AMS4123 3A 20V Step-Down Converter + 1A LDO General Description Features • • • • • • • • • • • • • • The AMS4123 combines a 3A Step-Down converter with a 1A LDO in a single SO-8 exposed paddle package. Both the LDO and Step-Down converter are low ESR, ceramic capacitor output, stable. The Step-Down converter is internally compensated with internal soft-start to minimize the number of external components. An Enable pin provides built-in externally programmable power-up sequencing. The Step-Down converter enable threshold is 2.0V and the LDO enable threshold is 2.5V. It also has hiccup current limit and thermal protection. Thermal protection shuts down both the Step-Down converter and LDO when the die temperature exceeds 135°C. Both regulators are adjustable using a 0.6V reference for low output voltage settings. The LDO has options for fixed output voltages from 0.6V to 5V in 100mV steps. The LDO external input can be powered from the Step-Down converter output, for improved efficiency, or from any voltage source that is less than or equal to the device supply voltage (Vin). With a dropout voltage of less than 350mV at 1A, the AMS4123 LDO makes the perfect solution for a low noise 1.8V power source developed from 2.5V Step-Down converter output. The AMS4123 is a complete solution for LCD TV power requirements when combined with the AMS4122 (2A Dual Switching Regulator in SO-8). Step-Down Converter + LDO in SO-8EP Internally Compensated Up to 95% Efficiency Low ESR Ceramic Output Capacitor Stable Soft Start Under-Voltage Lockout Dual Threshold Enable 300 kHz Switching Frequency Hiccup Current Limit Over-Temperature Shutdown Ultra-Low Dropout LDO 350mV @ 1A Up to 3A Step-Down Output Current Up to 1A LDO Output Current Excellent Light Load Efficiency Applications • • • • • • • • • Audio Power Amplifiers Portable (Notebook) Computers Point of Regulation for High Performance Electronics Consumer Electronics DVD, Blue-ray DVD writers LCD TVs and LCD monitors Distributed Power Systems Battery Chargers Pre-Regulator for Linear Regulation Typical Application Vin 4.5V to 20V U1 3 C5 220nF AMS4123 Vin SW LDOin BST 1 L1 10uH 2.5V at 2A SW out 2.5V 7 C1 10uF 2 1.8V at 1A 8 R1 20.0k 6 C3 2.2uF R2 10.0k LDOout FB SW FB LDO Enable EN D1 B340LB 5 C2 22uF 4 R3 10.0k R4 31.6k C8 4.7nF 3/5/2010 C9 100uF www.advanced-monolithic.com 1 R1 and R4 Voltage Options 1.8V 20.0k 2.5V 31.6k 3.3V 45.3k 5.0V 73.2k Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Pin Description Pin # Symbol 1 SW 2 BST 3 Vin 4 EN 5 FB SW 6 FB LDO 7 LDO in 8 LDO out 9 GND (PADDLE) Description Step-Down converter switching node that connects the internal power switch to the output inductor. The bootstrap capacitor tied to this pin is used as the bias source for the drive to the internal power switch. Use a 220nF or greater capacitor from the BST to the SW pin. Input Power. Supplies bias to the IC and is also the power input to the step-down converter main power switch. Bypass Vin with low impedance ceramic with sufficient capacitance to minimize switching frequency ripple as well as high frequency noise. Enable. A voltage greater than 2V at this pin enables the switching regulator. 2.5V enables the LDO section. Step-Down Converter Feedback input. A resistor network of two resistors is used to set-up the output voltage connected between VSW out and GND. The node between the two resistors is connected to Feedback Switch pin. LDO Feedback input. A resistive voltage divider is used to set the output voltage connected between the LDO output and GND. The node between the two resistors is connected to FB LDO pin. LDO Input. Connect to the output of the Step-Down converter. LDO IN can also be powered from any power supply as long as it is 2V less than Vin. LDO Output pin. Ground paddle to be connected to PCB ground plane. This is also the ground for internal voltage reference. Pin Configuration 8L SOIC SO Package (S) Top View 3/5/2010 www.advanced-monolithic.com 2 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Absolute Maximum Ratings (1) Recommended Operating Conditions (2) VIN Supply Voltage………………...