ACT4088 - Active-Semi, Inc.

Innovative PowerTM - 1 - www.active-semi.com

TYPICAL APPLICATION CIRCUIT

ACT4088

Rev0, 13-May-08

28V, 1.5A, 1.4MHz Step-Down DC/DC Converter in SOT23-6

FEATURES

• Wide 4.5V to 28V Input Voltage Range

• 1.5A Output Current (12VIN to 5VOUT)

• Output Adjustable Down to 0.81V

• 0.3Ω Internal Power MOSFET

• Up to 92% Efficiency

• Stable with Low ESR Ceramic Output Capacitors

• Fixed 1.4MHz Operating Frequency

• Internal Soft-Start Function

• Over Current Protection with Hiccup-Mode

• Thermal Shutdown

• Available in a SOT23-6 Package

APPLICATIONS

• TFT LCD Monitors

• Portable DVDs, Headphones, MP3 Players, etc.

• Car-Powered or Battery-Powered Equipment

• Set-Top Boxes

• Telecom Power Supplies

• DSL and Cable Modems and Routers

GENERAL DESCRIPTION

The ACT4088 is a current-mode step-down DC/DC

converter that supplies up to 1.5A into 5V from a

12V input. 1.4MHz switching frequency allows the

use of tiny external components, and internal loop

compensation provides simple, stable power sup-

plies with a minimum of external components. Opti-

mized for use with ceramic input and output capaci-

tors, the ACT4088 provides a very compact 1.5A

power supply for space constrained mobile and

consumer applications.

The ACT4088 operates over a wide input voltage

range and utilizes current-mode operation to pro-

vide excellent line and load transient response

while requiring no external compensation compo-

nents. Fault protection includes cycle-by-cycle cur-

rent limiting, frequency fold-back, hiccup mode, and

thermal shutdown. Internal soft-start provides a

controlled startup with no overshoot, even at light

loads.

The ACT4088 is available in a tiny SOT23-6 pack-

age and requires very few external components.

ACT4088

BST

VOUT

VIN

4.5V to 28V

OFF

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 2 - www.active-semi.com

ORDERING INFORMATION

PIN CONFIGURATION

PIN DESCRIPTIONS

PART NUMBER TEMPERATURE RANGE PACKAGE PINS PACKING

ACT4088US-T -40°C to 85°C SOT23-6 6 TAPE & REEL

TOP MARK

FRWJ

SOT23-6

PIN NUMBER PIN NAME PIN DESCRIPTION

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

2 IN

Power supply input. Bypass this pin with a 10µF ceramic capacitor to G, placed as

close to the IC as possible.

3 EN

Enable Input. EN is pulled up to 5V with a 2µA current, and contains a precise 1.24V

logic threshold. Drive this pin to a logic-high or leave unconnected to enable the IC.

Drive to a logic-low to disable the IC and enter micro-power shutdown mode.

4 FB

Feedback Input. The voltage at this pin is regulated to 0.81V. Connect to the center

point of a resistive voltage-divider between OUT and G to set the output voltage.

5 G

Ground and Heat sink. Connect this pin to a large, uncovered PCB copper area for

best heat dissipation.

6 BST

Bootstrap. This pin acts as the power supply for the high-side switch’s gate driver.

Connect a 22nF capacitor between this pin and SW.

