MAX1647 Datasheet, PDF (22/24 Pages) Maxim Integrated Products – Chemistry-Independent Battery Chargers

Chemistry-Independent
Battery Chargers
IF (TEMPWORD OR 0xBEFF) = 0xFFFF THEN GOTO
HAVEBATT:
GOTO ENDINT:
HAVEBATT:
{ A battery is installed. Turn the battery’s broadcast
mode off to monitor the charging process. Using the
BatteryMode( ) command, make sure the CHARGER_
MODE bit is set. }
WriteWord(SMBADDR = 0b00010110 = 0x16,
COMMAND = 0X03, DATA = 0x4000)
GOTO ENDINT:
NOBATT:
{ Notify the system that AC power is present, but no bat-
tery is present. }
GOTO ENDINT:
ENDINT:
{ This is the end of the interrupt routine. }
The following pseudo-code describes a polling routine
that queries the battery for its desired charge voltage and
charge current, checks to make sure that the requested
charge current and charge voltage are valid, and
instructs the MAX1647 to comply with the request.
DOPOLLING:
{ This is the beginning of the polling routine. }
{ Ask the battery what voltage it wants using the bat-
tery’s ChargingVoltage( ) command. }
TEMPVOLTAGE = ReadWord( SMBADDR =
0b00010111 = 0x17, COMMAND = 0x15 )
{ Ask the battery what current it wants using the bat-
tery’s ChargingCurrent( ) command. }
TEMPCURRENT = ReadWord( SMBADDR =
0b00010111 = 0x17, COMMAND = 0x14 )
{ Now the routine can check that the TEMPVOLTAGE
and TEMPCURRENT values make sense and that the
battery is not malfunctioning. }
{ With valid TEMPVOLTAGE and TEMPCURRENT val-
ues, instruct the MAX1647 to comply with the request. }
WriteWord( SMBADDR = 0b00010010 = 0x12 ,
COMMAND = 0x15, DATA = TEMPVOLTAGE )
WriteWord( SMBADDR = 0b00010010 = 0x12 ,
COMMAND = 0x14, DATA = TEMPCURRENT )
ENDPOL:
{ This is the end of the polling routine. }
Negative Input Voltage Protection
In most portable equipment, the DC power to charge
batteries enters via a two-conductor cylindrical power
jack. It is easy for the end user to add an adapter to
switch the DC power’s polarity. Polarized capacitor C6
would be destroyed if a negative voltage were applied.
Diode D4 in Figure 3 prevents this from happening.
If reverse-polarity protection for the DC input power is
not necessary, diode D4 can be omitted. This eliminates
the power lost due to the voltage drop on diode D4.
Selecting External Components for the
MAX1647 4A Application
The MAX1647 can be configured to charge at a maxi-
mum current of 4A (instead of 2A, as shown in Figure 3)
by changing the external power components and tying
SEL to REF. The following paragraphs discuss the selec-
tion requirements for each component in Figure 3 that
must be changed to accommodate the 4A application.
Diode D4 in Figure 3 has to support both the charge
current and the current required to operate the host
load (i.e., what the batteries normally power when not
charging). This means that the continuous current flow-
ing through D4 exceeds 4A. One possible choice for
D4 is the Motorola MBRD835L 8A Schottky barrier
diode in a DPAK surface-mount package. Care must
be taken in thermal management of the circuit board
when using the 4A application circuit, by mounting D4
on a three-square-inch piece of copper.
Motorola’s MBRD835L can also be used for D3. The
Siliconix Si4410DY is a good choice for M1 and M2 in the
4A application. Changing M2 from a 2N7002 (Table 1) to
a Si4410DY increases the power dissipated by the
MAX1647’s 20-pin SSOP.
High-current inductors are difficult to find in surface-mount
packages. Low-cost solutions use toroidal powdered-iron
cores with exposed windings of heavy-gauge wire. The
Coiltronics CTX20-5-52 20µH 5A inductor provides a high-
efficiency solution.
R1A must also dissipate more power in the 4A applica-
tion circuit than in the circuit of Figure 3. R1A’s value
decreases to 50mΩ in the 4A application. IRC’s
LR2512-01-R050-F meets this requirement with a 1W
maximum power-dissipation rating.
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