………..….-0.3V to 23V LDOIN Supply Voltage…………………..……...-0.3V to 20V LDOOUT Output Voltage………………….….....-0.3V to 20V BST Boot Strap Voltage………………....…. -0.3V to 27V FBLDO,FBSW feedback pins………....……-0.3V to +12V EN Enable Voltage………………………..….-0.3V to +20V Storage Temperature Range……………...-65⁰C to 150⁰C Lead Temperature…………………..……….….…… 260⁰C Junction Temperature………...………………..…… 150⁰C Input Voltage………………………………….………..4.5V to 20V Ambient Operating Temperature…… …………….-40⁰C to 85⁰C Electrical Characteristics Parameter Thermal Information (3) 8L SOIC EP θJA …………………………………….…...45⁰C/W θJC ...........................................................10⁰C/W Maximum Power Dissipation…………………………...….…….2W TA= 25 °C and VIN=12V (unless otherwise noted). Symbol Conditions Min. Typ. Max. Units 4.5 12 20 V Vin Vin LDO Feedback Voltage VFBLDO ILDO=0A tbd 0.586 tbd V Switcher Feedback Voltage VFBSW Isw=0A tbd 0.596 tbd V LDO Output Voltage tolerance VLDO Out VLDO out=0.6V to 5V in 100mV increments -1.5 1 1.5 % Step-Down Converter Bias Current IQSW 1.4 1.9 mA LDO+SW Bias Current IQSW+LDO 1.3 2.0 mA LDO Bias Current IQLDO VEN= 5V; VFBLDO = 1.5V 400 μA Shutdown Supply Current IVinsd VEN =0V 90 nA SW NPN Saturation Voltage Converter Current Limit VSAT ISW out=1A 0.66 V ILIMSW VSW out=5V 4.2 A LDO Current Limit ILIMLDO VLDO in=5V; Co=2.2μF 1.1 A VDO VLDOin=VLDOout-0.1V, Io=1A 350 mV ILDO = 0 to1A 0.5 % VLDOin = VLDOout+0.5V to 20V, Vin=20V 0.1 % LDO Dropout Voltage VLDOin =VEN =5V VFBSW= 1.5V VLDOin =VEN =5V VFBLDO =VFBSW= 1.5V Oscillator Frequency ΔVLDO Out / VLDO Out ΔVLDO Out / VLDO Out FOSC 300 340 kHz Maximum Duty Cycle DMAX VFB=0V 95 99 % Minimum Duty Cycle DMIN VFB=1.5V 0 Converter Enable Threshold VEN SW 2.0 Enable Hysteresis VENHYS 100 LDO Enable Threshold VEN LDO 2.5 Enable Pull-up Current IEN VEN = 0V 0.7 μA Under Voltage Lockout VUVLO Vin rising 4.2 V Under Voltage Lockout Hysteresis VUVLO HYS 200 mV Total Power dissipation PD 2.5 W Thermal Shutdown TSD 145 °C LDO Load Regulation LDO Line Regulation 3/5/2010 260 Note (4) www.advanced-monolithic.com 3 % 2.1 V mV 2.55 V Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Notes: 1. 2. 3. 4. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Operation outside of the recommended operating conditions is not guaranteed. Measured on approximately 1” square of 1 oz. copper. The total power dissipation for SO-8 EDP package is recommended to 2.5W rated at 25⁰C ambient temperature. The thermal resistance Junction to Case is 45⁰C/W. Total power dissipation for the switching regulator and the LDO should be taken in consideration when calculating the output current capability of each regulator. 3/5/2010 www.advanced-monolithic.com 4 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Typical Characteristics Efficiency (%) 90 80 VSW out Regulation (%) 100 Efficiency V SW out=5V, L=10µH, B340LB Schottky Vin =12V 70 Vin =23V 60 50 40 30 0.6 Vin =12V 0.2 -0.2 Vin =23V -0.6 -1.0 20 0.01 Load Regulation VSW out =5V, L=10µH 1.0 0.1 1 0.01 10 80 Efficiency VSW out=3.3V, L=10µH, B340LB Schottky Vin =12V 70 60 Vin =23V 50 40 30 1.0 0.6 Vin =23V 0.2 -0.2 Vin =12V -0.6 -1.0 20 0.01 0.1 1 0.01 10 80 VSW out Regulation (%) Efficiency (%) 90 Efficiency Vsw out=2.5V, L=10µH, B340LB Schottky Vin =12V 70 60 50 Vin =23V 40 30 20 0.01 3/5/2010 0.1 1 10 Output Current (A) Output Current (A) 100 10 Load Regulation VSW out=3.3V, L=10µH VSW out Regulation (%) Efficiency (%) 90 1 Output Current (A) Output Current (A) 100 0.1 1.0 Load Regulation Vsw out=2.5V, L=10 µH 0.6 V in =23V 0.2 -0.2 Vin =12V -0.6 -1.0 0.1 1 Output Current (A) 10 0.01 www.advanced-monolithic.com 5 0.1 1 10 Output Current (A) Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Typical Characteristics No Load Input Current vs. Input Voltage Vsw out = 2.5V, VLDO out = 1.8V 0.50 Output Error (%) Input Current (mA) 3.2 2.4 1.6 0.8 0.0 Output Voltage Error vs. Input Voltage VSW out = VLDO in = 2.5V, VLDO out =1.8V 0.25 V LDO out 0.00 Vsw out -0.25 ILDO=0.6A Isw =1.6A -0.50 0 5 10 15 20 25 0 5 Switching Frequency vs. Input Voltage Vsw out = 2.5V, VLDO out = 1.8V Dropout Voltage (V) 308 304 300 296 292 0 VLDO Out Voltage (V) 2 5 10 15 20 20 0.3 0.2 0.1 VLDO out programmed f or 1.8V Vin = 12V 0 0 0.2 0.4 0.6 0.8 LDO Output Current (V) VLDO Out Load Regulation VSW out = VLDO in =2.5V, VLDO Out=1.8V Feedback Voltage Temperature Variation Vin=15V 1 0.5 Vin=20V 0.61 0.4 0.6 0.8 1 1 FBSW 0.60 0.59 FBLDO 0.58 ILDO=Isw=0 V LDO out=1.8V, V SW out=2.5V 0.57 0 0.2 25 LDO Dropout Voltage vs. Load Current VLDO in = VLDO out - 0.1V 0.4 25 Vin=12V -50 Output Current (A) 3/5/2010 15 Input Voltage (V) 1.5 0 10 Input Voltage Vin (V) Feedback Voltage (V) Switching Frequency (kHz) Input Voltage (V) -10 30 70 110 150 Ambient Temperature (ºC) www.advanced-monolithic.com 6 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Typical Characteristics Step-Down Converter Output Ripple VSW out =2.5V, ISW out=1.8A, Vin=12V LDO 200mA to 800mA Transient Response, VLDO in=3.3V, Co=2.2µF, VLDO out = 1.8V, Vin=12V VSW out 20mVac /div VLDO out 100mVac /div IL 1A/div ILDO out 500mA /div VSW 5V/div 2 µsec/div 1 µsec/div Step-Down Converter Load Transient No Load to 2A,VSW out=2.5V, Vin=12V Step-Down Converter Load Transient 200mA to 2A, Vsw out = 2.5V, Vin=12V VSW out 100mVac /div VSW out 100mVac /div ISW out 500mA /div ISW out 1A/div 2 msec/div 40 µsec/div LDO Transient Response No Load to 1A, VLDO in=Vsw out =2.5V, Step-Down Converter Load Transient 200mA to 1.2A, Vsw out = 2.5V,Vin=12V VSW out 200mVac/ div VSW out 100mVac /div VLDO out 100mVac /div ISW out 500mA /div ILDO out 1A/div 20 µsec/div 3/5/2010 20 µsec/div www.advanced-monolithic.com 7 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Typical Characteristics VSW out 1V /div VLDO out 1V /div IL 2A/div Ven 5V /div Switching Frequency (kHz) Start-Up Response Vin=12V Switching Frequency Temperature Variation VSW out=2.5V, Vin =12V 300 280 260 240 220 -45 -10 25 60 95 130 Ambient Temperature (ºC) 400 µsec/div • VLDO out 1V /div IL 2A/div Vin 10V /div Feedback Voltage Error (%) Start-Up Response Enable=Vin=12V Feedback Voltage Temperature Variation 0.8 FBSW 0.0 FBLDO -0.8 ILDO=Isw=0 V LDO out=1.8V, V SW out=2.5V -1.6 -2.4 -50 -10 30 70 110 150 Ambient Temperature (ºC) 2 msec/div Start-Up Response Vin=20V 1.2 1V /div IL 2A/div Ven 5V /div Vcesat (V) VSW out 1V /div VLDO out Step-Down Converter Power Switch Saturation Voltage Vin =12V 0.9 0.6 0.3 Tamb = 25⁰ C Mounted on Eval. Board 0 0 0.7 1.4 2.1 2.8 3.5 Current (A) 1 msec/div 3/5/2010 www.advanced-monolithic.com 8 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Typical Characteristic LDO Current Limit VLDO in = 3.3V, V LDO out=1.8V 50 1.3 Ground Current (mA) Current Limit (A) 1.4 1.2 1.1 1 0.9 0.8 Voltage Mode Load V LDO out = 1.68V 0.7 0.6 5 9 13 17 21 LDO Ground Current VLDO in = 3.3V, VLDO out=1.8V 25 40 30 20 10 0 0 400 600 800 1000 Load Current (mA) V in Input Voltage (V) 3/5/2010 200 www.advanced-monolithic.com 9 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Functional Block Diagram Vin 3 UVLO 4.2V / 3.8V BST Reg. Internal Vcc Vcc 3.3V Regulator Isense Vref 0.6V EAout Σ 2 R 300kHz Oscillator FB SW S SET CLR BST Level Shift Q Q 1 SW SW out EAout 5 Vref 0.6V Switching Regulator Shutdown 