Stand-Alone, Switch-Mode

Li+ Battery Charger with Internal 14V Switch

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9

General Description

The MAX1757 includes all of the functions necessary to

charge 1, 2, or 3 Li+ battery cells in series. It includes a

step-down DC-DC converter that controls charging

voltage and current. It also includes input source cur-

rent limiting, battery temperature monitoring, battery

undervoltage precharging, battery fault indication, and

a state machine with timers for charge termination.

The DC-DC converter uses an internal power MOSFET

switch to convert the input voltage to the charging cur-

rent or voltage. Figure 1 shows the typical application

circuit. Figure 2 shows a typical charging sequence

and Figure 3 shows the functional diagram. The charg-

ing current is set by the voltage at ISETOUT. The bat-

tery regulation voltage is measured at the BATT pin.

The battery voltage limit is set to 4.2V per cell and can

be adjusted ±5% by changing the voltage at the VADJ

pin. By limiting the adjust range, the voltage limit accu-

racy is better than 1% while using 1% setting resistors.

27

26

1

22

10

9

20

19

21

18

D4

MBR5340

8

4

28

CSSP

DCIN

CSSN

VL

BST

HSD

HSD

LX

LX

PGND

CS

BATT

THM

REF

ISETIN

TO VL

SHDN

ISETOUT

VADJ

CELL

GND

CCV

CCI

CCS

TIMER1

TIMER2

R1

0.05

Ω

L1

22

µH

FAULT

FULLCHG

FULL CHARGE

FAST CHARGE

FAULT

FASTCHG

C2

0.1

µF

C5

1nF

C6

1nF

C1

0.1

µF

C3

1

µF

C12

0.22

µF

C10

22

µF

TO

SYSTEM

LOAD

INPUT

SUPPLY

C7

0.1

µF

C8

0.1

µF

R4

D2

D3

D1

R5

C4

0.1

µF

R6

10k

15

24

25

6

11

3

7

2

5

17

23

12

13

16

14

C15

68

µF

C9

0.1

µF

C13

4.7

µF

C14

0.1

µF

C16

0.1

µF

MAX1757

+

C11

22

µF

+

+

C17

1nF

MBR5340

C18

0.1

µF

THERM

Li+ BATTERY

1 TO 3 CELLS

Figure 1. Typical Application Circuit