® RT8553A Dual-Output Synchronous DC/DC Converter General Description Features The RT8553A is a dual-output DC/DC converter which is designed to provide the power for consumer products. It integrates a Boost converter and an inverting-Boost converter for enhancing the overall system efficiency of battery powered products. The RT8553A operates in Forced CCM mode for light load and heavy load condition. The high frequency allows for reduction of external components. In shutdown mode, the RT8553A consumes less than 1μA current. The RT8553A provides soft-start, OCP, OTP, OVP and UVLO function. The RT8553A is available in the tiny UDFN-12L 3x3 package to achieve best solution for saving PCB space and total BOM cost Boost Converter to Supply Positive Voltage from 4.6V to 5.4V Through External Feedback Resistors saving considerations. Buck-Boost Converter to Supply Negative Voltage from −4.6V to −5.4V Through External Feedback Resistors Maximum Output Current up to 200mA Typical Efficiency : 85% PWM Mode @ 1.5MHz Switching Frequency High Output Voltage Accuracy Excellent Line and Load Transient Excellent Line and Load Regulation Soft-Start to Limit Inrush Current Over-Temperature Protection (OTP) Over-Current Protection (OCP) Over-Voltage Protection (OVP) Low Quiescent Current < 1μ μA in Shutdown Mode RoHS Compliant and Halogen Free Ordering Information RT8553A Package Type QU : UDFN-12L 3x3 (U-Type) Applications Lead Plating System G : Green (Halogen Free and Pb Free) Note : Richtek products are : RoHS compliant and compatible with the current require- Cellular Phones Digital Cameras PDAs and Smart Phones Probable Instruments ments of IPC/JEDEC J-STD-020. Suitable for use in SnPb or Pb-free soldering processes. Simplified Application Circuit VIN CIN1 CIN2 L1 RT8553A LX1 VINP LX2 VINA VOUT1 L2 VOUT1 R1 Enable EN COUT1 FB1 R2 GND VOUT2 PGND VOUT2 R3 COUT2 FB2 AGND R4 VREF Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 1 RT8553A Marking Information Pin Configurations 3S=YM DNN YMDNN : Date Code LX1 VOUT1 PGND FB1 AGND EN 1 2 3 4 5 6 GND (TOP VIEW) 3S= : Product Code 13 12 11 10 9 8 7 VINP LX2 VOUT2 FB2 VREF VINA UDFN-12L 3x3 Functional Pin Description Pin No. Pin Name Pin Function 1 LX1 Switch Node of Boost Converter. 2 VOUT1 Boost Converter Output. 3 PGND Power Ground. 4 FB1 Feedback Voltage Input of Boost Converter. 5 AGND Analog Ground. 6 EN Enable Control Input. (Active High) 7 VINA Analog Supply Voltage Input. 8 VREF Reference Voltage. 9 FB2 Feedback Voltage Input of Inverting Boost Converter. 10 VOUT2 Buck-Boost Converter Output. 11 LX2 Switch Node of Buck-Boost Converter. 12 VINP Power Supply Voltage Input. 13 (Exposed Pad) GND Ground. The exposed pad must be soldered to a large PCB and connected to AGND for maximum power dissipation. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 2 is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A Function Block Diagram LX1 VIN Detection VINA Switching Well OVP1 VOUT1 UVLO P1 PWM Logic N1 + FB1 OCP1 + VINP - PGND VINP GM + EN Soft-Start VREF Band Gap Reference OCP2 VREF1 PWM Logic P2 LX2 Oscillator + + N2 - VOUT2 GM + VREF2 OVP2 FB2 OTP AGND Operation The RT8553A is a dual-output synchronous DC/DC converter for consumer product applications that it can support input voltage range from 2.5V to 4.5V and the output current up to 200mA. The RT8553A uses current mode architecture for the purpose of high efficiency and high transient response. The VOUT1 positive output voltage is produced from the DC/DC Boost converter, and output voltage can be adjusted by external feedback resistors. The VOUT2 negative output voltage is produced from the DC/DC Buck-Boost converter, and the negative output voltage can be adjusted by external feedback resistors. When the EN goes high, the positive output voltage will be enabled with an internal soft-start process. After the positive output voltage is ready, negative output voltage will be enabled with an internal soft-start process. Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 The RT8553A provides protection functions, such as OverCurrent Protection (OCP), Over-Temperature Protection (OTP) and Over-Voltage Protection (OVP) to protect application products and itself. The RT8553A employs an internal soft-start feature to avoid high inrush currents during start-up. Both the Boost and the Buck-Boost converters can operate in Force Continuous Conduction Mode (FCCM) for better stability and minimum output ripple. is a registered trademark of Richtek Technology Corporation. www.richtek.com 3 RT8553A Absolute Maximum Ratings (Note 1) Supply Input Voltage, VINA, VINP ----------------------------------------------------------------------------Boost Output Voltage, VOUT1 ---------------------------------------------------------------------------------Boost Switching Voltage, LX1 ----------------------------------------------------------------------------------Boost Feedback Voltage, FB1 ---------------------------------------------------------------------------------Reference Voltage, VREF ---------------------------------------------------------------------------------------Inverting Output Voltage, VOUT2 ------------------------------------------------------------------------------Inverting Boost Switching Voltage, LX2 ----------------------------------------------------------------------Inverting Boost Feedback Voltage, FB2 ----------------------------------------------------------------------Enable Input Voltage, EN ----------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C UDFN-12L 3x3 ------------------------------------------------------------------------------------------------------- 3.09W Package Thermal Resistance (Note 2) UDFN-12L 3x3, θJA ------------------------------------------------------------------------------------------------- 32.3°C/W UDFN-12L 3x3, θJC ------------------------------------------------------------------------------------------------ 6.6°C/W Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------ 260°C Junction Temperature ---------------------------------------------------------------------------------------------- 150°C Storage Temperature Range ------------------------------------------------------------------------------------- −65°C to 150°C ESD Susceptibility (Note 3) HBM (Human Body Model) --------------------------------------------------------------------------------------- 2kV MM (Machine Model) ---------------------------------------------------------------------------------------------- 200V Recommended Operating Conditions −0.3 to 6V −0.3 to 6V −0.3 to 6V −0.3 to 6V −0.3 to 6V −6.5 to 0.3V −6.5 to (VINP + 0.3V) −0.3 to 6V −0.3 to 6V (Note 4) Supply Voltage, VIN ----------------------------------------------------------------------------------------------- 2.5V to 4.5V Junction Temperature Range ------------------------------------------------------------------------------------- −40°C to 125°C Ambient Temperature Range ------------------------------------------------------------------------------------- −40°C to 