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MIC2168 Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC2168 Datasheet(HTML) 10 Page - Micrel Semiconductor |
10 / 14 page MIC2168 Micrel, Inc. M9999-040805 10 April 2005 I I max D (1 D) C (rms) OUT IN ≈ × × − ( ) The power dissipated in the input capacitor is: P I R DISS(C ) C (rms) ESR(C ) IN IN 2 IN = × Voltage Setting Components The MIC2168 requires two resistors to set the output voltage as shown in Figure 2. Error Amp 7 MIC2168 [adj.] FB VREF 0.8V R2 R1 Figure 2. Voltage-Divider Configuration Where: VREF for the MIC2168 is typically 0.8V The output voltage is determined by the equation: V V 1 R1 R2 O REF = × + A typical value of R1 can be between 3kΩ and 10kΩ. If R1 is too large, it may allow noise to be introduced into the voltage feedback loop. If R1 is too small, in value, it will decrease the efficiency of the power supply, especially at light loads. Once R1 is selected, R2 can be calculated using: R2 V R1 V V REF O REF = × − External Schottky Diode An external freewheeling diode is used to keep the inductor current flow continuous while both MOSFETs are turned off. This dead time prevents current from flowing unimpeded through both MOSFETs and is typically 15ns. The diode conducts twice during each switching cycle. Although the average current through this diode is small, the diode must be able to handle the peak current. The reverse voltage requirement of the diode is: V V DIODE(rrm) IN = The power dissipated by the Schottky diode is: P I V DIODE D(avg) F = × where: VF = forward voltage at the peak diode current The external Schottky diode, D1, is not necessary for circuit operation since the low-side MOSFET contains a parasitic body diode. The external diode will improve efficiency and decrease high frequency noise. If the MOSFET body diode is used, it must be rated to handle the peak and average cur- rent. The body diode has a relatively slow reverse recovery time and a relatively high forward voltage drop. The power lost in the diode is proportional to the forward voltage drop of the diode. As the high-side MOSFET starts to turn on, the body diode becomes a short circuit for the reverse recovery period, dissipating additional power. The diode recovery and the circuit inductance will cause ringing during the high-side MOSFET turn-on. An external Schottky diode conducts at a lower forward voltage preventing the body diode in the MOSFET from turning on. The lower forward voltage drop dissipates less power than the body diode. The lack of a reverse recovery mechanism in a Schottky diode causes less ringing and less power loss. Depending on the circuit components and operating conditions, an external Schottky diode will give a 1/2% to 1% improvement in efficiency. Feedback Loop Compensation The MIC2168 controller comes with an internal transcon- ductance error amplifier used for compensating the voltage feedback loop by placing a capacitor (C1) in series with a resistor (R1) and another capacitor C2 in parallel from the COMP pin to ground. See “Functional Block Diagram.” Power Stage The power stage of a voltage mode controller has an induc- tor, L1, with its winding resistance (DCR) connected to the output capacitor, COUT, with its electrical series resistance (ESR) as shown in Figure 3. The transfer function G(s), for such a system is: ESR COUT VO DCR L Figure 3. The Output LC Filter in a Voltage Mode Buck Converter (1 + ESR × s × C) - G (S) DCR × s × C + s2 × L × C + 1 + ESR × s × C Plottingthistransferfunctionwiththefollowingassumedvalues (L=2 µH, DCR=0.009Ω, COUT=1000µF, ESR=0.050Ω) gives lot of insight as to why one needs to compensate the loop by adding resistor and capacitors on the COMP pin. Figures 4 and 5 show the gain curve and phase curve for the above transfer function. |
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Similar Description - MIC2168_05 |
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