Datasheet MAX38889 (Maxim) - 13

MAX38889 2.5V to 5.5V, 3A Reversible Buck/Boost Regulator for Backup Applications Average charging current (ICAP_CHG) is determined by:  33kΩ  =  C I AP_CHG 1.5A    IS R ET  where RISET can be selected from 100kΩ to 33kΩ, and ICAP_CHG varies from 0.5A to 1.5A, respectively. Since charging current is efficiently drawn through a buck regulator, the average charging current from SYS (ISYS_CHG) will be:  33kΩ      C V AP 1 =        S I YS _ CHG 1.5A    IS R ET   S V YS   charging efficiency (  33kΩ  Peak charging current I =  LX_CHG ) 3A    IS R ET  When the supercapacitor voltage reaches its maximum voltage as determined by the FBCH threshold, the forced DCM of the charging stops. After this, only pulse charging takes place.
Backup
During backup, the boost regulator regulates the system voltage to the set the backup voltage, thus limiting the peak inductor current (ILX_BU) to:  33kΩ  I =  LX _ BU 3A    I R SET  where RISET can be selected from 100kΩ to 33kΩ, and ILX_BU varies from 1A to 3A, respectively. For a given system load in backup (ISYS_BU), the minimum supercapacitor voltage (VCAP_MIN) needed to support the load can be calculated as:  I  SYS _ BU  1  =     C V AP _MIN VSYS _MIN   I  −   LX_BU (0.5 d L l X ) backup efficiency Approximate a ripple current of 2A and 1A system load at 3.0V. Assuming an efficiency of 75%:  1A   1  =     = C V AP _MIN 3V  −  (  ) 2V   3A 0.5 2   75% 
Applications Information Capacitor Selection
Capacitors at the SYS and CAP pins reduce current peaks and increase efficiency. Ceramic capacitors are recommended because they have the lowest equivalent series resistance (ESR), smallest size, and lowest cost. Choose an acceptable dielectric such as X5R where the ambient temperature is less than +85°C or X7R where the ambient temperature is less than +125°C. Due to the ceramic capacitor’s capacitance derating at higher DC bias voltages, 2x47μF and 2x22μF ceramic capacitors are recommended at the SYS side and a 22μF ceramic capacitor at the CAP side for most applications. In addition, it is recommended to place a high-frequency filter capacitor of 1μF across the SYS and PGND pins as mentioned in the PCB Layout Guidelines.
Supercapacitor Selection
When the power source supplying the VSYS voltage is removed, power to the output is provided by the MAX38889 operating in the backup or boost mode of operation using the supercapacitor as its source. In order to ensure that the supply voltage stays in regulation, the amount of power the supercapacitor can deliver at its minimal voltage should be greater than that required by the system. The MAX38889 presents a constant power load to the supercapacitor where smaller current is pulled out of the supercapacitor near its maximum VCAP voltage. However, current drawn from the www.maximintegrated.com Maxim Integrated | 13 Document Outline Product Highlights Key Applications Simplified Application Diagram Pin Configuration Absolute Maximum Ratings Package Information Electrical Characteristics Typical Operating Characteristics Pin Configuration Pin Descriptions Functional Diagram Detailed Description Application Circuits Supercapacitor Voltage Configuration System Voltage Configuration Charge/Discharge Current Configuration Charging Backup Applications Information Capacitor Selection Supercapacitor Selection Inductor Selection Status Flags BKB Flag RDY Flag Enabling the Device Charger Enable Input (ENC) Backup Enable Input (ENB) PCB Layout Guidelines Ordering Information