AS1324-AD

AS1324 1 . 5 M H z , 6 0 0 m A , D C / D C St e p - D o w n R e g u l a t o r D a ta S he e t 1 General Description The AS1324 is a high-efficiency, constant-frequency synchronous buck converter available in adjustable- and fixed-voltage versions. The wide input voltage range (2.7V to 5.5V), automatic powersave mode and minimal external component requirements make the AS1324 perfect for any single Li-Ion battery-powered application. Typical supply current with no load is 30µA and decreases to ≤1µA in shutdown mode. The AS1324 is available as the standard versions listed in Table 1. Table 1. Standard Versions Model AS1324-AD AS1324-12 AS1324-15 AS1324-18 Output Voltage Adjustable via External Resistors Fixed: 1.2V Fixed: 1.5V Fixed: 1.8V 2 Key Features ! ! ! ! ! ! ! ! ! ! ! ! High Efficiency: Up to 96% Output Current: 600mA Input Voltage Range: 2.7V to 5.5V Constant Frequency Operation: 1.5MHz Variable- and Fixed-Output Voltages No Schottky Diode Required Automatic Powersave Operation Low Quiescent Current: 30µA Internal Reference: 0.6V Shutdown Mode Supply Current: ≤1µA Thermal Protection 5-pin TSOT-23 Package An internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. The internally fixed switching frequency (1.5MHz) allows for the use of small surface mount external components. Very low output voltages can be delivered with the internal 0.6V feedback reference voltage. The AS1324 is available in a 5-pin TSOT-23 package. 3 Applications The device is ideal for mobile communication devices, laptops and PDAs, ultra-low-power systems, threshold detectors/discriminators, telemetry and remote systems, medical instruments, or any other space-limited application with low power-consumption requirements. Figure 1. Typical Application Diagram – High Efficiency Step Down Converter VIN = 2.7V to 5.5V CIN 10µF 4 VIN 3 SW 4.7µH VOUT = 1.8V, 600mA COUT 10µF EN 1 5 VOUT AS132418 1 EN 5 VOUT GND 2 AS132418 SW 3 4 VIN 2 GND www.austriamicrosystems.com Revision 1.03 1 - 20 AS1324 Data Sheet - P i n o u t a n d P a c k a g i n g 4 Pinout and Packaging Pin Assignments Figure 2. Pin Assignments (Top View) EN 1 5 VFB EN 1 5 VOUT GND 2 AS1324 GND 2 AS1324-12/ AS1324-15/ AS1324-18 4 VIN SW 3 4 VIN SW 3 Pin Descriptions Table 2. Pin Descriptions Pin Number Pin Name Description Enable Input. Driving this pin above 1.5V enables the device. Driving this pin below 0.3V puts the device in shutdown mode. In shutdown mode all functions are disabled while SW goes high impedance, drawing 0V. Shutdown Connecting EN to GND or logic low places the AS1324 in shutdown mode and reduces the supply current to 0.1µA. In shutdown the control circuitry and the internal NMOS and PMOS turn off and SW becomes high impedance disconnecting the input from the output. The output capacitance and load current determine the voltage decay rate. For normal operation connect EN to VIN or logic high. Note: Pin EN should not be left floating. www.austriamicrosystems.com Revision 1.03 9 - 20 AS1324 Data Sheet - 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 The AS1324 is perfect for mobile communications equipment like cell phones and smart phones, digital cameras and camcorders, portabel MP3 and DVD players, PDA’s and palmtop computers and any other handheld instruments. Figure 22. Single Li-Ion 1.2V/600mA Regulator for High-Efficiency VIN 2.7 to 4.2V 4 CIN 2.2µF VIN 3 SW 4.7µH COUT 10µF 22pF 301kΩ R2 301kΩ VOUT 1.2V 600mA AS1324 1 EN 5 2 GND VFB R1 Figure 23. 