SI9731DQ VISHAY [Vishay Siliconix], SI9731DQ Datasheet - Page 8

• Manufacturer Part Number: SI9731DQ
• Description: UP Controlled Battery Charger For 1-Cell Li-ion or 1-Cell to 3-Cell NiCd/NiMH Batteries
• Manufacturer: VISHAY [Vishay Siliconix]

Si9731
Vishay Siliconix
DETAIL OPERATIONAL DESCRIPTION
Si9731 is a chemistry independent battery charger designed
for use with a system processor. For example, Si9731 can be
integrated within a cellular phone whereby the fast charge
and trickle (slow) charge modes can be software controlled
by the DSP. The device is designed to charge 1-cell Li-ion or
1-cell to 3-cell NiCd/NiMH batteries. A regulated or unregu-
lated external dc power source such as a wall adapter rated
at typically 4.5 V to 12 V is connected to Si9731’s V
input pin. Note that a typical low cost wall adaptor is com-
prised of a transformer, bridge rectifier and a reservoir
capacitor. The wall adaptor’s output voltage decreases lin-
early with increase in output current. When Si9731 is fast
charging the battery, the wall adaptor’s output voltage tracks
the battery voltage plus the voltage drop across Q1 (charging
current times MOSFET Q1’s r
features of Si9731 are described below.
Since the under voltage lock out (UVLO) point of Si9731 is
2.6 V (typical), it is essential to keep the charger voltage
above this level under all conditions, especially for fast
charging of single cell NiCd/NiMH. One simple solution is
adding a external resistor between V
which creates extra voltage drop to elevate the charger volt-
age. The value of the resistor is affected by the output V-I
characteristic of the ac charger.
Trickle Charge
The charge path is via N-Channel MOSFETs Q1 or Q2 (see
applications circuit of Figure 1). Si9731 defaults to trickle
(slow) charge mode if the battery voltage is too low to power
the main processor. With the main processor unable to drive
the MAINCHARGEEN pin as well as the TRICKLECHAR-
GEEN pins, Q1 is turned "OFF" preventing fast charging.
Meanwhile N-Channel MOSFET Q2 turns "ON" and estab-
lishes a trickle charge path from the external power source
V
an external current limiting resistor, R
mately I
voltage charges up to minimum battery operating voltage
3.4 V,
CHARGERPOWER_ON output changes state to wake up
the processor. The processor is now able to disable trickle
charge mode by taking the TRICKLECHARGEEN pin high
while taking control of fast charging via the MAINCHAR-
GEEN pin.
www.vishay.com
8
CHARGER
TRICKLE
the
to the battery. The trickle charge current is set by
internal
= (V
CHARGER
latch
- V
DS(on)
BAT+
is
, see Figure 1). The key
BAT+
)/R
triggered
ext
ext
pin and the battery,
, and is approxi-
. Once the battery
and
CHARGER
the
Fast Charge
Fast charging is accomplished by the low "ON" resistance
MOSFET, Q1. The application microprocessor is able to
"Pulse Charge" the battery via the MAINCHARGEREN con-
trol input of Si9731. The processor monitors the battery volt-
age via the system A/D converter and varies the pulse
charging duty cycle accordingly to maintain fast charging.
Note that even though charging current may be sufficiently
high, pulse charging with short "ON" time and long "OFF"
time ensures that heat generation due to thermal heating is
reduced.
In the case of NiCd or NiMH batteries, one of several charge
termination schemes may be used to terminate charge. For
example, the processor may disable fast charging by sens-
ing ΔV or dV/dt at the V
perature differential ΔT of the battery. Following fast charge,
trickle charge may be enabled to "top off" the battery.
When charging a 1-cell Li-ion battery, fast charging will oper-
ate in two modes, constant current mode followed by con-
stant voltage mode. In the constant current mode, a
discharged Li-ion battery is charged with constant current
available from the external dc source. The MOSFET pass
transistor (Q1) may be pulsed "ON" and "OFF" at varying
duty cycle by the control signal present at the MAINCHAR-
GEEN input pin. Once the battery voltage reaches it’s termi-
nation voltage of 4.1 V or 4.2 V (depending on the connection
of the 4.1 V
age" charging mode by taking the CVMODE pin high. Taking
CVMODE pin high disables trickle charging and enables the
internal battery voltage divider by turning ON Q5. Then the
error amplifier will compare divided V
internal precision 1.3 V bandgap reference voltage (see Fig-
ure 1). The output of the error amplifier drives the pass tran-
sistor Q1 to maintain V
voltage. This operation is same as a linear regulator.
True Load Disconnect
Both the fast charge FET (Q1 in Figure 1) and trickle charge
FET (Q2 in Figure 1) incorporate a floating body diode. In
their "OFF" state both FETs block current bidirectionally.
Note that because of the reverse blocking switches, a Schot-
tky diode in series with the external V
is not required.
TAP
), Si9731 may be placed in the "Constant Volt-
BAT+
BAT+
output or by monitoring the tem-
at the regulated termination
S-60869-Rev. B, 22-May-06
Document Number: 71321
BAT+
CHARGER
voltage against an
power supply