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MAX846AC

Cost-Saving Multichemistry Battery-Charger System

Maxim Integrated 

Cost-Saving Multichemistry

Battery-Charger System

Voltage/Current Regulator

The voltage/current regulator consists of a precision

attenuator, voltage loop, current-sense amplifier, and

current loop. The attenuator can be pin programmed to

set the regulation voltage for one or two Li-Ion cells

(4.2V and 8.4V, respectively). The current-sense ampli-

fier is configured to sense the battery current on the

high side. It is, in essence, a transconductance amplifi-

er converting the voltage across an external sense

resistor (RCS) to a current, and applying this current to

an external load resistor (RISET). Set the charge current

by selecting RCS and RISET. The charge current can

also be adjusted by varying the voltage at the low side

of RISET or by summing/subtracting current from the

ISET node (Figure 5). The voltage and current loops are

individually compensated using external capacitors at

CCV and CCI, respectively. The outputs of these two

loops are OR’ed together and drive an open-drain,

internal N-channel MOSFET transistor sinking current to

ground. An external P-channel MOSFET or PNP transis-

tor pass element completes the loop.

Stability

The Typical Operating Characteristics show the loop

gains for the current loop and voltage loop. The domi-

nant pole for each loop is set by the compensation

capacitor connected to each capacitive compensation

pin (CCI, CCV). The DC loop gains are about 50dB for

the current loop and about 33dB for the voltage loop,

for a battery impedance of 250mΩ.

The CCI output impedance (50kΩ) and the CCI capaci-

tor determine the current-loop dominant pole. In Figure

2, the recommended CCCV is 10nF, which places a

dominant pole at 300Hz. There is a high-frequency

pole, due to the external PNP, at approximately fT/ß.

This pole frequency (on the order of a few hundred kilo-

hertz) will vary with the type of PNP used. Connect a

10nF capacitor between the base and emitter of the

PNP to prevent self-oscillation (due to the high-imped-

ance base drive).

Similarly, the CCV output impedance (150kΩ) and the

CCV capacitor set the voltage-loop dominant pole. In

Figure 2, the compensation capacitance is 10nF, which

places a dominant pole at 200Hz.

The battery impedance directly affects the voltage-loop

DC and high-frequency gain. At DC, the loop gain is

proportional to the battery resistance. At higher fre-

quencies, the AC impedance of the battery and its con-

nections introduces an additional high-frequency zero.

A 4.7µF output capacitor in parallel with the battery,

mounted close to BATT, minimizes the impact of this

impedance. The effect of the battery impedance on DC

gain is noticeable in the Voltage-Loop-Gain graph (see

Typical Operating Characteristics). The solid line repre-

sents voltage-loop gain versus frequency for a fully

charged battery, when the battery energy level is high

and the ESR is low. The charging current is 100mA. The

dashed line shows the loop gain with a 200mA charg-

ing current, a lower amount of stored energy in the bat-

tery, and a higher battery ESR.

__________Applications Information

Stand-Alone Li-Ion Charger

Figure 2 shows the stand-alone configuration of the

MAX846A. Select the external components and pin

configurations as follows:

• Program the number of cells: Connect CELL2 to GND

for one-cell operation, or to VL for two-cell operation.

• Program the float voltage: Connect a 1% resistor from

VSET to GND to adjust the float voltage down, or to

VL to adjust it up. If VSET is unconnected, the float

voltage will be 4.2V per cell. Let the desired float volt-

age per cell be VF, and calculate the resistor value

as follows:

Table 1. Float-Voltage Accuracy

Internal-reference accuracy

ERROR SOURCE

VSET error due to external divider. Calculated from a 2% internal 20kΩ resistor tolerance and

a 1% external RVSET resistor tolerance. The total error is 3% x (adjustment). Assume max

adjustment range of 5%.

VSET amplifier and divider accuracy

TOTAL

ERROR

±0.5%

±0.15%

±0.25%

±0.9%

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