General Description

The MAX38888 is a storage capacitor or capacitor bank backup regulator designed to efficiently transfer power between a storage element and a system supply rail in reversible buck and boost operations using the same inductor.

When the main supply is present and above the minimum system supply voltage, the regulator operates in buck mode and charges the storage element at up to 500mA peak inductor current. Once the storage element is charged, the circuit draws only 2.5µA of current while it maintains the supercapacitor or other storage element in its ready state. When the main supply is removed, the regulator operates in boost mode and prevents the system from dropping below the minimum operating voltage, discharging the storage element at up to 2.5A peak inductor current.

The MAX38888 is externally programmable for minimum and maximum voltage of the storage element, such as supercapacitor, minimum system voltage, and maximum charge and discharge currents. The internal DC-DC converter requires only a 1µH inductor.

 

 Applications

  • Handheld Industrial Equipment
  • Portable Computers
  • Portable Devices with a Removable Battery

 

Benefits and Features

  • 2.5V to 5V System Output Voltage
  • 0.8V to 4.5V Cap Voltage Range
  • Up to 2.5A Peak Inductor Discharge Current
  • Programmable Voltage and Current Thresholds
  • ±2% Threshold Accuracy
  • Up to 95% Efficiency, Charge or Discharge
  • 2.5μA Ready Quiescent Current
  • Small Solution Size
  • 3mm x 3mm x 0.75mm TDFN Package

 

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 or X7R. Due to the ceramic capacitors’ capacitance derating with DC bias, standard 22μF ceramic capacitors are recommended at both pins for most applications.

Supercapacitor Selection When the power source supplying the VSYS voltage is removed, power to the output is provided by the MAX38888 operating in the back-up or boost mode of operation using the supercapacitor as its source. In order to ensure 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 MAX38888 will present a constant power load to the supercapacitor where smaller current will be pulled out of the supercapacitor near its maximum VCAP voltage. However, current drawn from the supercapacitor will increase as it discharges to maintain constant power at the load. The amount of energy required in the backup mode will be the product of the constant backup power and time defined as backup time, tBACKUP. The amount of energy available in the supercapacitor is calculated using the following formula: E=1 / 2 ×CSCAP×(VCAPMAX 2−VCAPMIN 2)(J) The amount of energy required to complete the backup equals: E=VSYS×ISYS×tBACKUP)(J) where, ISYS will be the system load during backup. Since the energy required at the system side during the backup event comes from the available energy in the supercapacitor, assuming conversion efficiency η and given tBACKUP, the required CSCAP will be determined by the following equation: CSCAP =(2xVSYS × ISYS × tBACKUP) www.maximintegrated.com / [(VCAPMAX 2– VCAPMIN 2) × η](F)

Inductor Selection

The MAX38888 works with a 1μH inductor in most applications. In applications where lower peak currents are desired, larger inductance may be used in order to reduce the ripple. The recommended inductance range is from 1μH to 4.7μH. Select 4.7μH for higher RISET value (100kΩ). 1μH is not supported for a 100kΩ RISET value.

Status Flags

The MAX38888 has two dedicated pins to report the device status to the host processor. The ready output (RDY) will be high when the supercapacitor is fully charged (i.e., FBCH > 0.5V). RDY is pulled low when FBCL < 0.475V. The other status flag is the Backup Output (BKUPB), which will be held low when the part is in the backup mode (i.e., when FBS < 0.5V and FBCL > 0.5V). BKUPB is released high when FBCL < 0.475V or FBS > 0.56V. Both output pins are open-drain type and require external pullup resistors. Recommended values for the pullup resistors are 1MΩ. The pins should be pulled up to the SYS rail.