ACT4075 - Active-Semi, Inc.

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TYPICAL APPLICATION CIRCUIT

ACT4075

Rev PrA, 13-Aug-07

Advanced Product Information – All Information Subject to Change

Wide Input 2.5A Step Down Converter

FEATURES

 2.5A Output Current

 Up to 95% Efficiency

 4.5V to 32V Input Range

 6µA Shutdown Supply Current

 410kHz Switching Frequency

 Adjustable Output Voltage

 Cycle-by-Cycle Current Limit Protection

 Thermal Shutdown Protection

 Internal Soft Start Function

 Frequency Fold Back at Short Circuit

 Stability with Wide Range of Capacitors,

Including Low ESR Ceramic Capacitors

 SOP-8/EP (Exposed Pad) Package

APPLICATIONS

 TFT LCD Monitors or Televisions and HDTV

 Portable DVD Players

 Car-Powered or Battery-Powered Equipment

 Set-Top Boxes

 Telecom Power Supplies

 DSL and Cable Modems and Routers

GENERAL DESCRIPTION

The ACT4075 is a current-mode step-down DC/DC

converter that generates up to 2.5A output current

at 410kHz switching frequency. The device utilizes

Active-Semi’s proprietary ISOBCD30 process for

operation with input voltage up to 32V.

Consuming only 6μA in shutdown mode, the

ACT4075 is highly efficient with peak efficiency at

95% when in operation. Protection features include

cycle-by-cycle current limit, thermal shutdown, and

frequency fold back at short circuit. The device also

includes an internal soft start function to prevent

overshoot.

The ACT4075 is available in SOP-8/EP exposed

pad package and requires very few external de-

vices for operation.

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ACT4075

Rev PrA, 13-Aug-07

ORDERING INFORMATION

PART NUMBER TEMPERATURE RANGE PACKAGE PINS PACKING

ACT4075YH -40°C to 85°C SOP-8/EP 8 TUBE

ACT4075YH-T -40°C to 85°C SOP-8/EP 8 TAPE & REEL

PIN CONFIGURATION

PIN DESCRIPTIONS

PIN NUMBER PIN NAME PIN DESCRIPTION

1 BS Bootstrap. This pin acts as the positive rail for the high-side switch’s gate driver.

Connect a 10nF between this pin and SW.

2 IN Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor

in Application Information section.

3 SW Switch Output. Connect this pin to the switching end of the inductor.

4 G Ground.

5 FB Feedback Input. The voltage at this pin is regulated to 1.222V. Connect to the

resistor divider between output and ground to set output voltage.

6 COMP Compensation Pin. See Compensation Technique in Application Information sec-

tion.

7 EN

Enable Input. When higher than 1.3V, this pin turns the IC on. When lower than

0.7V, this pin turns the IC off. Output voltage is discharged when the IC is off. This

pin has a small internal pull up current to a high level voltage when pin is not con-

nected.

8 N/C Not Connected.

EP EP

Exposed Pad shown as dashed box. The exposed thermal pad should be con-

nected to board ground plane and pin 4. The ground plane should include a large

exposed copper pad under the package for thermal dissipation (see package out-

line). The leads and exposed pad should be flush with the board, without offset

from the board surface.

SOP-8/EP

8

7

6

5

1

2

3

4

BS

IN

SW

GFB

COMP

EN

N/C

ACT4075

EP

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ACT4075

Rev PrA, 13-Aug-07

ABSOLUTE MAXIMUM RATINGS

PARAMETER VALUE UNIT

IN to G -0.3 to +34 V

EN to G -0.3 to VIN + 0.3 V

BS to SW -0.3 to +8 V

FB, COMP to G -0.3 to 6 V

Continuous SW Current Internally limited A

Junction to Ambient Thermal Resistance (θJA) 46 °C/W

Maximum Power Dissipation 1.8 W

Operating Junction Temperature -40 to 150 °C

Storage Temperature -55 to 150 °C

Lead Temperature (Soldering, 10 sec) 300 °C

SW to G -1 to VIN + 1 V

ELECTRICAL CHARACTERISTICS

(VIN = 12V, TA= 25°C, unless otherwise specified.)

