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Design Example 3, SLA Charging with LTC4121

Consider the design of a sealed lead acid charger with temperature compensation of the float voltage. Sealed Lead Acid batteries require the float voltage be decreased as cell temperature rises. With the LTC4121 this is achieved using an NTC thermistor in the feedback pin divider as shown in the figure above. Using the circuit above, the float voltage automatically decreases with temperature as shown in the graph below. The NTC pin is grounded in this example to disable NTC qualified charging and highlight the float voltage programming over a wide temperature range. With an NTC pin network connected, as in example 1 or example 4, the charger would be disabled below 0°C or above 40°C. The sealed lead acid charger above is configured to charge from a variable supply that can range from 6.2V up to 40V. The switch frequency is selected at 750kHz to meet minimum on time requirements at VBAT = 4.2V. And a 47μH switch inductor is selected to keep ripple current below 30% of ICHG at VIN = 40V.

Sealed Lead Acid Float Voltage