5V Input to 3.3V/600mA Buck Regulator VIN 5V 4 CIN 4.7µF VIN 3 SW 4.7µH COUT 10µF 22pF 301kΩ R2 66.5kΩ VOUT 3.3V 600mA AS1324 1 EN 5 VFB 2 GND R1 Figure 24. Single Li-Ion 1.5V/600mA Regulator for High-Efficiency VIN 2.7 to 4.2V 4 CIN 4.7µF VIN 3 SW 4.7µH COUT 10µF VOUT 1.5V 600mA AS132415 1 EN 5 VOUT 2 GND www.austriamicrosystems.com Revision 1.03 10 - 20 AS1324 Data Sheet - A p p l i c a t i o n I n f o r m a t i o n Figure 25. Single Li-Ion 1.8V/600mA Regulator for Low Output Ripple 4.7µH COUT 22µF VOUT 1.8V 600mA VIN 2.7 to 4.2V 4 CIN 10µF VIN 3 SW AS132418 1 EN 5 VOUT 2 GND External Component Selection Inductor Selection For most applications the value of the external inductor should be in the range of 2.2 to 6.8µH as the inductor value has a direct effect on the ripple current. The selected inductor must be rated for its DC resistance and saturation current. The inductor ripple current (ΔIL) decreases with higher inductance and increases with higher VIN or VOUT. In Equation (EQ 2) the maximum inductor current in PWM mode under static load conditions is calculated. The saturation current of the inductor should be rated higher than the maximum inductor current as calculated with Equation (EQ 3). This is recommended because the inductor current will rise above the calculated value during heavy load transients. V OUT 1 – -------------V IN Δ I L = V OUT × ----------------------L×f Δ IL I LMAX = I OUTMAX + ------2 Where: f = Switching Frequency (1.5 MHz typical) L = Inductor Value ILmax = Maximum Inductor current ΔIL = Peak to Peak inductor ripple current The recommended starting point for setting ripple current is ΔIL = 240mA (40% of 600mA). (EQ 2) (EQ 3) The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated inductor should be sufficient for most applications (600mA + 120mA). A easy and fast approach is to select the inductor current rating fitting to the maximum switch current limit of the converter. Note: For highest efficiency, a low DC-resistance inductor is recommended. Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage ripple, greater core losses, and lower output current capability. www.austriamicrosystems.com Revision 1.03 11 - 20 AS1324 Data Sheet - A p p l i c a t i o n I n f o r m a t i o n The total losses of the coil have a strong impact on the efficiency of the dc/dc conversion and consist of both the losses in the dc resistance and the following frequency-dependent components: 1. The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies) 2. Additional losses in the conductor from the skin effect (current displacement at high frequencies) 3. Magnetic field losses of the neighboring windings (proximity effect) 4. Radiation losses Table 5. Recommended Inductors Part Number L DCR Current Rating Dimensions (L/W/T) Manufacturer Murata www.murata.com LQH32CN2R2M33 LQH32CN4R7M33 LPS3008-222MLC LPS3015-222MLC MOS6020-222MLC MOS6020-472MLC CDRH3D16NP-2R2N CDRH3D16ND-4R7N 2.2µH 4.7µH 2.2µH 2.2µH 2.2µH 4.7µH 2.2µH 4.7µH 97mΩ 150mΩ 175mΩ 110mΩ 35mΩ 50mΩ 72mΩ 105mΩ 790mA 650mA 1100mA 2000mA 3260mA 1820mA 1200mA 900mA 3.2x2.5x2.0mm 3.2x2.5x2.0mm 3.1x3.1x0.8mm 3.1x3.1x1.5mm 6.0x6.8x2.4mm 6.0x6.8x2.4mm 4.0x4.0x1.8mm 4.0x4.0x1.8mm Coilcraft www.coilcraft.com Sumida www.sumida.com Figure 26. Efficiency Comparison of Different Inductors, VIN = 2.7V, VOUT = 1.8V and 1.2V 95 VOUT = 1.8V 95 VOUT = 1.2V 90 90 Efficiency (%) . Efficiency (%) . 