ACT8846 datasheet(34/44 Pages) ACTIVE-SEMI

34 / 44 page

ACT8846

Rev 7.0 06-Dec-2017

Innovative PowerTM

34 -

www.active-semi.com

ActivePMUTM is a trademark of Active-Semi.

I2CTM is a trademark of NXP.

General Description

REG1, REG2, REG3, and REG4 are fixed-frequency,

current-mode,

synchronous

PWM

step-down

converters that achieves peak efficiencies of up to

97%. These regulators operate with a fixed frequency

2.25MHz,

minimizing

noise

sensitive

applications and allowing the use of small external

components. Additionally, REG1, REG2, REG3, and

REG4 are available with a variety of standard and

custom output voltages, and may be software-

controlled via the I2C interface for systems that

require advanced power management functions.

100% Duty Cycle Operation

REG1, REG2, REG3, and REG4 are capable of

operating at up to 100% duty cycle. During 100%

duty cycle operation, the high-side power MOSFETs

are held on continuously, providing a direct

connection from the input to the output (through the

inductor), ensuring the lowest possible dropout

voltage in battery powered applications.

Operating Mode

By default, REG1, REG2, REG3, and REG4 operate

in fixed-frequency PWM mode at medium to heavy

loads, then transition to a proprietary power-saving

mode at light loads in order to save power.

Synchronous Rectification

REG1, REG2, REG3, and REG4 each feature

integrated

synchronous

rectifiers,

maximizing

efficiency and minimizing the total solution size and

cost by eliminating the need for external rectifiers.

Soft-Start

REG1, REG2, REG3, and REG4 include internal 400

us soft-start ramps which limit the rate of change of

the output voltage, minimizing input inrush current

and ensuring that the output powers up in a

monotonic manner that is independent of loading on

the outputs. This circuitry is effective any time the

regulator is enabled, as well as after responding to a

short-circuit or other fault condition.

Compensation

REG1, REG2, REG3, and REG4 utilize current-mode

control and a proprietary internal compensation

scheme

simultaneously

simplify

external

component

selection

and

optimize

transient

performance over their full operating range. No

compensation design is required; simply follow a few

simple guide lines described below when choosing

external components.

Input Capacitor Selection

The input capacitor reduces peak currents and noise

induced upon the voltage source. A 10μF ceramic

capacitor is recommended for each regulator in most

applications.

Output Capacitor Selection

REG1, REG2, REG3, and REG4 were designed to

take advantage of the benefits of ceramic capacitors,

namely small size and very-low ESR. REG1, REG2,

REG3 and REG4 are designed to operate with 33uF

or 44uF output capacitor over most of their output

voltage ranges, although more capacitance may be

desired depending on the duty cycle and load step

requirements.

Two of the most common dielectrics are Y5V and

X5R. Whereas Y5V dielectrics are inexpensive and

can provide high capacitance in small packages, their

capacitance varies greatly over their voltage and

temperature ranges and are not recommended for

DC/DC applications. X5R and X7R dielectrics are

more suitable for output capacitor applications, as

their characteristics are more stable over their

operating ranges, and are highly recommended.

Inductor Selection

REG1, REG2, REG3, and REG4 utilize current-mode

control and a proprietary internal compensation

scheme

simultaneously

simplify

external

component

selection

and

optimize

transient

performance over their full operating range. These

devices were optimized for operation with 2.2μH or

1μH 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%.

Configuration Options

Output Voltage Programming

Figure 4:

OUT1 Output Voltage Programming

STEP-DOWN DC/DC REGULATORS

RFB1

RFB2

OUT1

ACT8846

FB1

CFF

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