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ACT4910QW Scheda tecnica(PDF) 21 Page - Active-Semi, Inc |
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ACT4910QW Scheda tecnica(HTML) 21 Page - Active-Semi, Inc |
21 / 34 page ACT4910QW Rev 1.0, 15-Sept-2017 Innovative PowerTM www.active-semi.com ActiveSwitcherTM is a trademark of Active-Semi Copyright © 2017 Active-Semi, Inc. 21 then retries to softstart again. The following equation calculates the minimum allowable softstart time. _ ∗ .∗ Equation 3 Where COUT is the sum of the output capacitance and storage capacitance in mF, VOUT is the output voltage in volts, and IILIM is the eFuse current limit in Amperes programmed by the ILIM resistor. Buck Converter General Description The ACT4910 contains current-mode, synchronous PWM step-down converter that achieves peak efficiencies of 95%. The buck converter minimizes noise in sensitive applications and allows the use of small external components. It is highly flexible with external component selection and can be reconfigured via I2C registers. External components set the output voltage and compensation while I2C registers set the switching frequency and current limit. The buck converter operates in fixed frequency PWM mode. Its switching frequency is programmable between 320kHz to 1130 kHz via the I2C register BK_FREQ[2:0] which allows the system to be optimized for different applications. Its current limit is adjustable between 5A to 9A, allowing for further system optimization. The output voltage is externally programmable between 1.8V and 18V. The buck converter generates a regulated output voltage at the VOUT pin from the storage capacitors when the IC enters supplement mode. This provides the backup power when the system experiences fault conditions. After the IC exits the SOFTSTART state, the buck converter is enabled but remains turned off. It automatically turns on when the IC enters supplement mode, and remains on until the storage capacitors discharge to 3.6V. Frequency Setting Higher switching frequencies result in smaller solution sizes at the cost of slightly lower efficiency. Lower switching frequencies result in larger solution sizes with higher efficiency. The maximum allowable switching frequency for any given design is limited by the following equation. _ ∗ Equation 3 Where FSW_max is the maximum allowable frequency, VSTR is the storage voltage, and VOUT is the output voltage during supplement mode. Output Voltage Setting The buck converter output voltage is programmed by an external resistor divider connected between the VOUT pin and VSS, with the center tap connected to the FB pin. The buck output voltage can be set above, below, or equal to the input voltage supplement threshold. When the input voltage drops below this threshold, the IC enters supplement mode and regulates the output to the programmed buck voltage. Although the buck converter immediately starts up when the IC enters supplement mode, the output voltage still has a small, but finite drop in output voltage between the time the eFuse turns off and the buck converter is fully on. This voltage drop should be considered when setting the output voltage. The following equation calculates the correct resistor values to set the desired output voltage. 1 R2 ∗ 1 Equation 4 Where R1 is the top feedback resistor, R2 is the bottom feedback resistor, VOUT is the desired output voltage, and VFB is the fixed 1.2V reference voltage on the FB pin. Choose R2 in the range of 10kohm. Smaller resistor values are acceptable, but larger values will affect voltage accuracy due to bias currents into the FB pin. Protection The buck converter has several protection mechanisms to insure safe operation. It stops operation when input voltage from storage cap reaches STR_UVLO (3.6V) or when the output voltage drops below the power good threshold which is fixed at 93% of the output setpoint. It also stops operating when the output voltage is above the over voltage threshold which is fixed at 110% of the output voltage setpoint. The output undervoltage protection can be masked by the I2C register bit Mask_BK_UV REG0x38 [0]. The buck converter provides overcurrent and short circuit protection. Overcurrent protection is achieved with cycle-by-cycle current limiting. The peak current threshold is set between 5A and 9A by the BK_CLIM bits. If the peak current reaches the programmed threshold, the IC turns off the power FET. This condition typically results in shutdown due to an output voltage UV condition due to the shortened switching cycle. |
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