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Datasheet MAX1647 - Chemistry-Independent Battery Chargers - Maxim Integrated Products

Chemistry-Independent

Battery Chargers

internal current sources’ state, and the six most signifi-The PWM comparator compares the current-sensecant bits control the switching regulator’s current. Theamplifier’s output to the higher output voltage of eitherinternal current source supplies 1mA resolution to thethe GMV or the GMI amplifier (the error voltage). Thisbattery to comply with the smart-battery specification.current-mode feedback corrects the duty ratio of theWhen the current is set to a number greater than 32,switched voltage, regulating the peak battery currentthe internal current source remains at 31mA. This guar-and keeping it proportional to the error voltage. Sinceantees that battery-current setting is monotonic regard-the average battery current is nearly the same as theless of current-sense resistor choice and current-sensepeak current, the controller acts as a transconductanceamplifier offset.

amplifier, reducing the effect of the inductor on the out-The GMI amplifier’s noninverting input is driven by a 4:1put filter LC formed by the output inductor and the bat-resistive voltage divider, which is driven by the 6-bittery’s parasitic capacitance. This makes stabilizing theDAC. If an external 4.096V reference is used, this inputcircuit easy, since the output filter changes from a com-is approximately 1.0V at full scale, and the resolution isplex second-order RLC to a first-order RC. To preserve16mV. The current-sense amplifier drives the invertingthe inner current-control loop’s stability, slope compen-input to the GMI amplifier. It measures the voltagesation is also fed into the comparator. This damps outacross the current-sense resistor (RSEN) (which isperturbations in the pulse width at duty ratios greaterbetween the CS and BATT pins), amplifies it by approx-than 50%.

imately 5.45, and level shifts it to ground. The full-scaleAt heavy loads, the PWM controller switches at a fixedcurrent is approximately 0.2V / RSEN, and the resolutionfrequency and modulates the duty cycle to control theis 3.2mV / RSEN.

battery voltage or current. At light loads, the DC currentThe current-regulation-loop is compensated by addingthrough the inductor is not sufficient to prevent the cur-a capacitor to the CCI pin. This capacitor sets the cur-rent from going negative through the synchronous recti-rent-feedback loop’s dominant pole. The GMI amplifier’sfier (Figure 3, M2). The controller monitors the currentoutput is clamped to between approximately one-fourththrough the sense resistor RSEN; when it drops to zero,and three-fourths of the REF voltage. While the current isthe synchronous rectifier turns off to prevent negativein regulation, the CCV voltage is clamped to withincurrent flow.

80mV of the CCI voltage. This prevents the battery volt-age from overshooting when the DAC voltage setting isMOSFET Drivers

updated. The converse is true when the voltage is inThe MAX1647 drives external N-channel MOSFETs toregulation and the current is not at the current DAC set-regulate battery voltage or current. Since the high-sideting. Since the linear range of CCI or CCV is about 1.5VN-channel MOSFET’s gate must be driven to a voltageto 3.5V or about 2V, the 80mV clamp results in a rela-higher than the input source voltage, a charge pump istively negligible overshoot when the loop switches fromused to generate such a voltage. The capacitor C7voltage to current regulation or vice versa.

(Figure 3) charges to approximately 5V through D2when the synchronous rectifier turns on. Since one sidePWM Controller

of C7 is connected to the LX pin (the source of M1), theThe battery voltage or current is controlled by the cur-high-side driver (DHI) can drive the gate up to the volt-rent-mode, pulse-width-modulated (PWM), DC-DC con-age at BST, which is greater than the input voltage,verter controller. This controller drives two externalwhen the high-side MOSFET turns on.

N-channel MOSFETs, which switch the voltage from theThe synchronous rectifier behaves like a diode, but withinput source. This switched voltage feeds an inductor,a smaller voltage drop to improve efficiency. A smallwhich filters the switched rectangular wave. The con-dead time is added between the time that the high-sidetroller sets the pulse width of the switched voltage so thatMOSFET turns off and the synchronous rectifier turnsit supplies the desired voltage or current to the battery.on, and vice versa. This prevents crowbar currents (cur-The heart of the PWM controller is the multi-input com-rents that flow through both MOSFETS during the briefparator. This comparator sums three input signals totime that one is turning on and the other is turning off).determine the pulse width of the switched signal, set-Connect a Schottky rectifier from ground to LX (acrossting the battery voltage or current. The three signals arethe source and drain of M2) to prevent the synchronousthe current-sense amplifier’s output, the GMV or GMIrectifier’s body diode from conducting. The body diodeerror amplifier’s output, and a slope-compensation sig-typically has slower switching-recovery times, so allow-nal, which ensures that the controller’s internal current-ing it to conduct would degrade efficiency.

control loop is stable.

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MAX1647/MAX1648