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ACT8870QJ-T Datasheet(PDF) 28 Page - Active-Semi, Inc |
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ACT8870QJ-T Datasheet(HTML) 28 Page - Active-Semi, Inc |
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28 / 43 page  ACT8870QJ-T Rev 3.0, 02-Oct-2017 Innovative PowerTM www.active-semi.com ActiveSwitcherTM is a trademark of Active-Semi. Copyright © 2016-2017 Active-Semi, Inc. I2CTM is a trademark of NXP 28 Input Capacitor Selection Each regulator requires a high quality, low-ESR, ceramic input capacitor. Note that even though each buck converter has separate input pins, all input pins must be connected to the same voltage potential. 10uF capacitors are typically suitable, but this value can be increased without limit. Smaller capacitor values can be used with lighter output loads. Choose the input capacitor value to keep the input voltage ripple less than 50mV. Vripple ∗ ∗ 1 ∗ Be sure to consider the capacitor’s DC bias effects and maximum ripple current rating when using capacitors smaller than 0805. A capacitor’s actual capacitance is strongly affected by its DC bias characteristics. The input capacitor is typically an X5R, X7R, or similar dielectric. Use of Y5U, Z5U, or similar dielectrics is not recommended. Input capacitor placement is critical for proper operation. Each buck’s input capacitor must be placed as close to the IC as possible. The traces from VIN_Bx to the capacitor and from the capacitor to PGNDx should as short and wide as possible. Inductor Selection The Buck converters utilize current-mode control and a proprietary internal compensation scheme to simultaneously simplify external component selection and optimize transient performance over their full operating range. The ACT8870 is optimized for operation with 1.0-1.5μH inductors. Choose an inductor with a low DC-resistance, and avoid inductor saturation by choosing inductors with DC ratings that exceed the maximum output current by at least 30%. The following equation calculates the inductor ripple current. ∆ 1 ∗ ∗ Where VOUT is the output voltage, VIN is the input voltage, FSW is the switching frequency, and L is the inductor value. Output Capacitor Selection The ACT8870 is designed to use small, low ESR, ceramic output capacitors. Buck1 typically requires a 44uF output capacitor while Buck2, Buck3, and Buck4 require a 22uF output capacitor each. In order to ensure stability, the actual Buck1 capacitance should be greater than 33uF while Buck2, Buck3, and Buck4 should be greater than 15uF. The output capacitance can be increased to reduce output voltage ripple and improve load transients if needed. Design for an output ripple voltage less than 1% of the output voltage. The following equation calculates the output voltage ripple as a function of output capacitance. VRIPPLE ∆ 8∗ ∗ Where ΔIL is the inductor ripple current, FSW is the switching frequency, and COUT is the output capacitance after taking DC bias into account. Be sure to consider the capacitor’s DC bias effects and maximum ripple current rating when using capacitors smaller than 0805. A capacitor’s actual capacitance is strongly affected by its DC bias characteristics. The output capacitor is typically an X5R, X7R, or similar dielectric. Use of Y5U, Z5U, or similar dielectrics are not recommended due to their wide variation in capacitance over temperature and voltage ranges. LDO CONVERTERS General Description The ACT8870 contains three fully integrated low dropout linear regulators (LDO). LDO1 is an 800mA output, while LDO2 and LDO3 are 200mA outputs. The LDOs are require only two small external components (Cin, Cout) for operation. They ship with default output voltages that can be modified via the I2C interface for systems that require advanced power management functions. LDO1 has a dedicated input pin. LDO2 and LDO3 share an input pin. The LDOs can operate from different input voltages than the buck converters. LDO1 and LDO2/3 can operate from different input voltage from each other. Enable / Disable Control When power is applied to the IC, all LDOs automatically turn on according to a pre-programmed sequence. Once in normal operation (ACTIVE state), each con- verter can be independently disabled via I2C. Each CMI version requires a different set of command to disable a converter, so contact the factory for specific instructions if needed. Each converter contains an optional inte- grated discharge resistor that actively discharges the output capacitor when the regulator is disabled. Each LDO’s discharge function is enabled via its I2C bit DIS_PULLDOWN. Soft-Start Each LDO contains a softstart circuit that limits the rate of change of the output voltage, minimizing input inrush current and ensuring that the outputs power up in a monotonically. This circuitry is effective any time the |
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