ACT8870 datasheet(28/38 Pages) ACTIVE-SEMI

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ACT8870QJ-T

REV 0 23-DEC-2016

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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

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.

∆

∗

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

LDO is enabled, as well as after responding to a short

circuit or other fault condition. Each LDO’s softstart time

is adjustable via its I2C bits SS_RAMP between 525us

and 1000us.

Overcurrent and Short Circuit Protection

Each LDO provides overcurrent and short circuit protec-

tion. The overcurrent threshold is set by the LDOx_ILIM

I2C bits. In both an overload and a short circuit condition,

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