85°C Electrical Characteristics (VINA = VINP = VEN = 3.7V, CIN = COUT1 = COUT2 = 10μF, L1 = L2 = 4.7μH, TA = 25°C, unless otherwise specified) Parameter Symbol Test Conditions Min Typ Max Unit General Specification Input Voltage Range V IN VOUT1 = 5V 2.5 3.7 4.5 V Under-Voltage Lockout High Voltage V UVLOH VINA Rising -- 2.22 2.4 V Under-Voltage Lockout Low Voltage V UVLOL VINA Falling 1.9 2.18 -- V VIN Quiescent Current IQ No Load Condition, No Switching VOUT1 = 5V, VOUT2 = 5V -- 2 -- mA I SHDN VEN = GND -- -- 1 A Logic-High V IH VINA = 2.5V to 4.5V 1.2 -- -- V Logic-Low V IL VINA = 2.5V to 4.5V -- -- 0.6 V I EN VINA = VEN = 4.5V -- 150 -- k VIN Shutdown Current EN Input Voltage EN Pull Down Resistor Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 4 is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A Parameter Symbol Test Conditions Min Typ Max Unit 1.35 1.5 1.65 MHz Switching Frequency f SWPW M FCCM Mode Reference Voltage VREF VREF -- 1 -- V Boost Maximum Duty D1MAX Open Loop Condition -- 85 -- % Inv erting-Boost Maximum Duty D2MAX Open Loop Condition -- 85 -- % I OUT1 = IOUT2 = 10mA to 30mA, VOUT1 = 5V, VOUT2 = 5V -- 80 -- I OUT1 = IOUT2 = 30mA to 200mA, VOUT1 = 5V, VOUT2 = 5V -- 85 -- System Efficiency ESYS % Over-Temperature Protection OTP -- 140 -- C Over-Temperature Protection Hysteresis OTPHYST -- 15 -- C 4.6 5.0 5.4 V VINA = VINP = 2.9V to 4.5V I OUT1 = 5mA to 200mA I OUT2 = No Load 2 -- 2 % Boost Converter Positive Output Voltage Range VOUT1 Positive Output Voltage Total Variation Maximum Output Current IOUT1MAX VINA = VINP = 2.9V to 4.5V 200 -- -- mA Feedback Voltage of Boost Converter VFB1 VINA = VINP = 2.9V to 4.5V -- 1 -- V Over Voltage of Boost Converter VOVP1 -- 6 -- V I OUT1 = 3mA to 30mA and I OUT1 = 30mA to 3mA, T R = T F = 150s, Output Variation respect to VOUT1 -- ±20 -- mV I OUT1 = 10mA to 100mA and I OUT1 = 100mA to 10mA, T R = T F = 150s, Output Variation respect to VOUT1 -- ±25 -- mV VINA = VINP = 2.9V to 4.5V, I OUT1 = 5mA, I OUT2 = No Load -- 0.5 -- VINA = VINP = 2.9V to 4.5V, I OUT1 = 100mA, I OUT2 = No Load -- 0.5 -- I OUT1 = 5 to 100mA, IOUT2 = No Load, VINA = VINP = 2.9V -- 0.5 -- I OUT1 = 5 to 100mA, IOUT2 = No Load, VINA = VINP = 4.5V -- 0.5 -- VINA = VINP = 2.9V to 4.35V -- 1 -- A VINA = VINP = 3.7V, ILX1 = 100mA -- 0.2 -- VINA = VINP = 3.7V, ILX1 = 100mA -- 0.3 -- Load Transient Static Line Regulation Static Load Regulation Boost Switching Current Limit N1 N-MOSFET On-Resistance P1 P-MOSFET On-Resistance VOUT1LOAD_LT VOUT1LINE_SL VOUT1LOAD_SL ILX1 % RDS(ON)1 Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 % is a registered trademark of Richtek Technology Corporation. www.richtek.com 5 RT8553A Parameter Symbol Test Conditions Min Typ Max Unit 4.6 5 5.4 V VINA = VINP = 2.9V to 4.5V IOUT2 = 5mA to 300mA IOUT1 = No Load 2 -- 2 % Inverting-Boost Converter Negative Output Voltage Range V OUT2 Negative Output Voltage Total Variation Maximum Output Current I OUT2MAX VINA = VINP = 2.9V to 4.5V 200 -- -- mA Feedback Voltage of Inv erting Conv erter V FB2 VINA = VINP = 2.9V to 4.5V -- 0 -- V Over-Voltage of Inv erting-Boost Converter V OVP2 -- -6 -- V IOUT2 = 3mA to 30mA and IOUT2 = 30mA to 3mA, TR = TF = 150s, Output Variation respect to VOUT2 -- ±40 -- mV IOUT2 = 10mA to 100mA and IOUT2 = 100mA to 10mA, TR = TF = 150s, Output Variation respect to VOUT2 -- ±50 -- mV VINA = VINP = 2.9V to 4.5V, IOUT2 = 5mA, IOUT1 = No Load -- 0.5 -- % VINA = VINP = 2.9V to 4.5V, IOUT2 = 100mA, IOUT1 = No Load -- 0.5 -- % IOUT2 = 5mA to 100mA, I OUT1 = No Load, VINA = VINP = 2.9V -- 0.5 -- % IOUT2 = 5mA to 100mA, I OUT1 = No Load, VINA = VINP = 4.5V -- 0.5 -- % VINA = VINP = 2.9V to 4.5V -- 1.2 -- A VINA = VINP = 3.7V, ILX2 = 100mA -- 0.2 -- VINA = VINP = 3.7V, ILX2 = 100mA -- 0.3 -- Load Transient Static Line Regulation Static Load Regulation Inv erting-Boost Switching Current Limit N2 N-MOSFET On-Resistance P2 P-MOSFET On-Resistance V OUT2LOAD_LT V OUT2LINE_SL V OUT2LOAD_SL I LX2 RDS(ON)2 Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect device reliability. Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is measured at the exposed pad of the package. Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions. Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 6 is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A Typical Application Circuit L1 4.7µH RT8553A VIN 2.5V to 4.5V 12 CIN1 10µF CIN2 0.1µF 7 VINA LX1 LX2 VOUT1 6 Enable VINP L2 4.7µH 2 R1 800k 4 3 PGND FB2 AGND VOUT1 COUT1 5V 10µF R2 200k GND VOUT2 5 11 EN FB1 13 (Exposed Pad) 1 VREF 10 9 R3 100k 8 R4 20k VOUT2 COUT2 -5V 10µF Timing Diagram Power Sequence TSS1 < 2ms Toff_dly > 300us TSS2 < 2ms VIN 0 EN 0 5V VOUT1 (V) 0 0 0 0 VOUT2 (V) -5V tEN_Dly < 400us Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 7 RT8553A Typical Operating Characteristics Efficiency vs. Output Current VOUT1 vs. Output Current 100 5.004 95 4.999 85 4.994 VIN = 2.9V VIN = 3.7V VIN = 4.5V 80 75 VOUT1 (V) Efficiency (%) 90 70 4.989 VIN = 2.9V VIN = 3.7V VIN = 4.5V 4.984 65 60 4.979 55 VIN = 2.9V to 4.5V, VOUT1 = 5V, VOUT2 = −5V VIN = 2.9V to 4.5V, VOUT1 = 5V 50 4.974 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 10 20 Output Current (A) VOUT2 vs. Output Current -4.995 5.000 -5.000 4.995 -5.005 VIN = 2.9V VIN = 3.7V VIN = 4.5V -5.015 60 70 80 90 100 4.990 4.985 4.980 -5.020 4.975 VIN = 2.9V to 4.5V, VOUT2 = −5V -5.025 VOUT1 = 5V, IOUT1 = 100mA 4.970 0 10 20 30 40 50 60 70 80 90 100 2.9 3.1 3.3 3.5 Output Current (mA) 3.7 3.9 4.1 4.3 4.5 Input Voltage (V) Quiescent Current vs. Temperature VOUT2 vs. Input Voltage 2.0 -4.990 1.8 Quiescent Current (mA) -4.985 -4.995 VOUT2 (V) 50 VOUT1 vs. Input Voltage 5.005 VOUT1 (V) VOUT2 (V) 40 Output Current (mA) -4.990 -5.010 30 -5.000 -5.005 -5.010 VIN = 4.5V VIN = 3.7V VIN = 2.9V 1.6 1.4 1.2 1.0 0.8 -5.015 VOUT2 = −5V, IOUT2 = 100mA -5.020 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 Input Voltage (V) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 8 4.5 VIN = 2.9V to 4.5V, LX1/LX2 not switching 0.6 -50 -25 0 25 50 75 100 125 Temperature (°C) is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A VIN = 3.7V, VOUT2 = −5V, IOUT2 = 100mA LX2 Switching LX1 Switching VOUT2 (10mV/Div) VOUT1 (10mV/Div) VLX2 (3V/Div) VLX1 (2V/Div) ILX2 (200mA/Div) ILX1 (100mA/Div) VIN = 3.7V, VOUT1 = 5V, IOUT1 = 100mA Time (500ns/Div) Time (500ns/Div) VOUT1 Load Transient VOUT1 Line Transient VOUT1 (10mV/Div) VIN (500mV/Div) VOUT1 (10mV/Div) I LOAD (50mA/Div) I LOAD (50mA/Div) VIN = 3.7V, VOUT1 = 5V, TR = TF = 150μs, IOUT1 = 10mA to 100mA VIN = 2.9V to 3.4V, VOUT1 = 5V, IOUT1 = 100mA Time (100μs/Div) Time (500μs/Div) VOUT2 Load Transient VOUT2 Line Transient VOUT2 (10mV/Div) VIN (500mV/Div) VOUT2 (10mV/Div) I LOAD (50mA/Div) I LOAD (10mA/Div) VIN = 3.7V, VOUT2 = −5V, TR = TF = 150μs, IOUT2 = 3mA to 30mA Time (100μs/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 VIN = 2.9V to 3.4V, VOUT2 = −5V, IOUT2 = 100mA Time (500μs/Div) is a registered trademark of Richtek Technology Corporation. www.richtek.com 9 RT8553A Power On Sequence EN (2V/Div) EN (2V/Div) VOUT1 (2V/Div) VOUT2 (2V/Div) VOUT1 (2V/Div) VIN = 3.7V, VOUT1 = 5V, VOUT2 = −5V, No Load I IN (200mA/Div) VOUT2 (2V/Div) I IN (200mA/Div) Time (2ms/Div) Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 10 Power Off Sequence VIN = 3.7V, VOUT1 = 5V, VOUT2 = −5V, No Load Time (2ms/Div) is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A Application Information The RT8553A is a dual channel DC/DC converter capable of generating both positive and negative outputs by external feedback voltage-divider resistors from an input voltage range of 2.5V to 4.5V. Each converter works independently of one another and both outputs are separately controlled by a fixed frequency Pulse-Width-Modulated (PWM) regulator. Soft-Start The RT8553A employs a soft-start feature for both converters to limit the inrush current and prevent input voltage droop. When each converter is enabled, the implemented switch current limit ramps up slowly to its nominal programmed value in about 2ms for the Boost converter and Buck-Boost converter. Under-Voltage Lockout (UVLO) The Under-Voltage Lockout (UVLO) circuitry compares the input voltage with the UVLO threshold to ensure that the input voltage is high enough for reliable operation. Once the input voltage exceeds the UVLO rising threshold at 2.22V (typ.), start-up begins. A 40mV (typ.) hysteresis is included to prevent supply transients from causing a shutdown. Positive Output Voltage Setting The output voltage setting can be calculated according to the following equation : VOUT1 = VFB1 1 R1 R2 where VFB1 is the reference voltage with a typical value of 1V. Over-Voltage Protection (OVP) The RT8553A has over-voltage circuit protection mechanism which prevents feedback pin floating when the IC is enabled. When output voltage exceeds OVP threshold voltage, the IC would be clamped at fixed voltage with minimum duty. Over-Current Protection (OCP) The RT8553A includes a current sensing circuitry which monitors the inductor current during each charging cycle. If the current value becomes greater than the current limit, the converter will be shutdown and can only re-start normal operation after triggering EN pin or re-power on again. Over-Temperature Protection (OTP) The RT8553A includes an Over-Temperature Protection (OTP) feature to prevent the device from overheat due to excessive power dissipation. The OTP function shuts down all device operations when the junction temperature exceeds 140°C. Once the junction temperature cools down by approximately 15°C, the converter resumes operation. To maintain continuous operation, prevent the maximum junction temperature form rising above 125°C. Input Capacitor Selection Input ceramic capacitors with 10μF capacitance are suggested for the RT8553A applications. However, to achieve best performance with the RT8553A, larger capacitance can be used. For better voltage filtering, select ceramic capacitors with low ESR, X5R and X7R types are suitable because of their wider voltage and temperature ranges. Boost Inductor Selection The inductance depends on the maximum input current. As a general rule, the inductor ripple current range is 20% to 40% of the maximum input current. If 40% is selected as an example, the inductor ripple current can be calculated according to the following equations : VOUT IOUT(MAX) VIN IL = 0.4 IIN(MAX) IIN(MAX) = where η is the efficiency of the converter, IIN(MAX) is the maximum input current, and ΔIL is the inductor ripple current. The input peak current can then be obtained by