2.0V En 4 7 8 Pgnd 3/5/2010 P Paddle LDO In Vref Shutdown Comparators 2.5V PVin LDO Out 6 FB LDO www.advanced-monolithic.com 10 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Device Summary The AMS4123 is combines a high voltage 3 Amp fixed frequency step-down converter combined with a 1 Amp low drop out (LDO) linear regulator on a single die. The peak current mode step-down converter has internal compensation and is stable with a wide range of ceramic, tantalum, and electrolytic output capacitors. The step-down converter output voltage is sensed through an external resistive divider that feeds the negative input to an internal transconductance error amplifier. The output of the error amplifier is connected to the input to a peak current mode comparator. The inductor current is sensed as it passes through the power switch, amplified and is also fed to the current mode comparator. The error amplifier regulates the output voltage by controlling the peak inductor current passing through the power switch so that, in steady state, the average inductor current equals the load current. The step-down converter has an input voltage range of 4.5V to 20V with an output voltage as low as 0.6V. The LDO operates from an input voltage ranging from 1V to 20V and a typical dropout voltage of 350mV at 1A. The input to the LDO can be supplied by the output of the Step-Down converter or some other available power source that must be 2V less than the input voltage (Vin). The LDO is also stable for a wide range of ceramic output capacitors ranging from as low as 1µF. Enable The enable input has two levels so that the step-down converter can be enabled independently of the LDO. The enable threshold for the step-down converter is 2.0V while the enable threshold for the linear regulator output is 2.5V typical. Under Voltage Lockout The under-voltage lockout (UVLO) feature guarantees sufficient input voltage (Vin) bias for proper operation of all internal circuitry prior to activation. The input voltage (Vin) is internally monitored and the converter and LDO are enabled when the rising level of Vin reaches 4.2V. To prevent UVLO chatter 400mV of hysteresis is built in to the UVLO comparator so that the step-down converter and LDO are disabled when VIN drops to 3.8V. 3/5/2010 Fault Protection Short circuit and over-temperature shutdown disable the converter and LDO in the event of an overload condition. Application Inductor The step-down converter inductor is typically selected to limit the ripple current to 40% of the full load output current. Solve for this value at the maximum input voltage where the inductor ripple current is greatest. L= Vin-Vo · L= 15V-2.5V · Vo Vin·Io·0.4·Fs 2.5V =9.4µH 15V·2A·0.4·300kHz For most applications the duty cycle of the AMS4123 step down converter is less than 50% duty and does not require slope compensation for stability. This provides some flexibility in the selected inductor value. Given the above selected value, others values slightly greater or less may be examined to determine the effect on efficiency without a detrimental effect on stability. With and inductor value selected, the ripple current can be calculated: Ipp= (Vo+Vfwd)·(1-D) L·Fs Using the maximum input voltage values the ripple is: Ipp= (2.5V+0.2V)· 1-0.23 =0.7A 10μH·300kHz Once the appropriate value is determined, the component is selected based on the DC current and the peak (saturation) current. Select an inductor that has a DC current rating greater than the full load current of the application. The DC current rating is also reflected in the DC resistance (DCR) specification of the inductor. The inductor DCR should limit the inductor loss to less than 2% of the stepdown converter output power. The peak current at full load is equal to the full