OPERATION
The SGM4054 is a single cell lithium-ion battery charger
using a constant-current/constant-voltage algorithm. It can
deliver up to 800mA of charge current (using a good
thermal PCB layout) with a final float voltage accuracy of
±1%. The SGM4054 includes an internal P-channel power
MOSFET and thermal regulation circuitry. No blocking
diode or external current sense resistor is required; thus,
the basic charger circuit requires only two external
components. Furthermore, the SGM4054 is capable of
operating from a USB power source.
Charge Termination
A charge cycle is terminated when the charge current falls
to 1/10th the programmed value after the final float voltage
is reached. This condition is detected by using an internal,
filtered comparator to monitor the PROG pin. When the
1
PROG pin voltage falls below 100mV for longer than tTERM
(typically 1ms), charging is terminated. The charge current
is latched off and the SGM4054 enters standby mode,
where the input supply current drops to 200µA. (Note:
C/10 termination is disabled in trickle charging and
thermal limiting modes).
Normal Charge Cycle
When charging, transient loads on the BAT pin can cause
the PROG pin to fall below 100mV for short periods of time
before the DC charge current has dropped to 1/10th the
programmed value. The 1ms filter time (tTERM) on the
termination comparator ensures that transient loads of this
nature do not result in premature charge cycle termination.
Once the average charge current drops below 1/10th the
programmed value, the SGM4054 terminates the charge
cycle and ceases to provide any current through the BAT
pin. In this state, all loads on the BAT pin must be supplied
by the battery.
A charge cycle begins when the voltage at the V pin rises
+
above the UVLO threshold level and a 1% program resistor
is connected from the PROG pin to ground or when a
battery is connected to the charger output. If the BAT pin is
less than 2.9V, the charger enters trickle charge mode. In
this mode, the SGM4054 supplies approximately 1/10 the
programmed charge current to bring the battery voltage up
to a safe level for full current charging. (Note: The
SGM4054 does not include this trickle charge feature).
When the BAT pin voltage rises above 2.9V, the charger
enters constant-current mode, where the programmed
charge current is supplied to the battery. When the BAT pin
approaches the final float voltage (4.2V), the SGM4054
enters constant-voltage mode and the charge current begins
to decrease. When the charge current drops to 1/10 of the
programmed value, the charge cycle ends.
The SGM4054 constantly monitors the BAT pin voltage in
standby mode. If this voltage drops below the 4.05V
recharge threshold (VRECHRG), another charge cycle begins
and current is once again supplied to the battery. To
manually restart a charge cycle when in standby mode, the
input voltage must be removed and reapplied, or the
charger must be shut down and restarted using the PROG
pin. Figure 1 shows the state diagram of a typical charge
cycle.
Programming Charge Current
The charge current is programmed using a single resistor
from the PROG pin to ground. The battery charge current is
1000 times the current out of the PROG pin. The program
resistor and the charge current are calculated using the
following equations:
Charge Status Indicator (
)
CHRG
The charge status output has three different states: strong
pull-down (~10mA), weak pull-down (~20µA) and high
impedance. The strong pull-down state indicates that the
SGM4054 is in a charge cycle. Once the charge cycle has
terminated, the pin state is determined by undervoltage
1000V 1000V
, ICHG =
R
PROG
=
ICHG
RPROG
lockout conditions. A weak pull-down indicates that V
+
meets the UVLO conditions and the SGM4054 is ready to
charge. High impedance indicates that the SGM4054 is in
The charge current out of the BAT pin can be determined at
any time by monitoring the PROG pin voltage using the
following equation:
undervoltage lockout mode: either V
above the BAT pin voltage or insufficient voltage is applied
to the V pin. A microprocessor can be used to distinguish
+
is less than 100mV
VPROG
+
R
PROG
=
·1000
RPROG
between these three states—this method is discussed in the
Applications Information section.
7
SGM4054
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