Detailed Description

The MAX38889 is a flexible storage capacitor or capacitor bank backup regulator transferring power efficiently between a storage element and a system supply rail.

When the main supply is present and its voltage is above the minimum threshold system supply voltage, the regulator operates in charging mode and charges the storage element with a maximum 3A peak, 1.5A average inductor current. Once the storage element is charged, the circuit draws only 4µA of current while maintaining the storage element in its ready state. The supercapacitor needs to be fully charged to enable backup operation.

When the main supply is removed, the regulator prevents the system from dropping below the set system backup operating voltage, boosting the supercapacitor voltage to a regulated V_SYS by discharging the storage element with a maximum 3A peak inductor current. During this backup mode of operation, the MAX38889 utilizes an adaptive on-time, current-limited, pulse-frequency-modulation (PFM) control scheme. Once the MAX38889 is in backup mode, the BKB flag is low.

The external pins allow various settings such as maximum supercapacitor voltage, system backup voltage (V_SYS), and peak inductor charge and discharge current.

The MAX38889 implements a True Shutdown^TM feature, disconnecting SYS from CAP as well as protecting against a SYS short if V_CAP > V_SYS.

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.

Supercapacitor Voltage Configuration

The maximum supercapacitor voltage (V_{CAP_MAX}) during charging is determined by the resistor-divider driving the FBCH pin. When V_FBCH 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:

$R 1 + R 2 = R 3 \times (\frac{V_{C A P_M A X}}{0.5 v} - 1)$

Selecting V_{CAP_MAX} = 2.7V and R3 = 499kΩ:

_{R1 + R2} _{≈ 2.2}MΩ

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 (R_T) is:

R_T = R1 + R2 = R3 = 2.2MΩ + 499kΩ ≈ 2.7MΩ

RDY will go high when V_FBCRis greater than 0.5V.

Assuming we want RDY to go high when V_CAP reaches 1.5V:

V_{CAP_RDY}= 1.5V

R3 = 499kΩ

$R 2 = R_{T} \times (\frac{0.5 V}{V_{C A P_R D Y}}) - R 3 = 2.7 M Ω \times (\frac{0.5 V}{1.5 V}) - 499 k Ω = 401 k Ω$

Selecting R2 = 402kΩ:

R1 = R_T - R2 – R3 = 2.7MΩ – 402kΩ – 499kΩ = 1.8MΩ

Select R2 = 1.82MΩ for fine-tuning on the bench.

During backup, the supercapacitor voltage discharges and the boost converter automatically adjusts its duty cycle to regulate V_SYS. When V_SYS drops below UVLO threshold (V_UVLOF) or V_FBCH drops to 10% of the FBCH threshold voltage (_{VTH_FBCH}), the boost regulator stops delivering load current and the supercapacitor voltage is preserved with only the current from the resistor-divider discharging the supercapacitor further.

System Voltage Configuration

The backup system voltage is determined by a resistor-divider driving the FBS pin. Set the system backup voltage using a resistor-divider from SYS to FBS to GND. When V_FBS is above 1.23V, the DC-DC regulator draws power from the main battery through the SYS pin to charge the supercapacitor to the maximum voltage set by FBCH and be ready for backup. The peak charging current can be programmed to up to 3A, max. When the main battery is removed and V_FBS drops to 1.2V, the DC-DC regulator draws power from the supercapacitor and regulates the SYS pin to the programmed backup voltage with the programmed peak inductor current, which is a maximum of 3A.

In order to reduce the current flowing through the resistor-divider, select high-value resistors.

To set system the backup voltage to 3.0V, select the bottom resistor (R7) to 1.21MΩ. The top resistor value can be calculated by the following equation:

$R 6 = (3.0 \times \frac{R 7}{1.2}) - R 7$

$R 6 = (3.0 \times \frac{1.21 M Ω}{1.2}) - 1.21 M Ω = 1.815 M Ω$

Select R6 = 1.82MΩ for fine-tuning on the bench.

Charge/DischargeCurrent Configuration

The MAX38889 current configuration pin, ISET, gives the user the ability to optimize the efficiency around the maximum load requirements of the system during backup. A larger current setting allows the supercapacitor to be discharged to a lower voltage for a given load during backup.

A single resistor sets both the charging and backup current.

Charging

Supercapacitor charging current and discharging current is determined by the same resistor, R_ISET. The buck regulator operates in forced discontinuous conduction mode (DCM) until the supercapacitor gets fully charged.

Average charging current (I_{CAP_CHG}) is determined by:

$I_{C A P_C H G} = 1.5 A \times (\frac{33 k Ω}{R_{I S E T}})$

where R_ISET can be selected from 100kΩ to 33kΩ, and I_{CAP_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 (I_{SYS_CHG}) will be:

$I_{S Y S_C H G} = 1.5 A \times (\frac{33 k Ω}{R_{I S E T}}) \times (\frac{V_{C A P}}{V_{S Y S}}) \times (\frac{1}{c h a r g i n g e f f i c i e n c y})$

Peak charging current

$(I_{L X_C H G}) = 3 A \times (\frac{33 k Ω}{R_{I S E T}})$

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 (I_{LX_BU}) to:

$I_{L X_B U} = 3 A \times (\frac{33 k Ω}{R_{I S E T}})$

where R_ISET can be selected from 100kΩ to 33kΩ, and I_{LX_BU}varies from 1A to 3A, respectively.

For a given system load in backup (I_{SYS_BU}), the minimum supercapacitor voltage (V_{CAP_MIN}) needed to support the load can be calculated as:

$V_{C A P_M I N} = V_{S Y S_M I N} \times [\frac{I_{S Y S_B U}}{I_{L X_B U} - (0.5 \times d l_{L X})}] \times (\frac{1}{b a c k u p e f f i c i e n c y})$

Approximate a ripple current of 2A and 1A system load at 3.0V. Assuming an efficiency of 75%:

$V_{C A P_M I N} = 3 V \times [\frac{1 A}{3 A - (0.5 \times 2)}] \times (\frac{1}{75 %}) = 2 V$