ACT8941A datasheet(31/42 Pages) ACTIVE-SEMI

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ACT8941A

Rev 0, 12-Mar-13

- 31 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

I2CTM is a trademark of NXP.

Innovative PowerTM

Active-Semi Proprietary―For Authorized Recipients and Customers

General Description

The ACT8941A features three synchronous, fixed-

frequency,

current-mode

PWM

step

down

converters that achieve peak efficiencies of up to

97%. REG1 and REG2 are capable of supplying up

to 1100mA of output current, while REG3 supports

up to 1200mA. These regulators operate with a

fixed frequency of 2MHz, minimizing noise in

sensitive applications and allowing the use of small

external components.

100% Duty Cycle Operation

Each regulator is capable of operating at up to

100%

duty

cycle.

During

100%

duty-cycle

operation, the high-side power MOSFET is 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.

Synchronous Rectification

REG1, REG2, and REG3 each feature integrated n-

channel

synchronous

rectifiers,

maximizing

efficiency and minimizing the total solution size and

cost by eliminating the need for external rectifiers.

Soft-Start

When enabled, each output voltages tracks an

internal 400μs soft-start ramp, minimizing input

current during startup and allowing each regulator

to power up in a smooth, monotonic manner that is

independent of output load conditions.

Compensation

Each buck regulator utilizes current-mode control

and a proprietary internal compensation scheme to

simultaneously

simplify

external

component

selection and optimize transient performance over

its full operating range. No compensation design is

required; simply follow a few simple guidelines

described

below

when

choosing

external

components.

Input Capacitor Selection

The input capacitor reduces peak currents and

noise induced upon the voltage source. A 4.7μF

ceramic capacitor is recommended for each

regulator in most applications.

Output Capacitor Selection

For most applications, 22μF ceramic output

capacitors are recommended for REG1, REG2 and

REG3.

Despite the advantages of ceramic capacitors, care

must be taken during the design process to ensure

stable operation over the full operating voltage and

temperature

range.

Ceramic

capacitors

are

available in a variety of dielectrics, each of which

exhibits different characteristics that can greatly

affect performance over their temperature and

voltage ranges.

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, and REG3 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

inductors, although inductors in the 1.5μH to 3.3μH

range can be used. 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

By default, each regulator powers up and regulates

to its default output voltage. Output voltage is

selectable by setting VSEL pin that when VSEL is

low, output voltage is programmed by VSET1[-]

bits, and when VSEL is high, output voltage is

programmed by VSET2[-] bits. However, once the

system is enabled, each regulator's output voltage

may be independently programmed to a different

value, typically in order to minimize the power

consumption of the microprocessor during some

operating modes. Program the output voltages via

the I2C serial interface by writing to the regulator's

VSET1[-] register if VSEL is low or VSET2[-]

Enable / Disable Control

During normal operation, each buck may be

enabled or disabled via the I2C interface by writing

to that regulator's ON[ ] bit. The regulator accept

rising or falling edge of ON[ ] bit as on/off signal. To

enable the regulator, clear ON[ ] to 0 first then set to

STEP-DOWN DC/DC REGULATORS

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