MOSFET Drivers

The MAX745 drives external N-channel MOSFETs to

switch the input source generating the battery voltage or

current. Since the high-side N-channel MOSFET’s gate

must be driven to a voltage higher than the input source

voltage, a charge pump is used to generate such a volt-

age. The capacitor (C7) charges through D2 to approxi-

mately 5V when the synchronous rectifier (M1B) turns on

(Figure 1). Since one side of C7 is connected to LX (the

source of M1A), the high-side driver (DHI) drives the gate

up to the voltage at BST, which is greater than the input

voltage while the high-side MOSFET is on.

The synchronous rectifier (M1B) behaves like a diode but

has a smaller voltage drop, improving efficiency. A small

dead time is added between the time when the high-side

MOSFET is turned off and when the synchronous rectifier

is turned on, and vice versa. This prevents crowbar cur-

rents during switching transitions. Place a Schottky rectifier

from LX to ground (D1, across M1B’s drain and source) to

prevent the synchronous rectifier’s body diode from con-

ducting during the dead time. The body diode typically has

slower switching-recovery times, so allowing it to conduct

degrades efficiency. D1 can be omitted if efficiency is not

a concern, but the resulting increased power dissipation in

the synchronous rectifier must be considered.

Since the BST capacitor is charged while the synchro-

nous rectifier is on, the synchronous rectifier may not be

replaced by a rectifier. The BST capacitor will not fully

charge without the synchronous rectifier, leaving the

high-side MOSFET with insufficient gate drive to turn on.

However, the synchronous rectifier can be replaced with

a small MOSFET (such as a 2N7002) to guarantee that

the BST capacitor is allowed to charge. In this case, the

majority of the high charging currents are carried by D1,

and not by the synchronous rectifier.

Internal Regulator and Reference

The MAX745 uses an internal low-dropout linear regula-

tor to create a 5.4V power supply (VL), which powers its

internal circuitry. The VL regulator can supply up to 25mA.

Since 4mA of this current powers the internal circuitry,

the remaining 21mA can be used for external circuitry.

MOSFET gate-drive current comes from VL, which must

be considered when drawing current for other functions.

To estimate the current required to drive the MOSFETs,

multiply the sum of the MOSFET gate charges by the

switching frequency (typically 300kHz). Bypass VL with a

4.7µF capacitor to ensure stability.

The MAX745 internal 4.2V reference voltage must be

bypassed with a 0.1µF or greater capacitor.

Minimum Input Voltage

The input voltage to the charger circuit must be greater

than the maximum battery voltage by approximately 2V

so the charger can regulate the voltage properly. The

input voltage can have a large AC-ripple component when

operating from a wall cube. The voltage at the low point

of the ripple waveform must still be approximately 2V

greater than the maximum battery voltage.

Using components as indicated in Figure 1, the minimum

input voltage can be determined by the following formula:

BATT

D6

CHG

DS(ON)

L

IN

V

V

I

(R

R

R1

V

0.89



+

+

+

+



×

(typically 0.4V to 0.5V);

MOSFET M1A’s on-resistance;

R1 is the current-sense resistor R1’s value.

MAX745

Switch-Mode Lithium-Ion

Battery-Charger

www.maximintegrated.com

Maxim Integrated │ 8

Chip Information

TRANSISTOR COUNT: 1695

SUBSTRATE CONNECTED TO GND

18

17

16

15

14

13

19

20

1

2

3

4

5

6

7

8

TOP VIEW

12

11

9

10

BST

LX

DHI

DLO

PGND

CS

BATT

STATUS

CELL0

CELL1

CCV

VL

DCIN

IBAT

VADJ

REF

THM/SHDN

CCI

GND

SETI

SSOP

MAX745

Pin Configuration