ACT4088 [ACTIVE-SEMI]
28V, 1.5A, 1.4MHz Step-Down DC/DC Converter in SOT23-6; 28V ， 1.5A ， 1.4MHz的降压型DC / DC采用SOT23-6转换器
| 型号： | ACT4088 |
| 厂家： | 
ACTIVE-SEMI, INC |
| 描述： | 28V, 1.5A, 1.4MHz Step-Down DC/DC Converter in SOT23-6 |
| 文件： | 总11页 (文件大小：1071K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
ACT4088
Rev PrA, 24-Jul-07
Advanced Product Information – All Information Subject to Change
28V, 1.5A, 1.4MHz Step-Down DC/DC Converter in SOT23-6
GENERAL DESCRIPTION
FEATURES
Wide 4.5V to 28V Input Voltage Range
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.
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
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.
Over Current Protection with Hiccup-Mode
Thermal Shutdown
Available in a SOT23-6 Package
APPLICATIONS
TFT LCD Monitors
The ACT4088 is available in a tiny SOT23-6 pack-
age and requires very few external components.
Portable DVDs, Headphones, MP3 Players, etc.
Car-Powered or Battery-Powered Equipment
Set-Top Boxes
Telecom Power Supplies
DSL and Cable Modems and Routers
TYPICAL APPLICATION CIRCUIT
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ACT4088
Rev PrA, 24-Jul-07
ORDERING INFORMATION
PART NUMBER
TEMPERATURE RANGE
-40°C to 85°C
PACKAGE
PINS
PACKING
ACT4088US-T
SOT23-6
6
TAPE & REEL
PIN CONFIGURATION
SOT23-6
PIN DESCRIPTIONS
PIN NUMBER PIN NAME
PIN DESCRIPTION
1
SW
Switch Output. Connect this pin to the switching end of the inductor.
Power supply input. Bypass this pin with a 10µF ceramic capacitor to G, placed as
close to the IC as possible.
2
IN
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.
3
EN
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.
4
5
6
FB
G
Ground and Heat sink. Connect this pin to a large, uncovered PCB copper area for
best heat dissipation.
Bootstrap. This pin acts as the power supply for the high-side switch’s gate driver.
Connect a 2.2nF capacitor between this pin and SW.
BST
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ACT4088
Rev PrA, 24-Jul-07
ABSOLUTE MAXIMUM RATINGS
PARAMETER
IN Supply Voltage
VALUE
UNIT
-0.3 to 32
V
SW Voltage
-1 to VIN + 1
VSW - 0.3 to VSW + 7
-0.3 to 6
V
V
BST Voltage
EN, FB Voltage
V
Continuous SW Current
Internally Limited
220
A
Junction to Ambient Thermal Resistance (θJA)
Maximum Power Dissipation
Operating Junction Temperature
Storage Temperature
°C/W
W
0.5
-40 to 150
-55 to 150
300
°C
°C
°C
Lead Temperature (Soldering, 10 sec)
: Do not exceed these limits to prevent damage to the device. Exposure to absolute maximum rating conditions for long periods may
affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = 12V, TA = 25°C, unless otherwise specified.)
PARAMETER
Input Voltage
SYMBOL
VIN
TEST CONDITIONS
VOUT = 3.3V, ILOAD = 0A to 1.5A
Input Voltage Rising
MIN TYP MAX UNIT
4.5
4
28
V
V
Under Voltage Lockout Voltage
Under Voltage Lockout Hysteresis
Feedback Voltage
VUVLO
4.2
250
0.81
250
0.300
15
4.49
mV
V
VFB
4.75V ≤ VIN ≤ 20V, VCOMP = 1.5V
0.79
0.83
Frequency Foldback Threshold
High-side Switch On Resistance
Low-side Switch On Resistance
SW Leakage
mV
Ω
RONH
RONH
Ω
VEN = 0, VSW = 0V
1
10
µA
VIN = 12V, VOUT = 5V, or
EN = G, SW = G
Current Limit
ILTM
fSW
1.8
A
Switching Frequency
1.2
1.4
467
92
1.7
MHz
kHz
%
Foldback Switching Frequency
Maximum Duty Cycle
VFB = 0V, or FB = G
VFB = 0.6V
DMAX
Minimum On-Time
75
ns
EN Threshold Voltage
EN Rising
EN Rising
1.12
1.24
100
2
1.36
V
EN Hysteresis
mV
µA
µA
mA
°C
EN Internal Pull-up Current
Supply Current in Shutdown
Supply Current in Operation
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
VEN = 0V or EN = G
VEN = 2V, VFB = 1.0V
15
30
2
1
160
10
°C
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ACT4088
Rev PrA, 24-Jul-07
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 2, VIN = 12V, L = 4.7µH, C1 = 10µF, C2 = 22µF, TA = +25°C, unless otherwise specified.)
Efficiency vs. Load Current
Efficiency vs. Load Current
95
85
95
85
VIN = 12V
VIN = 12V
VIN = 18V
75
65
75
65
VIN = 18V
VIN = 24V
VIN = 24V
55
50
55
50
VOUT = 5V
VOUT = 3.3V
10
0.1
1
10
0.1
1
Load Current (A)
Load Current (A)
FB Voltage vs. Temperature
Oscillator Frequency vs. Temperature
820
816
812
808
804
1.60
1.50
1.40
1.30
1.20
800
-40
-20
0
20
40
60
80
100
120
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
Temperature (°C)
Peak Current Limit vs. Duty Cycle
Shutdown Current vs. Input Voltage
30
25
3.0
2.5
2.0
1.5
20
15
10
5
1.0
0.5
0.0
EN Pull-up Current
0
0
20
40
60
80
100
0
4
8
24
28
12
16
20
Duty Cycle
Input Voltage (V)
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ACT4088
Rev PrA, 24-Jul-07
TYPICAL PERFORMANCE CHARACTERISTICS CONT’D
(Circuit of Figure 2, VIN = 12V, L = 4.7µH, C1 = 10µF, C2 = 22µF, TA = +25°C, unless otherwise specified.)
