Texas Instruments BQ24450EVM Functions and Features, Changing Output Voltage and Charge Current

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5 Functions and Features

5.1Changing Output Voltage and Charge Current

This EVM is configured to charge a 3-Cell Lead Acid Battery with a maximum charge current of 450 mA. The EVM can be altered to charge lower or higher voltage batteries and can also be altered to have a lower or higher maximum charge current.

Power Dissipation

It is important to monitor the power dissipation in the external transistor, Q1.

WARNING

Q1 and the surrounding area of the PCB may become hot.

(Power Dissipation in Q1) = (Vin – Vout – 0.25 – 0.7) × Charge Current

With a large difference between Vin and Vbat, and/or a high charge current, there will be a significant amount of power dissipation in Q1. This causes both Q1 and the surrounding PCB to get hot. If any alterations are made to the EVM circuit, it is important to ensure a safe level of power dissipation in the external FET, Q1.

Changing Output Voltage

The output voltage may be altered by changing some on-board resistors (R7, R8, R9, R11, R12). See the bq24450 datasheet for information on how to size these resistors. If Vout is changed, Vin must also be changed. Vin should be high enough to allow for the necessary dropout (across R2, R4, Q1, and D1) and low enough to maintain a safe level of power dissipation.

Changing the Maximum Charge Current

The maximum charge current is set by sense resistor, R2. (Max Charge Current) = (0.25) / R2. Resistor R2 may be adjusted to set a different max charge current. However, more current will yield higher power dissipation in Q1. This may cause the board to become hot.

Changing Pre-Charge Current

Pre-Charge current is set by R10.

Pre-Charge Current = (Vin – 2 – Vout) / R10

Changing the value of Itaper

As populated, ITAPER is 10% of IMax-Chg. This is because VILIM is 250mV and VISNS is 25mV and both are being sensed across R2. If a different value of ITAPER is desired, R4 can be populated (with a non-zero resistor) and JP1 and JP2 can be used to connect pins ISNSM and IFB to opposite sides of R4. See figure below.

Figure 3. Calculating the Value of ITAPER

SLUU464 –November 2010	bq24450EVM	5
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