAD = POUT PLOSS/POUT × 100 = % I2 × R LOSS. With the duty cycle = 0.5, RINDUCTOR = 0.15, RSENSE = 0.05 and ILOAD = 0.5 A. The result would be a 3% I 2R loss. The effects of I 2R losses causes the efficiency to fall off at higher output currents. 4. At high current loads the Schottky diode can be a substantial point of power loss. The diode efficiency is further reduced by the use of high input voltages. To calculate the diode loss, the load current should be multiplied by the duty cycle of the diode times the forward voltage drop of the diode. ILOAD × % duty cycle × VDROP = Diode Loss Figure 6 indicates the distribution of losses versus load cur- rent in a typical ADP1147 switching regulator circuit. With medium current loads the gate charge current is responsible for a substantial amount of efficiency loss. At lower loads the gate charge losses become large in comparison to the load, and result in unacceptable efficiency levels. When low load currents are encountered the ADP1147 employs a power savings mode to reduce the effects of the gate loss. In the power savings mode of operation the dc supply current is the major source of loss and becomes a greater percentage as the output current decreases. Losses at higher loads are primarily due to I 2R and the Schottky diode. All other variables such as capacitor ESR dissipation, MOSFET switching, and inductor core losses typically contribute less than 2% additional loss. Circuit Design Example In using the design example below assumptions are as follows: VIN = 5 Volts VOUT = 3.3 Volts VDIODE drop (VD) = 0.4 Volts IMAX OUT = 1 Amp Max switching frequency (f) = 100 kHz. The values for RSENSE, CT and L can be calculated based on the above assumptions. RSENSE = 100 mV/1 Amp = 100 m Ω. tOFF time = (1/100 kHz) × [1 – (3.7/5.4)] = 3.15 µs. CT = 3.15 µs /(1.3 × 10 4) = 242 pF. L = 5.1 × 10 5 × 0.1 Ω × 242 pF × 3.3 V = 41 µH. If we further assume: 1. The data is specified at +25 °C. 2. MOSFET max power dissipation (PP) is limited to 250 mW. 3. MOSFET thermal resistance is 50 °C/W. 4. The normalized RDS(ON) vs. temperature approximation ( δ P) is 0.007/ °C. This results in 250 mW × 50°C per watt = 12.5°C of MOSFET heat rise. If the ambient temperature TA is 50°C, a junction temperature of 12.5 °C +50°C, T A = 62.5 °C. δP = 0.007 × (62.5 °C –25°C) = 0.2625 We can now determine the required RDS(ON) for the MOSFET: RDS(ON) = 5(0.25)/3.3 (1) 2 (1.2625) = 300 m Ω The above requirements can be met with the use of a P-channel IRF7204 or an Si9430. When VOUT is short circuited the power dissipation of the Schottky diode is at worst case and the dissipation can rise greatly. The following equation can be used to determine the power dissipation: PD = ISC(AVG) × VDIODE Drop A 100 m Ω R SENSE resistor will yield an ISC(AVG) of 1 A. With a forward diode drop of 0.4 volts a 400 milliwatt diode power dissipation results. The rms current rating needed for CIN will be at least 0.5 A over the temperature range. |
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Description similaire - ADP1147AN-5 |
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