ACT4088

BST

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 3 - www.active-semi.com

ABSOLUTE MAXIMUM RATINGSc

PARAMETER VALUE UNIT

IN Supply Voltage -0.3 to 32 V

SW Voltage -1 to VIN + 1 V

BST Voltage VSW - 0.3 to VSW + 7 V

EN, FB Voltage -0.3 to 6 V

Continuous SW Current Internally Limited A

Junction to Ambient Thermal Resistance (JA) 220 °C/W

Maximum Power Dissipation 0.5 W

Operating Junction Temperature -40 to 150 °C

Storage Temperature -55 to 150 °C

Lead Temperature (Soldering, 10 sec) 300 °C

PARAMETER SYMBOL TEST CONDITIONS MIN TYP MAX UNIT

Input Voltage VIN V

OUT = 3.3V, ILOAD = 0A to 1.5A 4.5 28 V

Under Voltage Lockout Voltage VUVLO Input Voltage Rising 4 4.2 4.49 V

Under Voltage Lockout Hysteresis 250 mV

Feedback Voltage VFB 4.75V  VIN  20V, VCOMP = 1.5V 0.79 0.81 0.83 V

Frequency Foldback Threshold 250 mV

High-side Switch On Resistance RONH 0.300 

Low-side Switch On Resistance RONH 15 

SW Leakage VEN = 0, VSW = 0V 1 10 µA

Current Limit ILTM VIN = 12V, VOUT = 5V, or

EN = G, SW = G 1.8 A

Switching Frequency fSW 1.1 1.4 1.6 MHz

Foldback Switching Frequency VFB = 0V, or FB = G 467 kHz

Maximum Duty Cycle DMAX V

FB = 0.6V 92 %

Minimum On-Time 75 ns

EN Threshold Voltage EN Rising 1.12 1.24 1.36 V

EN Hysteresis EN Rising 100 mV

EN Internal Pull-up Current 2 µA

Supply Current in Shutdown VEN = 0V or EN = G 15 30 µA

Supply Current in Operation VEN = 2V, VFB = 1.0V 1 2 mA

Thermal Shutdown Temperature 160 °C

Thermal Shutdown Hysteresis 10 °C

ELECTRICAL CHARACTERISTICS

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

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.

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 4 - www.active-semi.com

TYPICAL PERFORMANCE CHARACTERISTICS

(Circuit of Figure 2, VIN = 12V, L = 4.7µH, C1 = 10µF, C2 = 22µF, TA = +25°C, unless otherwise specified.)

Temperature (°C)

FB Voltage (mV)

FB Voltage vs. Temperature

800

804

808

812

816

820

-40 -20 0 20 40

60 80

100 120

Oscillator Frequency (MHz)

Oscillator Frequency vs. Temperature

1.20

1.30

1.40

1.50

1.60

ACT4088-001

ACT4088-002

ACT4088-003

ACT4088-004

Efficiency vs. Load Current

Efficiency (%)

0.1 1 10

Efficiency vs. Load Current

Efficiency (%)

95 VIN = 12V

VOUT = 5V VOUT = 3.3V

Duty Cycle

Peak Current Limit (A)

Peak Current Limit vs. Duty Cycle

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 80 100

Input Voltage (V)

Quiescent Supply Current (µA)

Shutdown Current vs. Input Voltage

0 4 8 24 28

ACT4088-005

ACT4088-006

Load Current (A)

VIN = 24V

VIN = 18V

0.1 1 10

Load Current (A)

VIN = 24V

VIN = 18V

VIN = 12V

Temperature (°C)

-40 -20 0 20 40

60 80

100 120

3.0

Supply Current

EN Pull-up Current

16 12 20

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 5 - www.active-semi.com

TYPICAL PERFORMANCE CHARACTERISTICS CONT’D

(Circuit of Figure 2, VIN = 12V, L = 4.7µH, C1 = 10µF, C2 = 22µF, TA = +25°C, unless otherwise specified.)

ACT4088-008

ACT4088-009

ACT4088-010

ACT4088-011

ACT4088-012

CH1

CH2

Load Transient Response

CH1

CH2

Start-up Waveforms

CH1

CH2

Start-up Waveforms

CH1: VOUT, 50mV/div, (AC COUPLED)

CH2: VSW, 10V/div

CH3: IL, 500mA/div

TIME: 400ns/div

CH1

CH2

CH3

Steady State Switching Waveforms

CH1: VOUT, 100mV/div, (AC COUPLED)

CH2: IL, 1A/div

TIME: 1ms/div

CH1

CH2

Hiccup Mode Switching Waveforms

ACT4088-007

CH1

CH2

Load Transient Response

ILOAD = 200mA to 800mA

CH1: VOUT, 2V/div

CH2: VSW, 10V/div

CH3: IL, 1A/div

TIME: 200µs/div

CH1: VEN, 2V/div

CH2: VOUT, 2V/div

CH3: VSW, 10V/div

CH4: IL, 1A/div

TIME: 400µs/div

CH1: VOUT, 50mV/div

CH2: ILOAD, 500mA/div

TIME: 100µs/div

CH1: VOUT, 50mV/div

CH2: ILOAD, 500mA/div

TIME: 100µs/div

ILOAD = 200mA to 1.5A

ILOAD = 0mA

CH3

ILOAD = 1A

CH3

CH4

ILOAD = 1A

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 6 - www.active-semi.com

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

The ACT4088 is a current-mode step-down DC/DC

converter that provides excellent transient response

with no extra external compensation components.

This device contains an internal, low-resistance,

high-voltage power MOSFET, and operates at a

high 1.4MHz operating frequency to ensure a com-

pact, high-efficiency design with excellent AC and

DC performance.

Setting the Output Voltage

An external voltage divider is used to set the output

voltage, as well as provide a known impedance

from VOUT to FB for compensation purposes. Con-

nect a 50k resistor from the output to FB to ensure

stable compensation, and select the bottom resistor

to provide the desired regulation voltage.