load DC current plus one half of the ripple current. As mentioned before, the ripple current varies with input www.advanced-monolithic.com 11 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO voltage and is a maximum at the maximum input voltage. Ipkmax=Io+ (Vo+Vfwd)·(1-Dmin) 2·L·Fs High Frequency Ripple The following equation determines the required low ESR ceramic output capacitance for a given inductor current ripple (Ipp). Vo Dmin= Vinmax The duty cycle can be more accurately estimated by including the drops of the external Schottky diode and the internal power switch: Dmin= Dmin= Vo+Vfwd Vinmax-Vo+Vfwd 2.5V+0.2V =0.23 15V-0.3V+0.2V Vfwd is the diode freewheeling diode drop and Vsw is the collector to emitter drop of the internal power switch. With a good estimate of the duty cycle (D) the inductor peak current can be determined: Ipkmax=2A+ (2.5V+0.2V)·(1-0.23) =2.35A 2·10µH·300kHz There are a wide range 2 and 3 Amp, shielded and non-shielded inductors available. Table 1 lists a few. Table 1. Inductor Selection Guide Dimensions (mm) Series Type W L H Coilcraft NonDO3316P 9.4 13 5.2 Shielded Non9.4 13 3.0 DO3308 Shielded Sumida CDRH6D26 Shielded 7 7 2.8 Non7.3 7.3 5.2 CDH74 Shielded Coiltronics SD8328 Shielded 8.3 9.5 3.0 3/5/2010 Step-Down Converter Output Capacitor The optimum solution for the switching regulator is to use a large bulk capacitor for large load transients in parallel with a smaller, low ESR, X5R or X7R ceramic capacitor to minimize the switching frequency ripple. C= Ipp 0.7A = =15μF Fs·8·dV 300kHz·8·20mV Large Signal Transient For applications with large load transients an additional capacitor may be required to keep the output voltage within the limits required during large load transients. In this case the required capacitance can be examined for the load application and load removal. For full load to no load transient the required capacitance is 2 L·Io2 10μH·(2A) Cbulk= = =36μF Vos2 -Vo2 (2.7V)2 -(2.5V)2 For the application of a load pulse the capacitance required form hold up depends on the time it takes for the power supply loop to build up the inductor current to match the load current. For the AMS4123 this can be estimated to be less than 10 µsec or about three clock cycles. Cbulk= Io·t 2A·10μsec = =100μF dV 0.2V For applications that do not have any significant load transient requirements a ceramic capacitor alone is typically sufficient. Boot Strap Capacitor An external capacitor is required for the high side switch drive. The capacitor is biased during the off time while the switch node is at ground by way of the freewheeling diode. During the on time portion of the switching cycle the switch node is tied to the input voltage by way of the internal power switch. The boot strap capacitor is always referenced to the switch node so the charge stored in the capacitor during the off time is then used to drive the internal power switch during the on time. www.advanced-monolithic.com 12 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Typical bootstrap capacitor values are in the 220nF to 470nF range. Insufficient values will not be able to provide sufficient base drive current to the power switch during the on time. Values less than 220nF are not recommended. This will result in excessive losses and reduced efficiency. Optional Snubber To reduce high frequency ringing at the switching node a snubber network is suggested. The values typically selected are 470pF ceramic in series with a 10Ω resistor. The power dissipation of the 10Ω resistor is about 32mW for a 15V input with a 300kHz switching frequency. PR1=C3·Vin2 ·Fsw Vin is the maximum input voltage and Fsw is the switching frequency. The snubber capacitor must be rated to withstand the input voltage. Step-Down Converter Input Capacitor The low