Battery-Based ICs Serve and Protect

Apr 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

The LX1741 from Microsemi Integrated Products is a compact, high-efficiency step-up boost controller that employs an external power MOSFET. It provides several design enhancements that improve overall performance under very light load currents by implementing control circuits that are optimized for portable systems. It provides a quiescent supply current of only 80µA (typical) and a shutdown current of less than 1µA. Fig. 1 shows a typical application.

The input voltage ranges from 1.6V to 6V, which allows a wide selection of system battery voltages. Start-up operation guarantees a 1.6V input.

The LX1741 is a pulse frequency modulated (PFM) boost converter optimized for large step-up voltage applications. It operates in a pseudo-hysteretic mode with a fixed switch “off time” of 300 ns. When the feedback voltage (V_FB) falls below the 1.29V reference, or VADJ, it enables converter switching. When this occurs, a comparator activates the off-time controller. The off-time controller and current limiter circuit activate another comparator, which toggles the NDRV output circuit. The NDRV output switches on (and remains on) until the inductor current ramps up to the peak current level. External resistor R_CS sets this current level; the CS and SRC inputs monitor the current level.

Energy stored in the output capacitor during the inductor charging cycle powers the load. Upon achieving the peak inductor current, the NDRV output turns off (off-time is typically 300 ns), delivering a portion of the energy stored in the inductor to the load. The output voltage continues to rise at the input to the feedback circuit. If the voltage at the FB input is still less than 1.29V at the end of the off-time period, the NDRV output switches the external FET on and the inductor charging cycle repeats until V_FB is greater than the internal reference.

Application of an external voltage source at the ADJ pin allows output voltage adjustment over a typical range of about ±15%. The designer can vary the reference voltage directly at the ADJ pin by applying a dc voltage of 0.9V to 1.5V. A second option is to connect a PWM logic signal to the ADJ pin. The LX1741 includes an internal 50pF capacitor to ground that works with an external resistor to create a low pass filter for the ac component of a PWM input ≥100 kHz.

The adjustment voltage level is selectable (with limited accuracy) by implementing the voltage divider created between the external series resistor and the internal 2.5MΩ resistor. If the dc voltage at the ADJ pin drops below 0.6V, the device will revert to the internal reference voltage level of 1.29V. You can disable the LX 1741 by driving the SHDN pin with a low-level logic signal, which reduces the device power consumption to less than 1µA.

Selecting the appropriate values for R1 and R2 in the voltage divider connected to the feedback pin programs the output voltage. Using a value of 49.9K for R2 works well in most applications.

Setting the level of peak inductor current to about two times the expected maximum dc input current will minimize the inductor size, the input ripple current, and the output ripple voltage. The designer should use inductors that will not saturate at the peak inductor current. A typical inductor value is 47 µH. A lower value choice emphasizes peak current overshoot, while choosing a higher value emphasizes output ripple voltage.

The device is available in both the 8-pin MSOP, and miniature 8-pin MLP. In 10K quantities, the LX1741 is priced at 66 cents in the MLP package and at 67 cents in the MSOP package.

Li-Ion Charge Management Controller

The MCP73826 from Microchip Technology is a linear charge management controller that provides the preferred charge algorithm for Li-ion cells. The SOT-23 IC can operate with a host microcontroller or in stand-alone applications. Fig. 2, on page 62, depicts a typical stand-alone application circuit.

One of the MCP73826's three charging phases is preconditioning. If the battery voltage is below the internal low-voltage threshold, a foldback current preconditions the battery — protecting the Li-ion cell and minimizing heat dissipation.

Following the preconditioning phase, the device enters the controlled current phase with a programmable charge current set by an external sense resistor. The charge current ramps up, based on the cell voltage from the foldback current to the peak charge current established by the sense resistor. The IC maintains this phase until the battery reaches the charge-regulation voltage.

In the final constant voltage phase, the voltage regulation accuracy is better than ±1% over the entire operating temperature range and supply voltage range.

Because the MCP73826 is a linear charger with relatively low efficiency, the most important factors are thermal design and cost, which depend on the input voltage, output current, and thermal impedance between the external P-channel pass transistor, Q1, and the ambient air. The worst-case situation is when the shorted output and the P-channel pass transistor must dissipate maximum power. This requires a trade-off between the charge current, cost, and thermal requirements of the charger.