PARAMETER SYMBOL TEST CONDITIONS MIN TYP MAX UNIT

Input Voltage VIN VOUT = 2.5V, ILOAD = 0A to 2.5A 4.5 32 V

Feedback Voltage VFB 1.198 1.222 1.246 V

High-Side Switch On Resistance RONH 100 mΩ

Low-Side Switch On Resistance RONL 10 Ω

SW Leakage VEN = 0, VIN = 12V, VSW = 0V 0 10 µA

Current Limit ILIM 3.5 5 A

COMP to Current Limit Transcon-

ductance GCOMP ΔILOAD/ΔICOMP 3 A/V

Error Amplifier Transconductance GEA ΔICOMP = ±10µA 550 µA/V

Error Amplifier DC Gain AVEA 4000 V/V

Switching Frequency fSW 340 410 490 kHz

Short Circuit Switching Frequency VFB = 0V 40 kHz

Maximum Duty Cycle DMAX VFB = 1.1V, PWM mode 90 %

Minimum Duty Cycle VFB = 1.4V, PFM mode 0 %

Enable Threshold Voltage Hysteresis = 0.1V 0.7 1 1.3 V

Enable Pull Up Current Pin pulled up to VIN when left uncon-

nected 2 µA

Supply Current in Shutdown VEN = 0 6 20 µA

IC Supply Current in Operation VEN = 3V, not switching 0.85 2 mA

Thermal Shutdown Temperature Hysteresis = 10°C 160 °C

: Do not exceed these limits to prevent damage to the device. Exposure to absolute maximum rating conditions for long periods may

affect device reliability.

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ACT4075

Rev PrA, 13-Aug-07

FUNCTIONAL BLOCK DIA GRAM

FUNCTIONAL DESCRIPTION

As seen in the Functional Block Diagram, the

ACT4075 is a current mode pulse width modulation

(PWM) converter. The converter operates as fol-

lows:

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 con-

nected 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 result-

ing 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 de-

crease 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 1.222V refer-

ence. 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 2.65V.

The Oscillator normally switches at 410kHz. How-

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

switching frequency decreases until it reaches a

typical value of 40kHz at VFB = 0V.

Shutdown Control

The ACT4075 has an enable input EN for turning

the IC on or off. When EN is less than 0.7V, the IC

is in 6μA 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.

Thermal Shutdown

The ACT4075 automatically turns off when its junc-

tion temperature exceeds 160°C and then restarts

once the temperature falls to 150°C.

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ACT4075

Rev PrA, 13-Aug-07

FB

ACT4075

RFB2

RFB1

VOUT

Output Voltage Setting

Figure 1 shows the connections for setting the out-

put voltage. Select the proper ratio of the two feed-

back resistors RFB1 and RFB2 based on the output

voltage. Typically, use RFB2 ≈ 10kΩ and determine

RFB1 from the output voltage:

The inductor maintains a continuous current to the

output load. This inductor current has a ripple that is

dependent on the inductance value: higher induc-

tance reduces the peak-to-peak ripple current. The

trade off for high inductance value is the increase in

inductor core size and series resistance, and the

reduction in current handling capability. In general,

select an inductance value L based on ripple current

requirement:

VOUT 1.5V 1.8V 2.5V 3.3V 5V

L 6.8μH 6.8μH 6.8μH 8.5μH 15μH













1

V222.1V

RR OUT

FB21FB -

The input capacitor needs to be carefully selected to

maintain sufficiently low ripple at the supply input of

the converter. A low ESR capacitor is highly recom-

mended. Since large current flows in and out of this

capacitor during switching, its ESR also affects effi-

ciency.

The input capacitance needs to be higher than

10µF. The best choice is the ceramic type; however,

low ESR tantalum or electrolytic types may also be

used provided that the RMS ripple current rating is

higher than 50% of the output current. The input

capacitor should be placed close to the IN and G

pins of the IC, with shortest traces possible. In the

case of tantalum or electrolytic types, they can be

further away if a small parallel 0.1µF ceramic ca-

pacitor is placed right next to the IC.

The output capacitor also needs to have low ESR to

keep low output voltage ripple. The output ripple

voltage is:

where IOUTMAX is the maximum output current, KRIPPLE

is the ripple factor, RESR is the ESR resistance of the

output capacitor, fSW is the switching frequency, L is

the inductor value, COUT is the output capacitance. In

the case of ceramic output capacitors, RESR is very

small and does not contribute to the ripple. There-

fore, a lower capacitance value can be used for ce-

ramic type. In the case of tantalum or electrolytic

type, the ripple is dominated by RESR multiplied by the

ripple current. In that case, the output capacitor is

chosen to have sufficiently low ESR.

For ceramic output type, typically choose a capaci-

tance of about 22µF. For tantalum or electrolytic type,

choose a capacitor with less than 50mΩ ESR.