esr ceramic capacitor required at the input to filter out high frequency noise as well as switching frequency ripple. Placement of the capacitor is critical for good high frequency noise rejection. See the PCB layout guidelines section for details. Switching frequency ripple is also filtered by the ceramic bypass input capacitor. Given a desired input voltage ripple (Vripple) limit, the required input capacitor can be estimated with: Dmax= C= Linear Regulator Output Capacitor The Linear regulator is stable with a wide range of ceramic capacitors. The ceramic output capacitor can range from 1uF to 100uF with either X5R or X7R temperature coefficient. The actual values selected within the range will depend on the expected load transients and the output voltage tolerance requirements during the load transient. Linear Regulator Input Capacitor Place a 2.2uF X5R or X7R or equivalent ceramic bypass capacitor at the LDO input. Feedback Resistor Selection The step down converter and LDO both use a 0.6V reference voltage at the positive terminal of the error amplifier. To set the output voltage a programming resistor form the feedback node to ground must first be selected (R2,R3 of figure 4). A 10kΩ resistor is a good selection for a programming resistor. A higher value could result in an excessively sensitive feedback node while a lower value will draw more current and degrade the light load efficiency. The equation for selecting the voltage specific resistor is: R4= Vout Vref -1 ·R3 = Dmax·Io·(1-Dmax) Fs·Vripple 0.6V -1V ·10kΩ=31.67kΩ Table 2. Feedback Resistor values Vout (V) 1.8 2.5 3.3 5.0 Vo+Vfwd Vinmin-Vo+Vfwd 2.5V R1,R4 (kΩ) (R2,R3=10kΩ) 20.0 31.6 45.3 73.2 2.5V 0.2V 2.5V +0.2V ·2A· 19V-0.3V+0.2V 9V-0.3V+0.2V = =7μF 300kHz·0.2V . For high voltage input converters the duty cycle is always less than 50% so the maximum ripple is at the minimum input voltage. The ripple will increase as the duty cycle approaches 50% where it is a maximum. Step-Down Converter Feedforward Capacitor For optimum start-up and improved transient response place a feed-forward capacitor (C6) across the feedback resistor R2. Typical values range from 220pF to 10nF. 3/5/2010 www.advanced-monolithic.com 13 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO PCB Layout The following guidelines should be followed to insure proper layout. 1. Vin Capacitor. A low ESR ceramic bypass capacitor must be placed as close to the IC as possible. 2. Schottky Diode. During the off portion of the switching cycle the inductor current flows through the Schottky diode to the output cap and returns to the inductor through the output capacitor. The trace that connects the output diode to the output capacitor sees a current signal with a very high di/dt. To minimize the associated spiking and ringing, the inductance and resistance of this trace should be minimized by connecting the diode anode to the output capacitor return with a short wide trace. 3. Feedback Resistors. The feedback resistors should be placed as close as possible the IC. Minimize the length of the trace from the feedback pin to the resistors. This is a high impedance node susceptible to interference from external RF noise sources. 4. Inductor. Minimize the length of the SW node trace. This minimizes the radiated EMI associated with the SW node. 5. Ground. The most quiet ground or return potential available is the output capacitor return. The inductor current flows through the output capacitor during both the on time and off time, hence it never sees a high di/dt. The only di/dt seen by the output capacitor is the inductor ripple current which is much less than the di/dt of an edge to a square wave current pulse. This is the best place to make a solid connection to the IC ground and input capacitor. This node is used as the star ground shown in Figure 1. This method of grounding helps to reduce high di/dt traces, and the detrimental effect associated with them, in a step-down converter. The inductance of these traces should always be minimized by using wide traces, ground planes, and proper component placement. 