EUP8084
is forced to drop below 10% of the full-scale current by
UVCL, STAT will stay in the strong pulldown state.
Battery Charger Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the
input voltage and keeps the battery charger off until ADP
rises above 3.6V and approximately 110mV above the
BAT pin voltage. The 3.6V UVLO circuit has a built-in
hysteresis of approximately 0.6V, and the 110mV
automatic shutdown threshold has a built-in hysteresis of
approximately 65mV. During undervoltage lockout
conditions, maximum battery drain current is 5µA and
maximum supply current is 10µA.
Charge Current Soft-Start and Soft-Stop
The EUP8084’s battery charger includes a soft-start
circuit to minimize the inrush current at the start of a
charge cycle. When a charge cycle is initiated, the charge
current ramps from zero to full-scale current over a
period of approximately 120µs. Likewise, internal
circuitry slowly ramps the charge current from full-scale
to zero when the battery charger is turned off or self
terminates. This has the effect of minimizing the transient
current load on the power supply during start-up and
charge termination.
Undervoltage Charge Current Limiting (UVCL)
The battery charger in the EUP8084 includes
undervoltage charge current limiting that prevents full
charge current until the input supply voltage reaches
approximately 300mV above the battery voltage
(∆VUVCL1). This feature is particularly useful if the
EUP8084 is powered from a supply with long leads (or
any relatively high output impedance). See Applications
Information section for further details.
Timer and Recharge
The EUP8084’s battery charger has an internal
termination timer that starts when the input voltage is
greater than the undervoltage lockout threshold and at
least 110mV above BAT, and the battery charger is
leaving shutdown.
At power-up or when exiting shutdown, the charge time
is set to 4.5 hours. Once the charge cycle terminates, the
battery charger continuously monitors the BAT pin
voltage using a comparator with a 2ms filter time. When
the average battery voltage falls below 4.05V (which
corresponds to 80%-90% battery capacity), a new charge
cycle is initiated and a 2.25 hour timer begins. This
ensures that the battery is kept at, or near, a fully charged
condition and eliminates the need for periodic charge
cycle initiations. The STAT output assumes a strong
pulldown state during recharge cycles until C/10 is
reached or the recharge cycle terminates.
Trickle Charge and Defective Battery Detection
At the beginning of a charge cycle, if the battery voltage
is below 2.95V, the battery charger goes into trickle
charge mode, reducing the charge current to 10% of the
programmed current. If the low battery voltage persists
for one quarter of the total time (1.125 hr), the battery is
assumed to be defective, the charge cycle terminates and
the STAT pin output pulses at a frequency of 2Hz with a
75% duty cycle. If, for any reason, the battery voltage
rises above 2.95V, the charge cycle will be restarted. To
restart the charge cycle (i.e., when the dead battery is
replaced with a discharged battery less than 2.95V), the
charger must be reset by removing the input voltage and
reapplying it or temporarily pulling the EN_BAT pin
below the enable threshold.
SWITCHING REGULATOR OPERATION:
The switching regulator in the EUP8084 can be turned on
by pulling the ENB pin above VIH.
Battery Charger Shutdown Mode
Main Control Loop
The EUP8084’s battery charger can be disabled by
pulling the EN_BAT pin below the shutdown threshold
(VIL). In shutdown mode, the battery drain current is
reduced to less than 2µA and the ADP supply current to
about 5µA provided the regulator is off. When the input
voltage is not present, the battery charger is in shutdown
and the battery drain current is less than 5µA.
The switching uses a slop-compensated constant
frequency, current mode PWM architecture. Both the
main (P-Channel MOSFET) and synchronous
(N-channel MOSFET) switches are internal. During
normal operation, the buck converter regulates output
voltage by switching at a constant frequency and then
modulating the power transferred to the load each cycle
using PWM comparator. It sums three weighted
differential signals: the output feedback voltage from an
external resistor divider, the main switch current sense,
and the slope-compensation ramp. It modulates output
power by adjusting the inductor-peak current during the
first half of each cycle. An N-channel, synchronous
switch turns on during the second half of each cycle (off
time). When the inductor current starts to reverse or
when the PWM reaches the end of the oscillator period,
the synchronous switch turns off. This keep excess
STAT Status Output Pin
The charge status indicator pin has three states: pulldown,
pulse at 2Hz (see Defective Battery Detection) and high
impedance. The pulldown state indicates that the battery
charger is in a charge cycle. A high impedance state
indicates that the charge current has dropped below 10%
of the full-scale current or the battery charger is disabled.
When the timer runs out (4.5 hrs), the STAT pin is also
forced to the high impedance state. If the battery charger
is not in constant-voltage mode when the charge current
DS8084 Ver1.0 Apr. 2008
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EUTECH [ EUTECH MICROELECTRONICS INC ]
