·For Li-Ion:

The voltage divider network for Li-Ion is very important. If the battery voltage is scaled too low, the battery will not attain its full capacity when charged, and if scaled too high, the battery may become damaged. Never exceed the recommended maximum voltage or current for a Li-Ion battery!

The dimensioning is done in the following manner.

First calculate the maximum battery voltage for the specific battery pack. See example below.

BatteryVoltage/Cell = 3.6V NumberOfCells = 2 → Battery-

PackVoltage = 3.6x2 = 7.2V

MaximumBatteryVoltage/Cell = 4.1V → MaximumBattery-

Voltage = 4.1x2 = 8.2V

When the maximum battery voltage has been determined, the voltage divider network has to be dimensioned using the following formula:

The LM3647 has two different regulation voltages, which the user can select. These are 2.675V (SEL3 tied to GND) and 2.740V (SEL3 tied to VCC). This selection pin can be used to configure the charger to regulate for different input voltages so that the charger can handle both 3.6V- and 3.7V-cells, without changing the resistor values in the divider network. SEL3 can also be used if there is problem in finding the right values in the resistor network. The recommended tolerance of the resistors are 0.1%, but 1% may be used with a marginal loss of battery capacity by subtracting the tolerance of the divider network from the maximum battery voltage.

·Using the LM3647 without current feedback, for Ni-Cd/Ni-MH only (slow PWM mode):

This mode uses an external constant-current power-source, which is switched on and off according to the charge-phase of the LM3647. The frequency is approximately 0.1 Hz. The advantage of this charge method is that operational amplifiers and the current feedback circuitry are not needed, which provides a low-cost solution. The dimensioning of the voltage divider network is performed the same way. The constant current source is dimensioned in the following manner:

Charge Phase:

Duty Cycle:

 

 

Soft Start

10%

 

 

Fast Charge

100%

 

 

Topping Charge

10%

 

 

Maintenance Charge

5%

 

 

AN-1164

AN101315-7

The LM3647 regulates the constant current source by turning the transistor Q1 on and off.

When the transistor is off, the LM317T regulator feeds a constant current to the battery (at V_OUT).

When the transistor is on, the output from the LM317 is limited to 1.25V (which should be greater than the battery voltage).

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National Instruments LM3647 user manual Charge Phase Duty Cycle
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LM3647 specifications

The National Instruments LM3647 is a versatile and advanced power management integrated circuit designed specifically for applications in mobile and portable devices. This device provides an efficient power supply solution that caters to the demands of today's sophisticated electronics.

One of the main features of the LM3647 is its ability to deliver a highly accurate output voltage. With a low output voltage ripple, it ensures that sensitive components receive stable power, thereby enhancing the performance and reliability of the system. The device typically uses a constant on-time control scheme, which allows for rapid transient response and improves overall efficiency, especially during dynamic load changes common in portable devices.

The LM3647 offers an impressive input voltage range, typically from 2.5V to 5.5V, making it compatible with a wide range of power sources, including lithium-ion batteries and USB power. Additionally, the device is highly efficient, boasting peak efficiencies of over 95%. This efficiency translates to extended battery life, a crucial factor for portable electronics.

Another significant characteristic of the LM3647 is its integrated power path management capability. This feature allows it to intelligently manage power distribution between the battery and the load, ensuring seamless transitions between battery and wall power, and providing system protection from overloads and short circuits.

The LM3647 also supports a flexible output configuration, featuring multiple outputs that can be independently programmed. This flexibility is particularly beneficial in applications such as smartphones, tablets, and wearables that require multiple voltage rails for various components, including processors, displays, and sensors.

Moreover, the LM3647 incorporates advanced thermal management techniques. With a package designed to dissipate heat efficiently, it helps maintain operational stability and longevity of the device even under heavy load conditions.

In summary, the National Instruments LM3647 stands out as a high-performance power management IC leveraging cutting-edge technologies to provide efficient power solutions for mobile and portable devices. Its features, including high efficiency, accurate output voltage, integrated power path management, and flexible output configurations, combined with robust thermal characteristics, make it an excellent choice for designers aiming for optimal power management in their electronic devices.
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