6. For good thermal performance vias are required to couple the exposed tab of the SO-8 package to the PCB ground plane. The via diameter should be 0.3mm to 0.33mm positioned on a 1.2mm grid. 3/5/2010 Ion+ Ioff Ion Ioff PCB Inductance High di/dt Ion Ioff Ion+Ioff Ion Ioff High di/dt trace reduction “Star Ground” Figure 1. Step Down Converter Layout www.advanced-monolithic.com 14 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Output Power and Thermal Limits The AMS4123 junction temperature, Step-Down converter and LDO current capability depends on the internal dissipation and the junction to case thermal resistance of the SO8 exposed paddle package. This gives the junction temperature rise above the device paddle and PCB temperature. The temperature of the paddle and PCB will be elevated above the ambient temperature due to the total losses of the step down converter and losses of other circuits and or converters mounted to the PCB. Tjmax=Pd·θjc+Tpcb+Tamb The losses associated with the AMS4123 overall efficiency are; 1. Output Diode Conduction Losses 2. Inductor DCR Losses 3. AMS4123 Internal losses a. Power Switch Forward Conduction and Switching Losses b. Quiescent Current Losses The internal losses contribute to the junction temperature rise above the case and PCB temperature. The junction temperature depends on many factors and should always be verified in the final application at the maximum ambient temperature. This will assure that the device does not enter over-temperature shutdown when fully loaded at the maximum ambient temperature. 3/5/2010 www.advanced-monolithic.com 15 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO Figure 2. AMS4123 Evaluation Board Top Side Figure 3. AMS4123 Evaluation Board Bottom Side 1 2 JP1 LDO Input R5 10 VLX L1 10uH Vout C7 470pF 1 C5 C2 J6 gnd Vin 2 220nF J3 C9 22uF 100uF 16V C4 22uF 35V C1 10uF 50V U1 3 4 SW LDO Out BST LDO In Vin FB LDO EN FB SW D1 B340LB J2 AMS4123_1 R4 gnd 45.3k J5 Enable J1 VLDOIn J4 8 7 R1 31.6k VLDOOut 6 5 C6 2.2uF R3 10.0k C8 R2 10.0k C3 2.2uF J7 gnd 4.7nF Figure 4. AMS4123 Evaluation Board Schematic Table 3. Evaluation Board Bill of Materials Component Value L1 C9 C2 C1 C3,C6 C3,C6 option C7 C5 C8 C4 R5 3/5/2010 Manufacturer Manufacturer Part Number Coilcraft DO3316P Kemet T491X107M016AS 10µF, 50V, X5R, 1210, Ceramic 2.2µF, 10V, X5R, 0805 2.2µF, 10V, X5R, 0603 Taiyo Yuden TDK Taiyo Yuden Murata Murata LMK212BJ226MG-T C3225X5R1A226M UMK325BJ106KM-T GRM216R61A225KE24 GRM39X5R225K10H52V 470pF 50V, 20%, X7R, 0603 220nF 25V, 10%, X7R, 0603 4.7nF 50V, 20%, X7R, 0603 22µF 35V Tantalum Case E 10Ω, 0.1W, 0603 5% Murata Murata Murata Vishay Vishay/Dale GRM188R71H471MA01 GRM188R71E224KA88 GRM188R71H472MA01 293D226X9035E2TE3 CRCW060310R0JNEA 10µH 3.9A 9.4mm x 13mm x 5.2mm 100µF, 16V, X case General Purpose Tantalum 22µF, 10V, X5R, 0805, Ceramic www.advanced-monolithic.com 16 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO R2,R3 R1,R4 D1 U1 10kΩ, 0.1W, 0603 1% See table 2 3A, 40V Schottky Step-Down Converter / LDO Various Various Diodes Inc. AMS CRCW060310K0FKEA CRCW0603xxKxFKEA B340LB AMS4123 ORDERING INFORMATION Package Type SOIC EDP AMS4123S TEMP. RANGE -25°C to 125°C PACKAGE DIMENSIONS inches (millimeters) unless otherwise noted. 8 LEAD SOIC PLASTIC PACKAGE (S) 3/5/2010 www.advanced-monolithic.com 17 Phone (925) 443-0722 Fax (925) 443-0723 AMS4123 3A 20V Step-Down Converter + 1A LDO 3/5/2010 www.advanced-monolithic.com 18 Phone (925) 443-0722 Fax (925) 443-0723