MAX745 Maxim, MAX745 Datasheet - Page 8

• Manufacturer Part Number: MAX745
• Description: Switch-Mode Lithium-Ion Battery-Charger
• Manufacturer: Maxim

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 synchro-
nous rectifier is turned on, and vice versa. This
prevents crowbar currents 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 conducting 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.
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 cir-
cuitry, the remaining 21mA can be used for external cir-
cuitry. 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 4.7
The MAX745 internal 4.2V reference voltage must be
bypassed with a 0.1
Switch-Mode Lithium-Ion
Battery Charger
8
_______________________________________________________________________________________
Internal Regulator and Reference
µ
µ
F or greater capacitor.
F capacitor to ensure stability.
MOSFET Drivers
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:
where: V
TRANSISTOR COUNT: 1695
SUBSTRATE CONNECTED TO GND
___________________Chip Information
__________________Pin Configuration
TOP VIEW
V
IN
x
V
(typically 0.4V to 0.5V);
I
R
MOSFET M1A’s on-resistance;
R
R1 is the current-sense resistor R1’s value.
CHG
[V
IN
D6
DS(ON)
L
THM/SHDN
is the the inductor’s series resistance;
BATT
is the input voltage;
is the voltage drop across D6
is the charging current;
VADJ
DCIN
IBAT
CCV
SETI
GND
REF
CCI
VL
+ V
is the high-side
10
1
2
3
4
5
6
7
8
9
D6
+ I
MAX745
SSOP
Minimum Input Voltage
0.89
CHG
( R
DS(ON)
20
19
18
17
16
15
14
13
12
11
BST
LX
DHI
DLO
PGND
CS
BATT
STATUS
CELL0
CELL1
+ R
L
+ R1)]