adding the maximum input current with half of the inductor ripple current as shown in the following equation : Ipeak = 1.2 x IIN(MAX) Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 11 RT8553A Ipeak Note that the saturated current of the inductor must be greater than Ipeak. ΔIL The inductance can eventually be determined according to the following equation : Input Current Inductor Current VIN (VOUT VIN ) 2 L= 0.4 VOUT IOUT(MAX) fOSC 2 Output Current where fOSC is the switching frequency. For better system performance, a shielded inductor is preferred to avoid EMI problems. Time (1-D)TS Output Ripple Voltage (ac) Boost Output Capacitor Selection The output ripple voltage is an important index for estimating chip performance. This portion consists of two parts. One is the product of the inductor peak current with the ESR of the output capacitor, while the other part is formed by the charging and discharging process of the output capacitor. As shown in Figure 1, ΔVOUT1 can be evaluated based on the ideal energy equalization. According to the definition of Q, the Q value can be calculated as the following equation : Q = 1 IIN + 1 IL IOUT + IIN 1 IL IOUT 2 2 2 V IN 1 = COUT VOUT1 VOUT fOSC where fOSC is the switching frequency and ΔIL is the inductor ripple current. Bring COUT to the left side to estimate the value of ΔVOUT1 according to the following equation : D IOUT VOUT1 = COUT fOSC where D is the duty cycle and η is the Boost converter efficiency. Finally, taking ESR into consideration, the overall output ripple voltage can be determined by the following equation : D IOUT VOUT1 = VESR + C OUT fOSC where VESR = IC RC_ESR = Ipeak RC_ESR Time ΔVOUT1 Figure 1. The Output Ripple Voltage without the Contribution of ESR Negative Output Voltage Setting The output voltage setting can be calculated according to the following equation : VOUT2 = VFB2 VREF VFB2 R3 R4 where VREF is the reference voltage with a typical value of 1V and VFB2 = 0V. Buck-Boost Converter Inductor Selection The first step in the design procedure is to verify whether the maximum possible output current of the Buck-Boost converter supports the specific application requirements. To simplify the calculation, the fastest approach is to estimate converter efficiency by taking the efficiency numbers from provided efficiency curves or to use a worst case assumption for the expected efficiency, e.g., 80%. The calculation must be performed for the minimum assumed input voltage where the peak switch current is the highest. The inductor and internal switch have to be able to handle this current. Converter Duty Cycle : The output capacitor, COUT, should be selected accordingly. D= Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 12 VOUT VIN + VOUT is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A Maximum output current : Thermal Considerations VIN(MIN) D IOUT(MAX) = ILX2 1 D 2 fOSC(MAX) L For continuous operation, do not exceed the maximum operation junction temperature 125°C. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature differential between junction to ambient. The maximum power dissipation can be calculated by following formula : Inductor peak current : Ipeak = IOUT VIN D + 1 D 2 fOSC L where, ILX2 is switching current limit. As for inductance, we are going to derive the transition point, where the converter toggles from CCM to DCM. We need to define the point at which the inductor current ripple touches zero, and as the power switch LX is immediately reactivated, the current ramps up again. Figure 