The preferred fast charge current for Li-ion cells is the 1C rate with an absolute maximum current at the 2C rate. For example, a 500 mAH battery pack has a preferred fast charge current of 500 mA. Charging at this rate provides the shortest charge cycle times without degradation to the battery pack performance or life.

One of the external components that is crucial to the integrity and reliability of the charging system is the current sense resistor (R_SENSE). For a 500 mAH battery pack, a 100mΩ, 1% resistor provides a typical peak fast charge current of 530mA and a maximum peak fast charge current of 758mA. Worst-case power dissipation in the sense resistor is 57.5mW.

A larger value sense resistor decreases the peak fast charge current and power dissipation in both the sense resistor and external pass transistor, but increases charge cycle times.

The gate-to-source threshold voltage, input voltage, output voltage, and peak fast charge current determine selection of the external P-channel MOSFET that must satisfy the thermal and electrical design requirements. The worst-case power dissipation in the MOSFET occurs when the input voltage is at its maximum, and the output is shorted. Power dissipation for a 5V, ±10% input voltage source, 100mΩ 1% sense resistor, and foldback current scale factor of 0.43 is 1.8W. A Fairchild NDS8434 or an International Rectifier IRF7404 mounted on a 1 in.² pad of 2 oz. copper would limit the junction temperature rise to about 90°C. This allows a maximum operating ambient temperature of 60°C. You can realize a higher ambient temperature by increasing the copper pad size, or lowering the sense resistor value.

You must consider the gate-to-source threshold voltage and R_DS(on) of the external P-channel MOSFET in the design phase. The worst case V_GS provided by the controller occurs when the input voltage is minimum and the charge current is at its maximum. The worst-case V_GS, with a 5V, ±10% input voltage source, 100mΩ sense resistor, and 1.6V maximum sink voltage is — 2.8V. At this worst-case V_GS, the MOSFET's R_DS(on) must be low enough so it doesn't affect the charging system performance. The maximum allowable R_DS(on) at the worst-case V_GS is 242mΩ.

The MCP73826 is stable with or without a battery load. To maintain good ac stability in the constant voltage mode, place a 10 µF capacitor from the V_BAT pin to GND. This provides compensation when there's no battery load. In addition, the battery and interconnections, which are in the control feedback loop during the constant voltage mode, appear inductive at high frequencies. Therefore, the bypass capacitor may be necessary to compensate for their inductive nature.

Virtually any good quality output filter capacitor can be used, independent of the capacitor's minimum ESR. The actual value of the capacitor and its associated ESR depends on the forward transconductance, gm, and capacitance of the external pass transitor. A 10µF tantalum or aluminum electrolytic capacitor at the output is usually sufficient to ensure stability for up to a 1A output.

The MCP73826 operates with an input voltage range from 4.5V to 5.5V. The MCP73826 is fully specified over the ambient temperature range of -20°C to +85°C.

Li-Ion Protection IC

The JV257 from JVD Inc. is a single chip Li-ion rechargeable battery protection IC that incorporates high-accuracy over-charge, over-discharge, and over-current protection circuits, as well as a low R_DS(on) control MOSFET. The MOSFET circuit supports latch-up free operation under all operating conditions. This IC has circuits that remove many of the inherent errors in CMOS analog measurements. It's connected to the application circuit or board through parallel solder bumps or wirebonds that minimize the on resistance.

Fig. 3 shows a typical application circuit for the JV257. To control charging and discharging of the battery it measures the voltage from V_dd to V_ss and V_det to V_ss. If the V_dd pin is between the discharge detection voltage and the over-charge detection voltage, and the V_det pin is below the over-current detection voltage and above the abnormal charge current voltage, its internal power MOSFET turns on. Charging and discharging can normally occur in this condition.

The IC handles two over-current conditions. One is over-current1 when the discharging current exceeds the over-current1 detection voltage. The other is over-current2 that detects larger over-currents faster, i.e., when the charger reverses and goes to +12V max.

JVD Inc., San Jose, Calif.
CIRCLE 346 on Reader Service Card
Microsemi Integrated Products, Garden Grove, Calif.
CIRCLE 347 on Reader Service Card
Microchip Technology, Chandler, Ariz.
CIRCLE 348 on Reader Service Card

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