OUT

2

SW

IN

RIPPLERIPPLEOUTMAXRIPPLE

LCf28 VRKIV







Rectifier Diode

Use a Schottky diode as the rectifier to conduct cur-

rent when the High-Side Power Switch is off. The

Schottky diode must have current rating higher than

the maximum output current and the reverse volt-

age rating higher than the maximum input voltage.

Output Capacitor

Inductor Selection

Table 1:

Typical Inductor Values

Input Capacitor

APPLICATIONS INFORMATION

where VIN is the input voltage, VOUT is the output

voltage, fSW is the switching frequency, IOUTMAX is the

maximum output current, and KRIPPLE is the ripple

factor. Typically, choose KRIPPLE = between 20% and

30% to correspond to the peak-to-peak ripple current

being a percentage of the maximum output current.

With this inductor value (Table 1), the peak inductor

current is IOUT × (1 + KRIPPLE / 2). Make sure that this

peak inductor current is less that the 5A current

limit. Finally, select the inductor core size so that it

does not saturate at 5A.

(1)





RIPPLEOUTMAXSWIN

OUTINOUT KIfV VVV

L-



(2)

(3)

Figure 1:

Output Voltage Setting

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ACT4075

Rev PrA, 13-Aug-07

Stability compensation

The feedback system of the IC is stabilized by the

components at COMP pin, as shown in Figure 2.

The DC loop gain of the system is determined by

the following equation:

COMPVEA

OUT

VDC GA

IV222.1

A

The dominant pole P1 is due to CCOMP:

COMPVEA

EA

1P CA2 G

f





COMPCOMP

1Z CR2 1

f





And finally, the third pole is due to RCOMP and

CCOMP2 (if CCOMP2 is used):

COMP2COMP

3P CR2 1

f





Follow the following steps to compensate the IC:

STEP 1. Set the cross over frequency at 1/10 of the

switching frequency via RCOMP:

but limit RCOMP to 15kΩ maximum.

OUTOUT

OUT

2P CV2 I

f





The first zero Z1 is due to RCOMP and CCOMP:

The second pole P2 is the output pole:

STEP 2. Set the zero fZ1 at 1/4 of the cross over

frequency. If RCOMP is less than 15kΩ, the equation

for CCOMP is:

If RCOMP is limited to 15kΩ, then the actual cross

over frequency is 4.8/(VOUTCOUT). Therefore:

OUTOUT

6

COMP CV10x8.8C 



STEP 3. If the output capacitor’s ESR is high

enough to cause a zero at lower than 4 times the

cross over frequency, an additional compensation

capacitor CCOMP2 is required. The condition for us-

ing CCOMP2 is:

And the proper value for CCOMP2 is:

COMP

ESROUTOUT

COMP RRC

C

Though CCOMP2 is unnecessary when the output

capacitor has sufficiently low ESR, a small value

CCOMP2 such as 220pF may improve stability

against PCB layout parasitic effects.

Table 2 shows some calculated results based on

the compensation method above.

Table 2:

Typical Compensation for Different Output

Voltages and Output Capacitors

VOUT COUT RCOMP CCOMP CCOMP2

1.8V 22μF Ceramic 4kΩ 3.3nF 220pF

2.5V 22μF Ceramic 5.6kΩ 3.3nF 220pF

5V 22μF Ceramic 12kΩ 1.5nF 220pF

1.8V 100μF SP CAP 15kΩ 1.5nF 220pF

2.5V 100μF SP CAP 15kΩ 2.2nF 220pF

5V 100μF SP CAP 15kΩ 4.7nF 220pF

Figure 3 shows a sample ACT4075 application

circuit generating a 2.5V/2.5A output.

(11)

(13)

(4)

(5)

(6)

(7)

(8)

OUTOUT

8

COMPEA

SWOUTOUT

COMP

CV10x25.1 V222.1GG10 fCVπ2

R



(Ω) (9)

V012.0,

C10x1.1

Min

R

OUT

6

ESROUT















(12)

(Ω)

COMP

5

COMP R10x6.1

C

(10)

(F)

: CCOMP2 is needed for board parasitic and high ESR output

capacitor.

: CCOMP2 is needed only for high ESR output capacitor

Figure 2:

Stability Compensation

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ACT4075

Rev PrA, 13-Aug-07

Figure 3:

ACT4075 2.5V/2.5A Output Application

: D1 is a 40V, 3A Schottky diode with low forward voltage, an IR 30BQ040 or SK34 equivalent. C4 can be either a ceramic capacitor

(Panasonic ECJ-3YB1C226M) or SP-CAP (Specialty Polymer) Aluminum Electrolytic Capacitor such as Panasonic EEFCD0J470XR.

