Datasheet MAX38889 (Maxim) - 10

link to page 10 link to page 10 MAX38889 2.5V to 5.5V, 3A Reversible Buck/Boost Regulator for Backup Applications The MAX38889 implements a True ShutdownTM feature, disconnecting SYS from CAP as well as protecting against a SYS short if VCAP > VSYS. Charging and backup can be disabled by keeping the ENC and ENB pins low, respectively. The backup system status can be monitored through two status outputs: the RDY flag that indicates when the supercapacitor is charged and the BKB flag that indicates when backup operation is occurring.
Application Circuits
The typical application of the MAX38889 is shown in Figure 1. CHARGE DISCHARGE 3A (MAX) 3A (MAX) L1 0.47µH VCAP VSYS 2.7V (MAX) 3.0V (BACKUP) CAP LX SYS C3 CSC 10F C1 R1 1µF R6 R8 R9 C5 C6 MAIN SUPER SYS 22µF 1.82M 1.82MΩ 1MΩ 1MΩ 2x47µF 2x22µF BATTERY CAP LOAD (REMOVABLE) FBCR FBS C2
MAX38889
R4 R2 0.47µF 20kΩ 402kΩ BKB BACKUP FBCH RDY READY CHARGE ENABLE ENC ENB ISET GND PGND C4 2.2nF R3 RISET R7 BACKUP ENABLE 499kΩ 33kΩ 1.21MΩ Figure 1. Application Circuit
Supercapacitor Voltage Configuration
The maximum supercapacitor voltage (VCAP_MAX) during charging is determined by the resistor-divider driving the FBCH pin. When VFBCH reaches 0.5V, further charging of the supercapacitor is halted. The threshold has a 2.5% (12.5mV) hysteresis. To keep the ready power drawn low, keep the quiescent current in the resistor-divider low by setting the resistor from FBCH to GND to 500kΩ. The resistance from CAP to FBCH (R1 + R2) can then be determined by the standard equation:   C V AP _MAX R1+ R2 = R3   −1  0.5V    Selecting VCAP_MAX = 2.7V and R3 = 499kΩ: R1 + R2 ≈ 2.2MΩ The RDY pin is a flag that goes high when the supercapacitor has crossed a user-defined voltage. Like the FBCH input, the FBCR pin is driven from CAP through a resistor-divider. To save quiescent current, the FBCR and FBCH resistor- dividers can be combined using a three-resistor-divider string, as shown in Figure 1. Total resistance from CAP to GND (RT) is: RT = R1 + R2 = R3 = 2.2MΩ + 499kΩ ≈ 2.7MΩ www.maximintegrated.com Maxim Integrated | 10 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