MAX1647 Maxim, MAX1647 Datasheet - Page 16

• Manufacturer Part Number: MAX1647
• Description: Chemistry-Independent Battery Chargers
• Manufacturer: Maxim

The synchronous rectifier may not be completely
replaced by a diode because the BST capacitor
charges while the synchronous rectifier is turned on.
Without the synchronous rectifier, the BST capacitor
may not fully charge, leaving the high-side MOSFET
with insufficient gate drive to turn on. However, the syn-
chronous rectifier may be replaced with a small MOS-
FET, such as a 2N7002, to guarantee that the BST
capacitor is allowed to charge. In this case, most of the
current at high currents is carried by the diode and not
by the synchronous rectifier.
The MAX1647 uses an internal low-dropout linear regula-
tor to create a 5.4V power supply (VL), which powers its
internal circuitry. VL can supply up to 20mA. A portion of
this current powers the internal circuitry, but the remain-
ing current can power the external circuitry. The current
used to drive the MOSFETs comes from this supply,
which must be considered when calculating how much
power can be drawn. To estimate the current required to
drive the MOSFETs, multiply the total gate charge of
each MOSFET by the switching frequency (typically
250kHz). The internal circuitry requires as much as 6mA
from the VL supply. To ensure VL stability, bypass the VL
pin with a 1µF or greater capacitor.
The MAX1647 has an internal ±2% accurate 3.9V refer-
ence voltage. An external reference can be used to
increase the charger’s accuracy. Use a 4.096V reference,
such as the MAX874, for compliance with the Intel/
Duracell smart-battery specification. Voltage-setting
accuracy is ±0.65%, so the total voltage accuracy is the
accuracy added to the reference accuracy. For 1% total
voltage accuracy, use a reference with ±0.35% or greater
accuracy. If the internal reference is used, bypass it with
a 0.1µF or greater capacitor.
The MAX1647 uses serial data to control its operation. The
serial interface complies with the SMBus specification (see
System Management Bus Specification , from Intel
Architecture Labs; http://www.intel.com/IAL/power-
mgm.html; Intel Architecture Labs: 800-253-3696).
Charger functionality complies with the Intel/Duracell
Smart Charger Specification for a level 2 charger.
The MAX1647 uses the SMBus Read-Word and Write-
Word protocols to communicate with the battery it is
charging, as well as with any host system that monitors
the battery to charger communications. The MAX1647
never initiates communication on the bus; it only
receives commands and responds to queries for status
information. Figure 7 shows examples of the SMBus
Write-Word and Read-Word protocols.
Chemistry-Independent
Battery Chargers
16
______________________________________________________________________________________
Internal Regulator and Reference
MAX1647 Logic
Figure 7. Write-Word and Read-Word Examples
CMD0
CMD1
CMD2
CMD3
CMD4
CMD5
CMD6
CMD7
START
ACK
ACK
ACK
ACK
D10
D11
D12
D13
D14
D15
D8
D9
D0
D1
D2
D3
D4
D5
D6
D7
W
1
0
0
1
0
0
0
START
ACK
ACK
BOLD LINE INDICATES THAT
THE MAX1647 PULLS SDA LOW
ChargingMode( ) = 0 x 12
ChargingVoltage( ) = 0 x 15
ChargingCurrent( ) = 0 x 14
AlarmWarning( ) = 0 x 16
ChargerStatus( ) = 0 x 13
W
1
1
0
0
1
0
0
0
1
0
0
1
0
0
0
THERMISTOR_COLD
CHARGE_INHIBITED
BATTERY_PRESENT
CURRENT_NOTREG
VOLTAGE_NOTREG
THERMISTOR_HOT
ALARM_INHIBITED
THERMISTOR_OR
THERMISTOR_UR
MASTER_MODE
CURRENT_OR
AC_PRESENT
VOLTAGE_OR
POWER_FAIL
REPEATED
LEVEL_2
LEVEL_3
START
ACK
ACK
ACK
R
1
0
0
1
0
0
0