feature

• Wide 4.5V to 28V input voltage range

• 1.5A output current (12VIN to 5VOUT)

• Output adjustable down to 0.81V

• 0.3Ω inner power MOSFET

• Efficiency up to 92%

• Low ESR Ceramic Output Capacitor Stable

•Fixed 1.4MHz operating frequency

• Internal soft-start function

• Overcurrent protection in Hiccup mode

• Thermal shutdown available in SOT33-6 package

application

• TFT LCD display

• Portable DVD, Headphones, MP3 Player and more.

• Car-powered or battery-powered equipment

• Set-top box

• Telecom power

• DSL, cable modems and routers

General Instructions

The ACT4088 is a current mode step-down DC/DC converter that provides up to 1.5A to 5V from a 12V input. The 1.4MHz switching frequency allows the use of tiny external components, and the internal loop compensation provides a simple, stable power supply with minimal external components. The ACT4088 is optimized for use with ceramic input and output capacitors to provide 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 folding, hiccup mode, and thermal shutdown. Internal soft-start provides controlled start-up without overshoot, even at light loads.

The ACT4088 is available in a tiny SOT23-6 package and requires few external components.

Typical Application Circuit

Typical performance characteristics

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

Functional block diagram

Function description

The ACT4088 is a current mode step-down DC/DC converter that provides excellent transient response without additional external compensation components. The device contains an internal low-resistance high-voltage power MOSFET that operates up to 1.4MHz to ensure a compact, high-efficiency design with excellent AC and DC performance.

Set 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. Connect a 50kΩ resistor at the output to FB to ensure stable compensation, and choose the bottom resistor to provide the desired regulation voltage.

Figure 1: Output Voltage Setting

The feedback resistor (RFB1) interacts with the internal compensation network and plays an important role in setting the transient response of the ACT4088 and ensuring stability. For most applications, choosing RFB1=49.9kΩ provides good results. For applications with an output voltage of 1.8V or lower, use a larger value of RFB1, such as 80.6kΩ. After selecting RFB1, select RFB2 using the following formula:

select sensor

The ACT4088 is optimized for use with a 4.7µH inductor. When choosing an inductor, choose one with a DC resistance less than 250mΩ and a DC current rating typically 30% higher than the maximum load current.

During typical operation, the inductor maintains a continuous current output load. The inductor current ripple depends on the inductor value.

Higher inductance reduces peak-to-peak ripple current. The tradeoff for high inductance values is an increase in inductor core size and series resistance, and a decrease in current handling capability.

If efficiency at light loads (eg, less than 100mA) is critical in the application, a larger inductor is recommended.

rectifier diode

When the high side power switch is open, a Schottky diode is used as a rectifier to conduct current. The Schottky diode current rating must be higher than the maximum output current, and the reverse voltage rating must be higher than the maximum input voltage (see Figure 2).

Choosing Input Capacitors For best performance, choose ceramic type capacitors with X5R or X7R dielectrics due to their low ESR and small temperature coefficient. However, low ESR tantalum or electrolytic types can also be used, provided the rms ripple current rating is higher than 50% of the output current. For most applications, a 10µF capacitor is sufficient. The input capacitors should be placed close to the input and G pins of the IC with as short a path as possible. For tantalum or electrolytic types, connect a small 0.1µF ceramic capacitor next to the IC.

Select Output Capacitors 22µF ceramic capacitors with X5R or X7R dielectrics provide the best results in a wide range of applications.

The output capacitor also needs to have low ESR to keep the output voltage ripple low. The output ripple voltage is:

where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor (usually 20% to 30%), the RESR resistor is the ESR of the output capacitor, fSW is the switching frequency, L is the inductance value, and COUT is the output capacitor.

In the case of ceramic output capacitors, the RESR is very small and does not cause ripple. In the case of tantalum or electrolytic types, the ripple is controlled by the RESR times the ripple current. In this case, the output capacitor is chosen to have a sufficiently low voltage due to the ESR, typically a capacitor of less than 50mΩ ESR.

External bootstrap diode

If the input voltage is less than 5.5V or a 5V system rail is available, an external boot diode (D2 in Figure 2) is recommended. This diode helps enhance 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.

shutdown control

The ACT4088 enable pins provide several functions for adjusting and sequencing power supplies. An internal 2µA current source pull-up and a precision 1.24V comparator with hysteresis. Using these components, the user has the flexibility to use the EN-pin as:

1) A digital on/off control that pulls down the EN current source by using an external open-drain transistor. The voltage at EN is internally clamped to 6V.

