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MIC3172YN Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC3172YN Datasheet(HTML) 8 Page - Micrel Semiconductor |
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8 / 17 page  MIC2172/3172 Micrel, Inc. M9999-042205 8 April 2005 Functional Description Refer to “Block Diagram MIC2172” and “Block Diagram MIC3172.” Internal Power The MIC2172/3172 operates when V IN is ≥ 2.6V (and VEN ≥ 2.0V for the MIC3172). An internal 2.3V regulator supplies biasing to all internal circuitry including a precision 1.24V band gap reference. The enable control (MIC3172 only) enables or disables the internal regulator which supplies power to all other internal circuitry. PWM Operation The 100kHz oscillator generates a signal with a duty cycle of approximately 90%. The current-mode comparator output is used to reduce the duty cycle when the current amplifier output voltage exceeds the error amplifier output voltage. The resulting PWM signal controls a driver which supplies base current to output transistor Q1. Current Mode Advantages The MIC2172/3172 operates in current mode rather than voltage mode. There are three distinct advantages to this technique. Feedback loop compensation is greatly simplified because inductor current sensing removes a pole from the closed loop response. Inherent cycle-by-cycle current limit- ing greatly improves the power switch reliability and provides automatic output current limiting. Finally, current-mode op- eration provides automatic input voltage feed forward which prevents instantaneous input voltage changes from disturb- ing the output voltage setting. Anti-Saturation The anti-saturation diode (D1) increases the usable duty cycle range of the MIC2172/3172 by eliminating the base to collector stored charge which would delay Q1’s turnoff. Compensation Loop stability compensation of the MIC2172/3172 can be accomplished by connecting an appropriate network from either COMP to circuit ground (Typical Applications) or COMP to FB. The error amplifier output (COMP) is also useful for soft start and current limiting. Because the error amplifier output is a transconductance type, the output impedance is relatively high which means the output voltage can be easily clamped or adjusted externally. Block Diagram MIC3172 Current Amp. Error Amp. 1.24V Ref. Logic Driver Anti-Sat. V SW Pin 7 100kHz Osc. EN Pin 4 Reg. FB Pin 3 COMP Pin 2 P GND 2 Pin 6 P GND 1 Pin 8 VIN Pin 5 S GND Pin 1 D1 Q1 2.3V Com- parator |
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Similar Description - MIC3172YN |
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