ACT4012SH-T データシートの表示（PDF） - Active-Semi, Inc

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ACT4012SH-T

Wide Input 2A Step Down Converter

Active-Semi, Inc 

IN

EN

COMP

1.293V

FB

2μA

ENABLE

ERROR

AMPLIFIER

+

–

REGULATOR

&

REFERENCE

CURRENT SENSE

AMPLIFIER

+

–

+– +–

PWM

COMPARATOR

FOLDBACK

CONTROL

OSCILLATOR

&

RAMP

LOGIC

THERMAL

SHUTDOWN

Figure 2. Functional Block Diagram

ACT4012

BS

0.4Ω

HIGH-SIDE

POWER

SWITCH

SW

10Ω LOW-SIDE

POWER SWITCH

G

FUNCTIONAL DESCRIPTION

As seen in Figure 2, Functional Block

Diagram, the ACT4012 is a current mode pulse

width modulation (PWM) converter. The

converter operates as follows:

A switching cycle starts when the rising edge

of the Oscillator clock output causes the High-

Side Power Switch to turn on and the Low-Side

Power Switch to turn off. With the SW side of the

inductor now connected to IN, the inductor

current ramps up to store energy in the its

magnetic field. The inductor current level is

measured by the Current Sense Amplifier and

added to the Oscillator ramp signal. If the

resulting summation is higher than the COMP

voltage, the output of the PWM Comparator

goes high. When this happens or when

Oscillator clock output goes low, the High-Side

Power Switch turns off and the Low-Side Power

Switch turns on. At this point, the SW side of the

inductor swings to a diode voltage below ground,

causing the inductor current to decrease and

magnetic energy to be transferred to output. This

state continues until the cycle starts again.

The High-Side Power Switch is driven by

logic using BS bootstrap pin as the positive rail.

This pin is charged to VSW + 6V when the Low-

Side Power Switch turns on.

The COMP voltage is the integration of the

error between FB input and the internal 1.293V

reference. If FB is lower than the reference

voltage, COMP tends to go higher to increase

current to the output. Current limit happens when

COMP reaches its maximum clamp value of

2.55V.

The Oscillator normally switches at 410kHz.

However, if FB voltage is less than 0.7V, then the

switching frequency decreases until it reaches a

minimum of 50kHz at VFB = 0.5V.

SHUTDOWN CONTROL

The ACT4012 has an enable input EN for

turning the IC on or off. When EN is less than

0.7V, the IC is in 8μA low current shutdown

mode and output is discharged through the Low-

Side Power Switch. When EN is higher than

1.3V, the IC is in normal operation mode. EN is

internally pulled up with a 2μA current source

and can be left unconnected for always-on

operation. Note that EN is a low voltage input

with a maximum voltage of 6V; it should never

be directly connected to IN.

THERMAL SHUTDOWN

The ACT4012 automatically turns off when its

junction temperature exceeds 160°C.

Active-Semi, Inc.

-4-

www.active-semi.com

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