The SP-Cap is based on aluminum electrolytic capacitor technology, but uses a solid polymer electrolyte and has very stable capaci-

tance characteristics in both operating temperature and frequency compared to ceramic, polymer, and low ESR tantalum capacitors.

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ACT4075

Rev PrA, 13-Aug-07

TYPICAL PERFORMANCE CHARACTERISTICS

(Circuit of Figure 3, unless otherwise specified.)

Input Voltage (V)

8 10 12 14 16 18 20 22 24 26 28 30

ACT4075-0004

Switching Frequency vs. Input Voltage

360

410

310

Switching Frequency (kHz)

460

510

ACT4075-006

Output Current (A)

Surface Temperature vs. Output Current

Surface Temperature (°C)

20

80

100

120

140

40

60

0 0.5 1 1.5 2 2.5

VOUT = 5V

L = 15µH

CIN = 22µF

COUT = 22µF

VIN = 30V

VIN = 20V

VIN = 12V

ACT4075-0005

Input Voltage (V)

Shutdown Supply Current (µA)

4

5 10 15 20 25 30

Shutdown Supply Current vs. Input Voltage

0

2

6

8

10

12

14

16

18

40

70

80

90

100

50

60

Efficiency (%)

Output Current (A)

ACT4075-0001

30

20

10

0

0.01 0.1 1 10

Efficiency vs. Output Current

VOUT = 5V

L = 15µH

CIN = 22µF

COUT = 22µF

VIN = 20V

VIN = 30V

VIN = 8V

VIN = 12V

40

70

80

90

100

50

60

Efficiency (%)

0.01 0.1 1 10

Output Current (A)

ACT4075-0002

Efficiency vs. Output Current

30

20

10

VOUT = 2.5V

L = 10µH

CIN = 22µF

COUT = 22µF

0

VIN = 8V

VIN = 30V

VIN = 12V

VIN = 20V

-40

Temperature (°C)

-20 0 20 40 60 80 100

ACT4075-0003

1.21

1.25

1.27

1.23

1.19

1.17

Feedback Voltage (V)

Feedback Voltage vs. Temperature

0

35

32

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ACT4075

Rev PrA, 13-Aug-07

TYPICAL PERFORMANCE CHARACTERISTICS

(Circuit of Figure 3, unless otherwise specified.)

ACT4075-0007

VIN = 12V

Load Transient Response

VOUT

200mV/div

1A

IOUT

0A

100µs/div

ACT4075-0008

VIN = 12V

Load Transient Response

1A

IOUT

0A

VOUT

200mV/div

100µs/div

ACT4075-0009

VIN = 12V

Load Transient Response

VOUT

200mV/div

3A

IOUT

2A

100µs/div

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ACT4075

Rev PrA, 13-Aug-07

Active-Semi, Inc. reserves the right to modify the circuitry or specifications without notice. Us ers should e valuate each

product to make sure that it is suitable for their applications. Active-Semi products a re not intended or authorized for use

as critical components in life-support devices or systems. Active-Semi, Inc. does not assume any liability arising out of

the use of any product or circuit described in this datasheet, nor does it convey any patent license.

Active-Semi and its logo are trademarks of Active-Semi, Inc. For more information on this and ot her products, contact

sales@active-semi.com or visit http://www.active-semi.com. For other inquiries, please send to:

1270 Oakmead Parkway, Suite 310, Sunnyvale, California 94085-4044, USA

PACKAGE OUTLINE

SOP-8/EP PACKAGE OUTLINE AND DIMENSIONS

SYMBOL DIMENSION IN

MILLIMETERS DIMENSION IN

INCHES

MIN MAX MIN MAX

A 1.350 1.750 0.053 0.069

A1 0.050 0.150 0.002 0.006

A2 1.350 1.550 0.053 0.061

b 0.330 0.510 0.013 0.020

c 0.170 0.250 0.007 0.010

D 4.700 5.100 0.185 0.200

D1 3.202 3.402 0.126 0.134

E 3.800 4.000 0.150 0.157

E1 5.800 6.200 0.228 0.244

E2 2.313 2.513 0.091 0.099

e 1.270 TYP 0.050 TYP

L 0.400 1.270 0.016 0.050

θ 0° 8° 0° 8°