AS1331

Datasheet AS1331 300mA Buck-Boost Synchronous DC/DC Converters 1 General Description This special device is a synchronous buck-boost DC/DC converter which can handle input voltages above, below, or equal to the output voltage. Due to the internal structure of the AS1331 which is working continuously through all operation modes this device is ideal for dual or triple cell alkaline/NiCad/NiMH as well as single cell Li-Ion battery applications. Because of the implemented Power Save Mode, the solution footprint and the component count is minimized and also over a wide range of load currents a high conversion efficiency is provided. The device includes two N-channel MOSFET switches and two P-channel switches. Also following features are implemented: a quiescent current of typically 22µA (ideal for battery power applications), a shutdown current less than 1µA, current limiting, thermal shutdown and output disconnect. The AS1331 is available in a 10-pin 3x3mm TDFN package with fixed and adjustable output voltage. 2 Key Features ! ! Input Voltage Range: 1.8V to 5.5V Output Voltages: - Fixed: 2.5V, 3.0V, 3.3V - Adjustable: 2.5V to 3.3V Output Current: 300mA @ 3.3V Up to 90% efficiency Power Good Output Disconnection in Shutdown Automatic transition between Buck and Boost mode Ultra Low Quiescent Current: 22µA, Shutdown Current VOUT: The inductor current is going up to 600mA. VIN ~ VOUT: The device stops after 2µs. VIN < VOUT: The inductor current falls down to 0mA. If the inductor current is not 0mA, the transistors SW B and SW D are closed to ramp down the current to zero. If VOUT is still below the threshold voltage the next cycle is started. If IMAX (600mA) wasn’t reached in the previous cycle, SW A and SW D are closed until the inductor current is 600mA. Note: The 4-switch-mode (SW A+SW C => SW B+SW D => SW A + SW C...) and also the buck-mode (SW A+SW D => SW B+SW D => SW A+SW D...) are never used. www.austriamicrosystems.com Revision 1.03 10 - 16 AS1331 Datasheet - D e t a i l e d D e s c r i p t i o n Start-Up Mode At start-up the switch SW D is disabled and its diode is used to transfer current to the output capacitor until VOUT reaches approximately 2.15V. The inductor current is controlled by an alternate algorithm during start-up. Note: Do not apply loads >1mA until VOUT = 2.3V is reached. Other AS1331 Features Shutdown The part is in shutdown mode while the voltage at pin EN is below 0.4V and is active when the voltage is higher than 1.4V. Note: EN can be driven above VIN or VOUT, as long as it is limited to less than 5.5V. Output Disconnect and Inrush Limiting During shutdown VOUT is going to 0V so that no current from the input source is running thru the device. The inrush current is also limited at turn-on mode to minimize the surge currents seen by the input supply. These features of the AS1331 are realized by opening both P-channel MOSFETs of the rectifiers, allowing a true output disconnect. Power-OK and Low-Battery-Detect Functionality LBO goes low in startup mode as well as during normal operation if: 1) The voltage at the LBI pin is below LBI threshold (1.25V). This can be used to monitor the battery voltage. 2) LBI pin is connected to GND and VOUT is below 92.5% of its nominal value. LBO works as a power-OK signal in this case. The LBI pin can be connected to a resistive-divider to monitor a particular definable voltage and compare it with a 1.25V internal reference. If LBI is connected to GND an internal resistive-divider is activated and connected to the output. Therefore, the Power-OK functionality can be realised with no additional external components. The Power-OK feature is not active during shutdown and provides a power-on-reset function that can operate down to VIN = 1.8V. A capacitor to GND may be added to generate a power-on-reset delay. To obtain a logic-level output, connect a pull-up resistor from pin LBO to pin VOUT. Larger values for this resistor will help to minimize current consumption; a 100kΩ resistor is perfect for most applications (see Figure 25 on page 12). For the circuit shown in the left of Figure 24 on page 12, the input bias current into LBI is very low, permitting largevalue resistor-divider networks while maintaining accuracy. Place the resistor-divider network as close to the device as possible. Use a defined resistor for R2 and then calculate R1 as: V IN (EQ 1) R 1 = R 2 ⋅ ⎛ ------------------- – 1⎞ ⎝ V SENSE ⎠ Where: VSENSE (the internal sense reference voltage) is 1.25V. R2 (the predefined resistor in the resistor devider) has to be ≤ 270kΩ. In case of the LBI pin is connected to GND, an internal resistor-devider network is activated and compares the output voltage with a 92.5% voltage threshold. For this particular Power-OK application, no external resistive components are necessary. Thermal Shutdown To prevent the AS1331 from short-term misuse and overload conditions the chip includes a thermal overload protection. To block the normal operation mode all switches will be turned off. The device is in thermal shutdown when the junction temperature exceeds 145°C. To resume the normal operation the temperature has to drop below 135°C. A good thermal path has to be provided to dissipate the heat generated within the package. Otherwise it’s not possible to operate the AS1331 at its useable maximal power. To dissipate as much heat as possible away from the package into a copper plane with as much area as possible, it’s recommended to use multiple vias in the printed circuit board. It’s also recommended to solder the Exposed Pad (pin 11) to the GND plane. Note: Continuing operation in thermal overload conditions may damage the device and is considered bad practice. www.austriamicrosystems.com Revision 1.03 11 - 16 AS1331 Datasheet - D e t a i l e d D e s c r i p t i o n Output Voltage Selection The AS1331 is available in two versions (see Ordering Information on page 15). One version can only operate at one fixed output voltage (see Figure 25) and the other version can operate with user-adjustable output voltages from 2.5V to 3.3V by connecting a voltage divider between the pins VOUT and FB (see Figure 24). Figure 24. LiIon to Adjustable Output Voltage L1 6.8µH 4 SW1 LiIon C1 10µF 5 2 SW2 8 VIN 7 LBO R3 VOUT 2.5V to 3.3V R1 C2 22µF LBI On Off 6 AS1331-AD 1 VOUT 10 EN 3 PGND 9 GND FB R2 The output voltage can be adjusted by selecting different values for R1 and R2. Calculate VOUT by: R1 V OUT = V FB × ⎛ 1 + -----⎞ ⎝ R 2⎠ Where: (EQ 2) VFB = 1.25V, VOUT = 2.5V to 3.3V; Figure 25. LiIon to 3.3V with POK - Fixed Output Voltage L1 6.8µH 4 SW1 LiIon C1 10µF 5 2 SW2 8 VIN 7 LBO R3 C2 22µF LBI On Off 6 AS13313.3V 1 VOUT 10 VOUT 3.3V 300mA EN 3 PGND 9 GND FB www.austriamicrosystems.com Revision 1.03 12 - 16 AS1331 Datasheet - A p p l i c a t i o n I n f o r m a t i o n 9 Application Information Component Selection Only three power components are required to complete the design of the buck-boost converter. For the adjustable version VOUT programming resistors are needed. The high operating frequency and low peak currents of the AS1331 allow the use of low value, low profile inductors and tiny external ceramic capacitors. Inductor Selection For best efficiency, choose an inductor with high frequency core material, such as ferrite, to reduce core losses. The inductor should have low DCR (DC resistance) to reduce the I²R losses, and must be able to handle the peak inductor current without saturating. A 6.8µH inductor with a >600mA current rating and