2 portrays the input current activity of the Buck-Boost converter. IIN Ipeak SON VIN L PD(MAX) = (TJ(MAX) − TA) / θJA where T J(MAX) is the maximum operation junction temperature 125°C, TA is the ambient temperature and the θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125°C. The junction to ambient thermal resistance, θJA, is layout dependent. For UDFN-12L 3x3 package, the thermal resistance θJA is 32.3°C/W on the standard JEDEC 51-7 four-layers thermal test board. The maximum power dissipation at TA = 25°C can be calculated by following formula : IIN,AVG t DTSW TSW Figure 2. The Buck-Boost input signature in BCM The inductance can eventually be determined according to the following equation : D VOUT + VESR fOSC RLOAD COUT where VESR = IC RC_ESR = Ipeak RC_ESR PD(MAX) = ( 125°C − 25°C) / 32.3°C/W = 3.09W for UDFN-12L 3x3 package The maximum power dissipation depends on the operating ambient temperature for fixed T J(MAX) and thermal resistance, θJA. The derating curve in Figure 3 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation. ΔVESR can be neglected in many cases since ceramic capacitors provide very low ESR. Maximum Power Dissipation (W)1 V = 3.5 Four-Layer PCB 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 25 50 75 100 125 Ambient Temperature (°C) Figure 3. Derating Curve of Maximum Power Dissipation Copyright © 2015 Richtek Technology Corporation. All rights reserved. DS8553A-00 January 2015 is a registered trademark of Richtek Technology Corporation. www.richtek.com 13 RT8553A Layout Consideration For good regulation, place the power components as close to the IC as possible. The traces should be wide and short especially for the high current output loop. The feedback voltage-divider resistors must be placed near the Feedback pin. The divider center trace must be shorter and avoid the trace near any switching nodes. The input and output bypass capacitor should be placed as close to the IC as possible and connected to the ground plane of the PCB. Separate power ground (PGND) and analog ground (AGND). Connect the AGND and the PGND islands at a single end. Connect the exposed pad to a strong ground plane for maximum thermal dissipation. For the best performance of RT8553A the following PCB layout guidelines should be strictly followed. Minimize the size of the LX1, LX2 nodes and keep the traces wide and short. Care should be taken to avoid running traces that carry any noise-sensitive signals near LX or high-current traces. VIN L1 CIN1 VOUT1 COUT1 R1 LX1 1 12 VINP VOUT1 2 11 LX1 PGND 3 L2 10 VOUT2 GND FB1 4 9 FB2 AGND 5 8 VREF EN 6 7 COUT2 R2 VINA R4 CIN2 R3 VOUT2 Figure 4. PCB Layout Guide Copyright © 2015 Richtek Technology Corporation. All rights reserved. www.richtek.com 14 is a registered trademark of Richtek Technology Corporation. DS8553A-00 January 2015 RT8553A Outline Dimension 2 1 2 1 DETAIL A Pin #1 ID and Tie Bar Mark Options Note : The configuration of the Pin #1 identifier is optional, but must be located within the zone indicated. Symbol Dimensions In Millimeters Dimensions In Inches Min. Max. Min. Max. A 0.500 0.600 0.020 0.024 A1 0.000 0.050 0.000 0.002 A3 0.100 0.175 0.004 0.007 b 0.150 0.250 0.006 0.010 D 2.950 3.050 0.116 0.120 D2 2.300 2.650 0.091 0.104 E 2.950 3.050 0.116 0.120 E2 1.400 1.750 0.055 0.069 e L 0.450 0.350 0.018 0.450 0.014 0.018 U-Type 12L DFN 3x3 Package Richtek Technology Corporation 14F, No. 8, Tai Yuen 1st Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries. DS8553A-00 January 2015 www.richtek.com 15