Figure 1:

Output Voltage Setting

The feedback resistor (RFB1) interacts with the inter-

nal compensation network, and plays an important

in setting the ACT4088's transient response and

ensuring stability. For most applications, choosing

RFB1 = 49.9k provides good results. For applica-

tions with output voltages of 1.8V or lower, use a

larger RFB1 value such as 80.6k. Once RFB1 is

chosen, use the following equation to choose RFB2:

Selecting the Inductor

The ACT4088 was optimized for use with a 4.7µH

inductor. When choosing an inductor, choose one

with a DC resistance of less than 250m and a DC

current rating that is typically 30% higher than the

maximum load current.

During typical operation, the inductor maintains a

continuous current to output load. The inductor

current has a ripple that is dependent on the

inductance value.

Higher inductance 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 a reduction in current handling

capability.

If efficiency at light loads (such as less than 100mA)

is critical in the application, a larger inductor is

recommended.

RFB1

RFB2

VOUT

ACT4088

REFERENCE

& THERMAL

SHUTDOWN

CONTROL

SOFT-START

PWM Comparator

ILIM Comparator

HICCUP

REGULATOR

& UVLO

OSCILLATOR

COMPENSATION

DRIVER

CSA

BST

⎟

⎠

⎞

⎜

⎝

⎛−

V81.0

ROUT

1FB

2FB (1)

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 7 - www.active-semi.com

OUT

ESRRIPPLEOUTMAXRIPPLE LCf8 V

RKIV ×

+= (2)

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

(see Figure 2).

Selecting the Input Capacitor

For best performance choose a ceramic type ca-

pacitor with X5R or X7R dielectrics due to their low

ESR and small temperature coefficients. 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. For most

applications, a 10µF capacitor is sufficient. The

input capacitor should be placed close to the IN and

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

the case of tantalum or electrolytic types, connect a

small parallel 0.1µF ceramic capacitor right next to

the IC.

Selecting the Output Capacitor

A 22µF ceramic capacitor with X5R or X7R dielec-

tric provides the best results over a wide range of

applications.

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 (typically 20% to 30%),

RESR resistance is the ESR of the output capacitor,

fSW is the switching frequency, L is the inductor

value, and COUT is the output capacitance.

In the case of ceramic output capacitors, RESR is

very small and does not contribute to the ripple. 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 due to ESR, typically choose a ca-

pacitor with less than 50m ESR.

External Bootstrap Diode

An external bootstrap diode (D2 in Figure 2) is rec-

ommended if the input voltage is less than 5.5V or if

there is a 5V system rail available. This diode helps

strengthen gate drive at lower input voltages, result-

ing in lower on-resistance and higher efficiency.

Low cost diodes, such as 1N4148 or BAT54, are

suitable for this application.

Shutdown Control

The ACT4088 enable pin provides several features

for adjusting and sequencing the power supply. An

internal 2µA current source pull-up, and a precision

1.24V comparator with hysteresis. With these com-

ponents, a user has the flexibility of using the EN

pin as:

1) A digital on/off control by pulling down the EN

current source with an external open-drain tran-

sistor. The voltage at EN is internally clamped to

6V.

2) A sequenced power supply by tying the EN pin

through a resistor to the output of another power

supply. The IC will be enabled when the voltage

at EN exceeds 1.24V, or a resistor divider can be

used to adjust the turn-on threshold.

3) An always-on converter by floating the EN pin or

pulling EN to a desired voltage with a high value

(1M) external resistor. EN is internally clamped

at 6V and will dissipate power if an external re-

sistor attempts to pull EN above the 6V clamp

voltage.

4) Line UVLO. If desired, to achieve a UVLO volt-

age that is higher than the internal UVLO, an

external resistor divider from VIN to EN to GND

can be used to disable the ACT4088 until a

higher input voltage is achieved. For example, it

is not useful for a converter with 9V output to

start up with a 4.2V input voltage, as the output

cannot reach regulation. To enable the ACT4088

when the input voltage reaches 12V, a 9k/1k

resistor divider from IN to GND can be con-

nected to the EN pin. Both the precision 1.2V

threshold and 80mV hysteresis are multiplied by

the resistor ratio, providing a proportional 6.67%

hysteresis for any startup threshold. For the ex-

ample of a 12V enable threshold, the turn off

threshold would be 11.2V.

5) Power supply sequencing. By connecting a small

capacitor from EN to GND, the 2µA current

source and 1.24V threshold can provide a stable

and predictable delay between startup of multiple

power supplies. For example, a startup delay of

roughly 10mS is provided using 150nF, and

roughly 20mS by using 330nF. The EN current

source is active anytime an input supply is ap-

plied, so disabling the IC or resetting the delay

requires an external open-drain pull-down device

to reset the capacitor and hold the EN pin low for

shutdown.

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 8 - www.active-semi.com

Soft-Start

The ACT4088 provides an internal soft-start fea-

ture, which ramps the output voltage and output

current are from 0 to the full value over 0.5 millisec-

onds. This feature prevents output voltage over-

shoot at light loads as well as to prevent large in-

rush currents upon startup. The soft-start circuitry is

internally reset anytime the IC is disabled using the

EN pin, as well as if the IC reaches hiccup mode or

thermal shutdown. In all of these cases, soft-start

provides a smooth, controlled restart after the fault

is removed.

