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A8670EESTR-T Datasheet(PDF) 10 Page - Allegro MicroSystems |
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A8670EESTR-T Datasheet(HTML) 10 Page - Allegro MicroSystems |
10 / 27 page Fixed Frequency, 2 A Synchronous Buck Regulator With Fault Warnings and Power OK A8670 10 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com As mentioned in the Output Capacitor Selection section, the effects of voltage biasing should be taken into account when choosing the capacitor voltage rating. If ceramic capacitors are being used, then there is generally no need to consider the effects of ESR heating. Soft-Start and Output Overloads The soft-start routine controls the rate of rise of the reference voltage, which in turn controls the FB pin, and thereby the out- put voltage (VOUT )(see figure 1). This function minimizes the amount of inrush current drawn from the input voltage (VIN) and potential voltage overshoot on the output rail (VOUT ). A soft-start routine is initiated when the enable pin (EN) is high, no overvoltage exists on the output, the thermal protection cir- cuitry is not activated, and VIN is above the undervoltage thresh- old. Immediately after EN goes high, the soft-start capacitor is charged via an internal 10 μA source and PWM switching action occurs. During the Soft-Start Ramp Time (see A in figure 1), the reference is ramped from 0 up to 0.6 V, and the output voltage (VOUT) tracks the reference voltage. The POK flag is held low until the output voltage reaches 90% (typical) of the target volt- age and a delay of 90 μs (typical) occurs. When an output overcurrent event occurs, the regulator imme- diately limits the valley current at a constant level on a pulse-by pulse basis. The output voltage will tend to fold back, depending on how low the output impedance is. When the output voltage drops below 85% (typical) of the target voltage, the POK flag goes low. If the overload occurs for shorter than the Hiccup Overload Duration (<50 μs; B in figure 1), the output will auto- matically recover to the target level. If the overload occurs for longer than the Hiccup Overload Duration (>50 μs; C in figure 1), the regulator will shut down, the soft-start capacitor will be discharged, and (assuming no other fault conditions exist and the enable pin is still high) the regulator will be delayed by the Hic- cup Shutdown Duration (D in figure 1). The Hiccup Shutdown Duration ensures that prolonged overload conditions do not cause excessive junction temperatures to occur. After the Hiccup Shutdown Duration has elapsed, the output voltage is again brought up, controlled by the soft-start function. However, if the overload condition still exists and still remains after the Soft-Start Ramp Time has elapsed, the regulator will shut down and the process will repeat until the fault is removed. The Soft-Start Ramp Time, tss, can be found from the following formula: tSS CSS 0.6 10 ×10 –6 = (12) where CSS is C5 in the Typical Applications section circuit dia- grams. Although the A8670 is optimized for ceramic output capacitors, large value electrolytic capacitors can be used where either spe- cial hold-up, or power sequencing is required. Note the guidelines for selecting large value capacitors in the Control Loop section. Enable (EN) Soft-Start (SS) Output Voltage Load Current 0 V 0 V power OK (POK) 0 V 0A 0 V Soft-Start Ramp Time <50 μs 90 μs POK Delay 90 μs POK Delay 90 μs POK Delay Valley Current Limit Maximum load >50 μs Hiccup Overload Duration Hiccup Overload Duration Target output voltage 90% of Target Soft-Start Ramp Time Maximum load Target output voltage > 200 μs Hiccup Shutdown Duration A A B C D 90% of Target 85% of Target Figure 1. Operation of the soft-start function |
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