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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 0.3 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
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 supplies with a minimum of external components. Optimized for use with ceramic input and output capacitors, 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 provide excellent line and load transient response while requiring no external compensation components. Fault protection includes cycle-by-cycle current 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 package and requires very few external components.
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
TYPICAL APPLICATION CIRCUIT
VIN
4.5V to 28V
IN
BST
ACT4088
ON OFF
EN
SW
VOUT
FB G
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ACT4088
Rev0, 13-May-08
ORDERING INFORMATION
PART NUMBER
ACT4088US-T
TEMPERATURE RANGE
-40C to 85C
PACKAGE
SOT23-6
PINS
6
PACKING TAPE & REEL
TOP MARK
FRWJ
PIN CONFIGURATION
SW
1
6
BST
IN
2
ACT4088
5
G
EN
3
4
FB
SOT23-6
PIN DESCRIPTIONS
PIN NUMBER
1 2 3 4 5 6
PIN NAME
SW IN EN FB G BST
PIN DESCRIPTION
Switch Output. Connect this pin to the switching end of the inductor. Power supply input. Bypass this pin with a 10F ceramic capacitor to G, placed as close to the IC as possible. Enable Input. EN is pulled up to 5V with a 2A 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. 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. 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 22nF capacitor between this pin and SW.
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ACT4088
Rev0, 13-May-08
ABSOLUTE MAXIMUM RATINGS
PARAMETER
IN Supply Voltage SW Voltage BST Voltage EN, FB Voltage Continuous SW Current Junction to Ambient Thermal Resistance (JA) Maximum Power Dissipation Operating Junction Temperature Storage Temperature Lead Temperature (Soldering, 10 sec)
VALUE
-0.3 to 32 -1 to VIN + 1 VSW - 0.3 to VSW + 7 -0.3 to 6 Internally Limited 220 0.5 -40 to 150 -55 to 150 300
UNIT
V V V V A C/W W C C 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.
ELECTRICAL CHARACTERISTICS
(VIN = 12V, TA = 25C, unless otherwise specified.)
PARAMETER
Input Voltage Under Voltage Lockout Voltage Under Voltage Lockout Hysteresis Feedback Voltage Frequency Foldback Threshold High-side Switch On Resistance Low-side Switch On Resistance SW Leakage Current Limit Switching Frequency Foldback Switching Frequency Maximum Duty Cycle Minimum On-Time EN Threshold Voltage EN Hysteresis EN Internal Pull-up Current Supply Current in Shutdown Supply Current in Operation Thermal Shutdown Temperature Thermal Shutdown Hysteresis
SYMBOL
VIN VUVLO VFB RONH RONH
TEST CONDITIONS
VOUT = 3.3V, ILOAD = 0A to 1.5A Input Voltage Rising 4.75V VIN 20V, VCOMP = 1.5V
MIN
4.5 4 0.79
TYP
4.2 250 0.81 250 0.300 15
MAX
28 4.49 0.83
UNIT
V V mV V mV
VEN = 0, VSW = 0V ILTM fSW VFB = 0V, or FB = G DMAX VFB = 0.6V EN Rising EN Rising VEN = 0V or EN = G VEN = 2V, VFB = 1.0V 1.12 VIN = 12V, VOUT = 5V, or EN = G, SW = G 1.1
1 1.8 1.4 467 92 75 1.24 100 2 15 1 160 10
10
A A
1.6
MHz kHz % ns
1.36
V mV A
30 2
A mA C C
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ACT4088
Rev0, 13-May-08
TYPICAL PERFORMANCE CHARACTERISTICS
(Circuit of Figure 2, VIN = 12V, L = 4.7H, C1 = 10F, C2 = 22F, TA = +25C, unless otherwise specified.)
Efficiency vs. Load Current
95 VIN = 12V 95 ACT4088-001
Efficiency vs. Load Current
ACT4088-002
85
85
VIN = 12V
Efficiency (%)
75
VIN = 18V VIN = 24V
Efficiency (%)
75 VIN = 24V
VIN = 18V
65
65
55 50 0.1 1
VOUT = 5V 10
55 50 0.1 1 VOUT = 3.3V 10
Load Current (A)
Load Current (A)
FB Voltage vs. Temperature
820 1.60
Oscillator Frequency vs. Temperature Oscillator Frequency (MHz)
ACT4088-003 ACT4088-004
816
FB Voltage (mV)
1.50
812
1.40
808
804
1.30
800 -40 -20 0 20 40 60 80 100 120
1.20 -40 -20 0 20 40 60 80 100 120
Temperature (C)
Temperature (C)
Peak Current Limit vs. Duty Cycle Quiescent Supply Current (A)
3.0 30 25 20 15 10 5 0 0 ACT4088-005
Shutdown Current vs. Input Voltage
ACT4088-006
Peak Current Limit (A)
2.5 2.0 1.5 1.0 0.5 0.0 0 20 40 60 80 100
S
ly upp
nt rre Cu
EN Pull-up Current
4
8
12
16
20
24
28
Duty Cycle
Input Voltage (V)
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ACT4088
Rev0, 13-May-08
TYPICAL PERFORMANCE CHARACTERISTICS CONT'D
(Circuit of Figure 2, VIN = 12V, L = 4.7H, C1 = 10F, C2 = 22F, TA = +25C, unless otherwise specified.)