85 85 80 80 75 LQH32CN2R2 LPS3015-222 LQH32CN4R7 LPS3008-222 M OS6020-222 M OS6020-472 75 LQH32CN2R2 LPS3015-222 LQH32CN4R7 LPS3008-222 M OS6020-22 2 M OS6020-472 70 1 10 100 1000 70 1 10 100 1000 Output Current (mA) Output Current (mA) Output Capacitor Selection The advanced fast-response voltage mode control scheme of the AS1324 allows the use of tiny ceramic capacitors. Because of their lowest output voltage ripple low ESR ceramic capacitors are recommended. X7R or X5R dielectric output capacitor are recommended. At high load currents, the device operates in PWM mode and the RMS ripple current is calculated as: V OUT 1 – -------------V IN 1 = V OUT × ----------------------- × ---------------I RMSC OUT L×f 2× 3 (EQ 4) While operating in PWM mode the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging the output capacitor: V OUT 1 – -------------(EQ 5) V IN 1 Δ V OUT = V OUT × ----------------------- × ⎛ ------------------------------- + ESR⎞ ⎝ 8 × C OUT × f ⎠ L×f www.austriamicrosystems.com Revision 1.03 12 - 20 AS1324 Data Sheet - A p p l i c a t i o n I n f o r m a t i o n Higher value, low cost ceramic capacitors are available in very small case sizes, and their high ripple current, high voltage rating, and low ESR make them ideal for switching regulator applications. Because the AS1324 control loop is not dependant on the output capacitor ESR for stable operation, ceramic capacitors can be used to achieve very low output ripple and accommodate small circuit size. At light loads, the converter operates in powersave mode and the output voltage ripple is in direct relation to the output capacitor and inductor value used. Larger output capacitor and inductor values minimize the voltage ripple in powersave mode and tighten DC output accuracy in powersave mode. Input Capacitor Selection In continuous mode, the source current of thePMOS is a square wave of the duty cycle VOUT/VIN. To prevent large voltage transients while minimizing the interference with other circuits caused by high input voltage spikes, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given as: V OUT × ( V IN – V OUT ) I RMS = I MAX × ----------------------------------------------------------V IN (EQ 6) where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current, IMAX = ILIM - ΔIL/2 This formula has a maximum at VIN = 2VOUT where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations only provide negligible affects. The input capacitor can be increased without any limit for better input voltage filtering. Take care when using small ceramic input capacitors. When a small ceramic capacitor is used at the input, and the power is being supplied through long wires, such as from a wall adapter, a load step at the output, or VIN step on the input, can induce ringing at the VIN pin. This ringing can then couple to the output and be mistaken as loop instability, or could even damage the part by exceeding the maximum ratings. Ceramic Input and Output Capacitors When choosing ceramic capacitors for CIN and COUT, the X5R or X7R dielectric formulations are recommended. These dielectrics have the best temperature and voltage characteristics for a given value and size. Y5V and Z5U dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequencies and therefore should not be used. Table 6. Recommended Input and Output Capacitor Part Number C TC Code Rated Voltage Dimensions (L/W/T) Manufacturer Taiyo Yuden www.t-yuden.com Murata www.murata.com JMK212BJ226MG-T GRM188R60J106ME47 GRM21BR71A475KA73 22µF 10µF 4.7µF X5R X5R X7R 6.3V 6.3V 10V 0805 0603 0805 Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended that either a higher input capacity or a capacitance with a higher rated voltage is used. Feedback Resistor Selection In the AS1324-AD, the output voltage is set by an external resistor divider connected to VFB (see Figure 27). This circuitry allows for remote voltage sensing and adjustment. www.austriamicrosystems.com Revision 1.03 13 - 20 AS1324 Data Sheet - A p p l i c a t i o n I n f o r m a t i o n Figure 27. Setting the AS1324 Output Voltage 0.6V ≤ VOUT ≤ 5.5V R2 5 R1