Frequency Foldback

The voltage at FB is monitored by a comparator to

detect an extreme output overload condition. If the

voltage at the FB pin falls to below 0.3V, the inter-

nal oscillator slows to a decreased frequency of

467kHz, 33% of the nominal value. This prevents

the inductor current from rising excessively during a

dead-short condition, potentially resulting in induc-

tor saturation.

Figure 2:

ACT4088 Typical 5V/1.5A Output Application

ACT4088

BST

10µF

B240A

22nF L1

4.7µH

22µF

VOUT

VIN

RFB2

RFB1

49.9kO

9.53kO

OFF

1N4148

(Optional)

Optional Connection:

IN if VIN < 5.5V

OUT if VOUT < 5.5V



ACT4088

Rev0, 13-May-08

Innovative PowerTM - 9 - www.active-semi.com

ACT4088-013

CH1: ILOAD, 500mA/div

CH2: VOUT, 100mV/div (AC Coupled)

TIME: 200µs/div

ACT4088-014

CH1

CH2

Circuit of Figure 3

ILOAD = 150mA to 850mA

CH1: VSW, 10V/div

CH2: VOUT, 20mV/div (AC Coupled)

TIME: 400ns/div

Circuit of Figure 3

ILOAD = 1A

Figure 3:

ACT4088 Optimized for Minimal External Components

The ACT4088 with provides excellent AC and DC results across a wide range of external component combinations. The circuit of

Figure 3 can be used to generate a 5V output from a 12V input utilizing a smaller (i.e. lower-cost) output capacitor while maintaining

good performance.

CH1

CH2

Figure 4:

Circuit of Figure 3 (ILOAD = 150mA to 850mA)

Figure 5:

Circuit of Figure 3 (ILOAD = 1A)

ACT4088

BST

10µF

B240A

22nF L1

4.7µH

4.7µF

VOUT

VIN

RFB2

RFB1

150k?

28.7k?

OFF



ACT4088

Rev0, 13-May-08

Innovative PowerTM - 10 - www.active-semi.com

Hiccup Mode

If the ACT4088 transitions from normal operation to

a severe overload condition (the voltage at FB falls

below 0.3V), the controller automatically enters

"Hiccup Mode" to provide maximum protection to

the system. In hiccup mode, the IC stops switching,

clears the soft-start circuitry, then attempts to re-

start. If the overload condition has been removed,

the IC will start up normally and continue regulating.

In the case of a sustained overload, however, the

IC will attempt to regulate for a period of time equal

to 3x the soft-start period (1.5ms). If the overload

condition persists until the end of this period, the IC

will begin another hiccup cycle. This hiccup-mode

control scheme minimizes power dissipation during

severe overload conditions, and ensures that the

ACT4088 responds quickly to instantaneous severe

overload conditions while providing immunity to

false hiccups that may occur with a heavily loaded

output.

Thermal Shutdown

The ACT4088 automatically turns off when the IC

junction temperature exceeds 160°C, and re-

enables when the IC junction temperature drops by

10°C (typ).

PC Board Layout

The high current paths at G, IN and SW should be

placed very close to the device with short, direct

and wide traces. The input capacitor needs to be as

close as possible to the IN and G pins. The external

feedback resistors should be placed next to the FB

pin. Keep the switch node traces short and away

from the feedback network and use shielded

inductors.

ACT4088

Rev0, 13-May-08

Innovative PowerTM - 11 - www.active-semi.com

Active-Semi, Inc. reserves the right to modify the circuitry or specifications without notice. Users should evaluate each

product to make sure that it is suitable for their a pplications . Active-Semi products ar e not intende d or authori zed 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 other 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, Ca lifornia 94085-4044, USA

PACKAGE OUTLINE

SOT23-6 PACKAGE OUTLINE AND DIMENSIONS



LL1

0.2 SYMBOL

DIMENSION IN

MILLIMETERS

DIMENSION IN

INCHES

MIN MAX MIN MAX

A 1.050 1.250 0.041 0.049

A1 0.000 0.100 0.000 0.004

A2 1.050 1.150 0.041 0.045

b 0.300 0.500 0.012 0.020

c 0.100 0.200 0.004 0.008

D 2.820 3.020 0.111 0.119

E 1.500 1.700 0.059 0.067

E1 2.650 2.950 0.104 0.116

e 0.037 TYP

e1 1.800 2.000 0.071 0.079

L 0.700 REF 0.028 REF

L1 0.300 0.600 0.012 0.024

 0° 8° 0° 8°

0.950 TYP