Load Transient Response
ACT4088-007 ILOAD = 200mA to 800mA
Load Transient Response
ACT4088-008 ILOAD = 200mA to 1.5A CH1
CH1
CH2 CH2
CH1: VOUT, 50mV/div CH2: ILOAD, 500mA/div TIME: 100s/div
CH1: VOUT, 50mV/div CH2: ILOAD, 500mA/div TIME: 100s/div
Start-up Waveforms
ACT4088-009 ILOAD = 0mA CH1 ILOAD = 1A CH1 CH2
Start-up Waveforms
ACT4088-010
CH2
CH3
CH4 CH3
CH1: VOUT, 2V/div CH2: VSW, 10V/div CH3: IL, 1A/div TIME: 200s/div
CH1: VEN, 2V/div CH2: VOUT, 2V/div CH3: VSW, 10V/div CH4: IL, 1A/div TIME: 400s/div
Steady State Switching Waveforms
ACT4088-011 ILOAD = 1A CH1
Hiccup Mode Switching Waveforms
ACT4088-012
CH1
CH2
CH2 CH3 CH1: VOUT, 50mV/div, (AC COUPLED) CH2: VSW, 10V/div CH3: IL, 500mA/div TIME: 400ns/div CH1: VOUT, 100mV/div, (AC COUPLED) CH2: IL, 1A/div TIME: 1ms/div
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ACT4088
Rev0, 13-May-08
FUNCTIONAL BLOCK DIAGRAM
IN CSA EN REGULATOR & UVLO BST
OSCILLATOR
CONTROL SOFT-START PWM Comparator ILIM Comparator HICCUP
Q1 DRIVER
SW REFERENCE & THERMAL SHUTDOWN EA FB G COMPENSATION
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 compact, high-efficiency design with excellent AC and DC performance.
in setting the ACT4088's transient response and ensuring stability. For most applications, choosing RFB1 = 49.9k provides good results. For applications 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:
RFB2 =
RFB1 VOUT -1 0.81V
(1)
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. Connect 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
VOUT
Selecting the Inductor
The ACT4088 was optimized for use with a 4.7H 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.
ACT4088
FB
RFB1
RFB2
The feedback resistor (RFB1) interacts with the internal compensation network, and plays an important
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ACT4088
Rev0, 13-May-08
Rectifier Diode
Use a Schottky diode as the rectifier to conduct current when the High-Side Power Switch is off. The Schottky diode must have current rating higher than the maximum output current and the reverse voltage rating higher than the maximum input voltage (see Figure 2).
Shutdown Control
The ACT4088 enable pin provides several features for adjusting and sequencing the power supply. An internal 2A current source pull-up, and a precision 1.24V comparator with hysteresis. With these components, 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 transistor. 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 resistor attempts to pull EN above the 6V clamp voltage. 4) Line UVLO. If desired, to achieve a UVLO voltage 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 connected 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 example 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 2A 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 applied, 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.
Selecting the Input Capacitor
For best performance choose a ceramic type capacitor 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 10F 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 0.1F ceramic capacitor right next to the IC.
Selecting the Output Capacitor
A 22F ceramic capacitor with X5R or X7R dielectric 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:
VRIPPLE = IOUTMAX K RIPPLE RESR +
VIN 2 8 x fSW LCOUT
(2)
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 capacitor with less than 50m ESR.
External Bootstrap Diode
An external bootstrap diode (D2 in Figure 2) is recommended 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, resulting 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
Rev0, 13-May-08
Soft-Start
The ACT4088 provides an internal soft-start feature, which ramps the output voltage and output current are from 0 to the full value over 0.5 milliseconds. This feature prevents output voltage overshoot at light loads as well as to prevent large inrush 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 internal 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 inductor 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) C3 22nF
VIN C1 10F
2
IN
BST
6
ACT4088
ON OFF 3 EN
SW
1 D1 B240A
L1 4.7H VOUT 5V C2 22F RFB1
FB G 5
4 9.53kO 1%
RFB2
49.9kO 1%
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ACT4088
Rev0, 13-May-08 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.
VIN C1 10F
2
IN
BST
6 C3 22nF 1
L1 4.7H VOUT 5V C2 4.7F RFB1
ACT4088
ON OFF 3 EN
SW D1 B240A FB 4 28.7k? 1%
G 5
RFB2
150k? 1%
Figure 4: Circuit of Figure 3 (ILOAD = 150mA to 850mA)
ACT4088-013 Circuit of Figure 3 ILOAD = 150mA to 850mA
Figure 5: Circuit of Figure 3 (ILOAD = 1A)
ACT4088-014 Circuit of Figure 3 ILOAD = 1A
CH1
CH1
CH2
CH2
CH1: ILOAD, 500mA/div CH2: VOUT, 100mV/div (AC Coupled) TIME: 200s/div
CH1: VSW, 10V/div CH2: VOUT, 20mV/div (AC Coupled) TIME: 400ns/div
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ACT4088
Rev0, 13-May-08
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 restart. 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 160C, and reenables when the IC junction temperature drops by 10C (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
Rev0, 13-May-08
PACKAGE OUTLINE
SOT23-6 PACKAGE OUTLINE AND DIMENSIONS
D b L1 0.2
SYMBOL
A
DIMENSION IN MILLIMETERS MIN
1.050 0.000 1.050 0.300 0.100 2.820 1.500 2.650
DIMENSION IN INCHES MIN
0.041 0.000 0.041 0.012 0.004 0.111 0.059 0.104
MAX
1.250 0.100 1.150 0.500 0.200 3.020 1.700 2.950
MAX
0.049 0.004 0.045 0.020 0.008 0.119 0.067 0.116
E1
E
A1 A2 b
L
e e1
c
c D E E1
A1
A2
e e1 L L1
0.950 TYP 1.800 2.000
0.037 TYP 0.071 0.079
A
0.700 REF 0.300 0 0.600 8
0.028 REF 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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