MAX1898 Ver la hoja de datos (PDF) - Maxim Integrated

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MAX1898

Linear Charger for Single-Cell Li+ Battery

Maxim Integrated 

Linear Charger for Single-Cell Li+ Battery

Selecting External Components

The MAX1898 drives an external transistor to control

charging current. The most important specifications for the

pass transistor (MOSFET or bipolar) are current rating and

package power dissipation. Since the MAX1898 operates

as a linear charger, heat will dissipate in the external tran-

sistor. The worst-case power dissipation will be:

PDISS(MAX) = IFASTCHG (VIN - 2.5V)

The pass transistor and heatsinking (typically an expand-

ed PC board plane) must be rated for PDISS(MAX).

The pass device will most commonly be a P-channel

MOSFET. Its operating voltage must exceed the

expected input voltage, but MOSFET on-resistance is

important only in low-dropout designs where VIN is only

slightly higher than the battery regulation voltage. In

most charger designs, a fairly low-cost FET (around

100mΩ to 200mΩ RDS(ON)) performs more than ade-

quately. Lower resistance FETs perform well also, but

are not worth the added cost in this case.

A PNP bipolar transistor may also be used as a pass

device. DRV can sink up to 4mA, so reasonable high

current gain (hFE or Beta) is required to allow DRV to

drive the base of a bipolar pass transistor. For a 500mA

fast-charge current, an hFE of 125 is required. Although

their current gain is high, Darlington PNP transistors are

not recommended due to stability limitations.

If VIN is less than VBATT, the body diode inherent in the

external enhancement-mode FET allows current to flow

from the cell back to the source. To prevent this, add a

Schottky diode between the drain and BATT, as shown

in Figure 4.

Chip Information

TRANSISTOR COUNT: 1816

PROCESS: BiCMOS

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