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MC34652EF bảng dữ liệu(PDF) 11 Page - Freescale Semiconductor, Inc |
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MC34652EF bảng dữ liệu(HTML) 11 Page - Freescale Semiconductor, Inc |
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11 / 25 page  Analog Integrated Circuit Device Data Freescale Semiconductor 11 34652 FUNCTIONAL DEVICE OPERATION OPERATIONAL MODES FUNCTIONAL DEVICE OPERATION OPERATIONAL MODES START-UP SEQUENCE When power is first applied to the 34652 by connecting the VIN terminal to the negative voltage rail and the VPWR terminal to the positive voltage rail, the 34652 keeps the Power MOSFET turned off, deactivates the power good output signals, and resets the retry counter. If the device is disabled, no further activities will occur and power-up would not start. If the device is enabled, it starts to establish an internally regulated supply voltage required for the internal circuitry. The Power MOSFET will stay off until the start of the charging process. After Power-ON Reset (POR) and once the Undervoltage Lockout (UVLO) threshold is cleared, the 34652 checks for external components on four terminals—the UV, the ILIM, the ICHG, and 128 µs later the OV—to set the levels of the Undervoltage Threshold, the Overcurrent Limit, the Charging Current Limit, and the Overvoltage Threshold, respectively. The device also checks for external components on the TIMER terminal to decide on the Start-Up and Retry Delay Timer value, and the device keeps checking the TIMER terminal continuously throughout the operation. The device then initiates the start-up timer (Point A in Figure 6) and checks for the start-up conditions (see next paragraph). The duration of the timer is either a default or a user-programmable value. For undervoltage and overvoltage faults during power up the 34652 retries infinitely until normal input voltage is attained. If the die temperature ever increased beyond the thermal shutdown threshold or the device is disabled, then the start-up timer resets and the retry counter increments. If after 10 retries the die temperature is still high and the device is still disabled, the 34652 will not retry again and the power in the device must be recycled or the device must be disabled to reset the retry counter. Figure 6. Start-Up Sequence Start-Up Conditions The start-up conditions are as follows: • Input voltage is below the overvoltage turn-off threshold. This threshold is either a default or user-programmable value. • Input voltage is above the undervoltage turn-off threshold. This threshold is either a default or user-programmable value. • Die temperature is less than thermal shutdown temperature. • Device is enabled. If the start-up conditions are satisfied for a time equal to the length of the start-up timer and the retry counter is less than or equal to 10, the device starts to turn on the Power MOSFET gradually to control the inrush current that charges up the load capacitor to eventually switch on the load (Point B in Figure 6). Charging Process When charging a capacitor from a fixed voltage source, a definite amount of energy will be dissipated in the control circuit, no matter what the control algorithm is. This energy is equal to the energy transferred to the capacitor—½CV2. With this in mind, the Power MOSFET in the 34652 cannot absorb this pulse of energy instantaneously, so the pulse must be dissipated over time. To limit the peak power dissipation in the Power MOSFET and to spread out the duration of the energy dissipation in the Power MOSFET, the circuit uses a two-level current approach to controlling the inrush current and switching on the load as explained in the following paragraphs. When the Power MOSFET is turned on, the current limit is set gradually from 0 A to ICHG (between Points A and B in Figure 7, page 12). The low charging current value and the gradual rise time of ICHG are either defaults or they can be user programmable (2.0 ms rise time in the example in Figure 7). The low charging current value of ICHG is intended to limit the temperature increase during the load capacitor charging process, and the gradual rise to ICHG is to prevent transient dips in the input voltage due to sharp increases in the current. This prevents the input voltage from drooping due to current steps acting on the input line inductance, and that in turn prevents a premature activation of the UV detection circuit. |
Số phần tương tự - MC34652EF |
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Mô tả tương tự - MC34652EF |
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