Load Transient Response
Load Transient Response
ILOAD = 200mA to 1.5A
ILOAD = 200mA to 800mA
CH1
CH1
CH2
CH2
CH1: VOUT, 50mV/div
CH2: ILOAD, 500mA/div
TIME: 100µs/div
CH1: VOUT, 50mV/div
CH2: ILOAD, 500mA/div
TIME: 100µs/div
Start-up Waveforms
Start-up Waveforms
ILOAD = 0mA
ILOAD = 1A
CH1
CH2
CH1
CH2
CH3
CH3
CH4
CH1: VOUT, 2V/div
CH2: VSW, 10V/div
CH3: IL, 1A/div
CH1: VEN, 2V/div
CH2: VOUT, 2V/div
CH3: VSW, 10V/div
CH4: IL, 1A/div
TIME: 200µs/div
TIME: 400µs/div
Steady State Switching Waveforms
Hiccup Mode Switching Waveforms
ILOAD = 1A
CH1
CH2
CH1
CH2
CH3
CH1: VOUT, 100mV/div, (AC COUPLED)
CH2: IL, 1A/div
CH1: VOUT, 50mV/div, (AC COUPLED)
CH2: VSW, 10V/div
TIME: 1ms/div
CH3: IL, 500mA/div
TIME: 400ns/div
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ACT4088
Rev PrA, 24-Jul-07
FUNCTIONAL BLOCK DIAGRAM
in setting the ACT4088's transient response and
ensuring stability. For most applications, choosing
RFB1 = 49.9kΩ provides good results. For applica-
tions with output voltages of 1.8V or lower, use a
larger RFB1 value such as 80.6kΩ. Once RFB1 is
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.
chosen, use the following equation to choose RFB2
:
RFB1
RFB2
(1)
V
OUT
1
0.81V
Selecting the Inductor
Setting the Output Voltage
The ACT4088 was optimized for use with a 4.7µH
inductor. When choosing an inductor, choose one
with a DC resistance of less than 250mΩ and a DC
current rating that is typically 30% higher than the
maximum load current.
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 50kΩ resistor from the output to FB to ensure
stable compensation, and select the bottom resistor
to provide the desired regulation voltage.
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.
Figure 1:
Output Voltage Setting
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.
The feedback resistor (RFB1) interacts with the inter-
nal compensation network, and plays an important
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ACT4088
Rev PrA, 24-Jul-07
Rectifier Diode
Shutdown Control
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).
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.
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.
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
(1MΩ) 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.
Selecting the Output Capacitor
A 22µF ceramic capacitor with X5R or X7R dielec-
tric provides the best results over a wide range of
applications.
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 9kΩ/1kΩ
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.
The output capacitor also needs to have low ESR
to keep low output voltage ripple. The output ripple
voltage is:
VIN
VRIPPLE IOUTMAX KRIPPLE RESR
(2)
8 fSW 2LCOUT
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 50mΩ ESR.
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.
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.
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ACT4088
Rev PrA, 24-Jul-07
Soft-Start
Frequency Foldback
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.
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
Optional Connection:
IN if VIN < 5.5V
OUT if VOUT < 5.5V
D2
1N4148
(Optional)
2
6
VIN
IN
BST
C1
10µF
C3
22nF
L1
4.7µH
1
VOUT
5V
ACT4088
SW
FB
D1
B240A
C2
22µF
3
ON
OFF
EN
RFB1
4
G
5
49.9kΩ
1%
9.53kΩ
1%
RFB2
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ACT4088
Rev PrA, 24-Jul-07
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.
Figure 5:
Figure 4:
Circuit of Figure 3 (ILOAD = 1A)
Circuit of Figure 3 (ILOAD = 150mA to 850mA)
Circuit of Figure 3
ILOAD = 150mA to 850mA
Circuit of Figure 3
ILOAD = 1A
CH1
CH2
CH1
CH2
CH1: ILOAD, 500mA/div
CH1: VSW, 10V/div
CH2: VOUT, 100mV/div (AC Coupled)
TIME: 200µs/div
CH2: VOUT, 20mV/div (AC Coupled)
TIME: 400ns/div
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ACT4088
Rev PrA, 24-Jul-07
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.
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ACT4088
Rev PrA, 24-Jul-07
PACKAGE OUTLINE
SOT23-6 PACKAGE OUTLINE AND DIMENSIONS
D
DIMENSION IN
MILLIMETERS
DIMENSION IN
INCHES
θ
b
0.2
SYMBOL
MIN
1.050
0.000
1.050
0.300
0.100
2.820
1.500
2.650
MAX
1.250
0.100
1.150
0.500
0.200
3.020
1.700
2.950
MIN
0.041
0.000
0.041
0.012
0.004
0.111
0.059
0.104
MAX
0.049
0.004
0.045
0.020
0.008
0.119
0.067
0.116
A
A1
A2
b
c
c
e
D
e1
E
E1
e
0.950 TYP
0.037 TYP
e1
L
1.800
2.000
0.071
0.079
0.700 REF
0.028 REF
L1
θ
0.300
0°
0.600
8°
0.012
0°
0.024
8°
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 applications. Active-Semi products are 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 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, California 94085-4044, USA
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