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MIC24420 Datasheet(PDF) 15 Page - Micrel Semiconductor |
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MIC24420 Datasheet(HTML) 15 Page - Micrel Semiconductor |
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15 / 34 page 
Micrel, Inc. MIC24420/MIC24421 June 2012 15 M9999-062012-C Voltage Setting Components The regulator requires two external resistors to set the output voltage as shown in Figure 8. Figure 8. Setting the Output Voltage The output voltage is determined by the equation below. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + × = R2 R1 1 V V REF OUT Where: VREF is 0.7V nominal. If the voltage divider resistance is used to provide the minimum load (see EN/DLY section) then R1 should be low enough to provide the necessary impedance. Once R1 is selected, R2 can be calculated with the following formula. REF OUT REF V V R1 V R2 − × = Ω < + 2k R1 R2 And Minimum Pulse Width Output voltage is regulated by adjusting the on-time pulse width of the high-side MOSFET. This is accomplished by comparing the error amplifier output with a sawtooth waveform (see block diagram). The pulse width output of the comparator becomes smaller as the error amplifier voltage decreases. Due to propagation delay and other circuit limitations, there is a minimum pulse width at the output of the comparator. If the error amplifier voltage drops any further, the output of the comparator will be low. The PWM circuit will skip pulses if a smaller duty cycle is required to maintain output voltage regulation. This effectively cuts the output frequency in half. Thermal Protection The internal temperature of the regulator is monitored to prevent damage to the device. Both outputs are inhibited from switching if the over-temperature threshold is exceeded. Hysteresis in the circuit allows the regulator to cool before turning back on. |
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