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MIC2171 Datasheet(PDF) 6 Page - Micrel Semiconductor |
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MIC2171 Datasheet(HTML) 6 Page - Micrel Semiconductor |
6 / 10 page MIC2171 Micrel 4-8 1997 The device operating losses are the dc losses associated with biasing all of the internal functions plus the losses of the power switch driver circuitry. The dc losses are calculated from the supply voltage (V IN) and device supply current (IQ). The MIC2171 supply current is almost constant regardless of the supply voltage (see “Electrical Characteristics”). The driver section losses (not including the switch) are a function of supply voltage, power switch current, and duty cycle. P= V I V I I (bias+driver) IN Q IN(min) SW IN ()+× × () ∆ where: P (bias+driver) = device operating losses V IN(min) = supply voltage = VIN – VSW I Q = typical quiescent supply current I CL = power switch current limit ∆I IN = typical supply current increase As a practical example refer to Figure 1. V IN = 5.0V I Q = 0.007A I CL = 2.21A δ = 66.2% (0.662) Then: V = 5 – (2.21 0.37) = 4.18V IN(min) × P = ( 0.007) + (4.18 2.21 .009) (bias driver) + ×× × 5 P (bias+driver) = 0.1W Power switch dissipation calculations are greatly simplified by making two assumptions which are usually fairly accurate. First, the majority of losses in the power switch are due to on-losses. To find these losses, assign a resistance value to the collector/emitter terminals of the device using the satura- tion voltage versus collector current curves (see Typical Performance Characteristics). Power switch losses are calculated by modeling the switch as a resistor with the switch duty cycle modifying the average power dissipation. P SW = (ISW) 2 R SW δ where: δ = duty cycle δ = V+ V – V V+ V OUT F IN(min) OUT F V SW = ICL (RSW) V OUT = output voltage V F = D1 forward voltage drop at IOUT From the Typical performance Characteristics: R SW = 0.37Ω Then: P SW = (2.21) 2 × 0.37 × 0.662 P SW) = 1.2W P (total) = 1.2 + 0.1 P (total) = 1.3W Applications Information Soft Start A diode-coupled capacitor from COMP to circuit ground slows the output voltage rise at turn on (Figure 3). MIC2171 IN COMP C2 R1 VIN D2 C1 D1 Figure 3. Soft Start The additional time it takes for the error amplifier to charge the capacitor corresponds to the time it takes the output to reach regulation. Diode D1 discharges C1 when V IN is removed. Current Limit MIC2171 IN COMP C2 R3 VIN Q1 R2 GND R1 C1 SW FB VOUT Note: Input and output returns not common. ICL ≈ 0.6V/R2 Figure 4. Current Limit The maximum current limit of the MIC2171 can be reduced by adding a voltage clamp to the COMP output (Figure 4). This feature can be useful in applications requiring either a com- plete shutdown of Q1’s switching action or a form of current fold-back limiting. This use of the COMP output does not disable the oscillator, amplifiers or other circuitry, therefore the supply current is never less than approximately 5mA. Thermal Management Although the MIC2171 family contains thermal protection circuitry, for best reliability, avoid prolonged operation with junction temperatures near the rated maximum. The junction temperature is determined by first calculating the power dissipation of the device. For the MIC2171, the total power dissipation is the sum of the device operating losses and power switch losses. |
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