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MC34270FB 数据表(PDF) 6 Page - Motorola, Inc |
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MC34270FB 数据表(HTML) 6 Page - Motorola, Inc |
6 / 12 page MC34270 MC34271 6 MOTOROLA ANALOG IC DEVICE DATA VDD LEVEL (V) 0 0 1000 Figure 9. Frequency versus Timing TIMING RESISTANCE (k Ω, s) Figure 10. VA, Vref versus VDD 100 10 5.0 4.0 3.0 2.0 1.0 0 100 1000 1.0 2.0 3.0 4.0 5.0 6.0 VDD = 6.0 V TA = 25°C RT = 169 k TA = 25°C VA Vref OPERATING DESCRIPTION The MC34270 and MC34271 series are monolithic, fixed frequency power switching regulators specifically designed for dc to dc converter and battery powered applications. These devices operate as fixed frequency, voltage mode regulators containing all the active functions required to directly implement step–up, step–down and voltage inverting converters with a minimum number of external components. Potential markets include battery powered, handheld, automotive, computer, industrial and cost sensitive consumer products. A description of each section is given below with the representative block diagram shown in Figure 9. Oscillator The oscillator frequency is programmed by resistor RT. The charge to discharge ratio is controlled to yield a 95% maximum duty cycle at the switch outputs. During the fall time of the internal sawtooth waveform, the oscillator generates an internal blanking pulse that holds the inverting input of the AND gates high, disabling the output switching MOSFETs. The internal sawtooth waveform has a nominal peak voltage of 3.3 V and a valley voltage of 1.7 V. Pulse Width Modulators Both pulse width modulators consist of a comparator with the oscillator ramp voltage applied to the noninverting input, while the error amplifier output is applied to the inverting input. A third input to the comparator has a 0.5 mA typical current source that can be used to implement soft start. Output switch conduction is initiated when the ramp waveform is discharged to the valley voltage. As the ramp voltage increases to a voltage that exceeds the error amplifier output, the latch resets, terminating output MOSFET conduction for the duration of the oscillator ramp. This PWM/latch combination prevents multiple output pulses during a given oscillator cycle. Each PWM circuit is enabled by a logic input. When disabled, the entire block is turned off, drawing only leakage current from the power source. Shared circuits, like the reference and oscillator, can be activated by either EN1 or EN2. Circuit #1 has an ELD output which may be used to drive an LCD or backlight. Its output frequency is the oscillator frequency divided by 1024. Error Amplifiers and Reference Each error amplifier is provided with access to both inverting and noninverting inputs, and the output. The Error Amplifiers’ Common Mode Input Range is 0 to 2.5 V. The amplifiers have a minimum dc voltage gain of 60 dB. The 1.25 V reference has an accuracy of ±4.0% at room temperature. External loop compensation is required for converter stability. A simple low–pass filter is formed by connecting a resistive divider from the output to the error amplifier inverting input, and a series resistor–capacitor from the error amplifier output also to the to the inverting input. The step down converter is easiest to compensate for stability. The step–up and voltage inverting configurations, when operated as continuous conduction boost or flyback converters, are more difficult to compensate, and may require a lower loop design bandwidth. MOSFET Switch Outputs The output MOSFETs are designed to switch a maximum of 60 V, with a peak drain current capability of 500 mA. In circuit #1 an additional DRV1 output is provided for interfacing with an external MOSFET.The gates of the MOSFETs are held low when the circuit is disabled. Auxiliary Output Voltages Output voltages V0 through V4 are provided for use as references or bias voltages. V0 is the circuit #2 output voltage, when an internal FET switch is activated. The other auxiliary output voltages are proportional to VB. The amplifiers for V1 and V2 are powered from V0, while the amplifiers for V3 and V4 are powered from VDD. |
类似零件编号 - MC34270FB |
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类似说明 - MC34270FB |
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