2) A sequential power supply that connects the EN pin to the output of another power supply through a resistor. When the voltage at EN exceeds 1.24V, the IC will be enabled, or a resistive divider can be used to adjust the turn-on threshold.

3), through a floating EN pin or a normally open converter (1MΩ) external resistor that pulls EN to the desired high voltage value. EN is internally clamped at 6V and if an external resistor tries to pull EN above the 6V clamp, power will be dissipated.

4), line up. If desired, in order to obtain a UVLO voltage 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 obtained. For example, for a converter with an output voltage of 9V, startup at an input voltage of 4.2V is not useful because the output cannot reach the specified value. To enable the ACT4088 when the input voltage reaches 12V, a 9kΩ/1kΩ resistor divider from input to ground can be connected to the EN pin. The accurate 1.2V threshold and 80mV hysteresis are both multiplied by the resistor ratio to provide a proportional hysteresis of 6.67% for any start-up threshold. Taking the 12V enable threshold as an example, the shutdown threshold is 11.2V.

5), power sequence. By connecting a small capacitor from EN to GND, the 2µA current source and 1.24V threshold provide a stable and predictable delay between startup of multiple supplies. For example, using 150nF provides a startup delay of about 10ms, and using 330nF provides a startup delay of about 20ms. The EN current source is active as long as the input supply is on, so disabling the IC or the reset delay requires an external open-drain pull-down to reset the capacitor and hold the EN pin low for shutdown.

soft start

The ACT4088 provides an internal soft-start function that ramps the output voltage and output current from 0 to their maximum value in 0 to 0.5 milliseconds. This feature prevents output voltage overshoot at light loads and prevents large inrush currents at startup. The soft-start circuit is internally reset anytime the IC is disabled using the EN pin, and when the IC reaches hiccup mode or thermal shutdown. In all these cases, soft start provides a smooth, controlled restart after troubleshooting.

frequency folding

The voltage at FB is monitored by a comparator to detect extreme output overload conditions. If the voltage at the FB pin drops below 0.3V, the frequency of the internal oscillator will be reduced to 467kHz, or 33% of the nominal value. This prevents the inductor current from rising excessively under dead short conditions, which could cause the inductor to saturate.

Figure 2: Typical 5V/1.5A output application of ACT4088

Figure 3: ACT4088 optimized for minimal external components

The ACT4088 has excellent AC and DC results in a broad portfolio of external components. The circuit in Figure 3 can be used to generate a 5V output from a 12V input with a smaller (i.e. lower cost) output capacitor while maintaining good performance.

Figure 4: Circuit of Figure 3 (ILOAD=150mA to 850mA)

Figure 5: Circuit of Figure 3 (ILOAD=1A)

hiccup mode

If the ACT4088 transitions from normal operation to a severe overload condition (the voltage at FB drops below 0.3V), the controller automatically enters "hiccup mode" to provide maximum protection for the system. In hiccup mode, the IC stops switching, clears the soft-start circuit, and then attempts to restart. If the overload condition has been removed, the IC will start normally and continue to regulate. However, in the case of continuous overload, the IC will try to regulate for a period of time equal to 3 times the soft-start time (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 under severe overload conditions and ensures that the ACT4088 responds quickly to transient severe overload conditions while providing immunity to false hiccup that can occur at heavily loaded outputs.

Thermal shutdown

The ACT4088 automatically shuts down when the IC junction temperature exceeds 160°C, and can be re-enabled when the IC junction temperature drops by 10°C (typ).

PC board layout

The high-current paths at G, IN, and SW should be very close to devices with short, direct, and wide traces. The input capacitors need to be as close as possible to the input and G pins. The external feedback resistor should be placed next to the FB pin. Keep switch node traces short and away from feedback networks, and use shielded inductors.

Packaging Outline

SOT33-6 Packaging Dimensions

Active Semiconductor Corporation reserves the right to modify circuits or specifications without notice. Users should evaluate each product to ensure it is suitable for their application. Active semi-finished products are not intended or authorized for use as critical components in life support devices or systems. Active Semi, Inc. assumes no liability whatsoever arising out of the use of any product or circuit described in this data sheet, and does not assign any patent licenses.