LTC4001 Datasheet, PDF (16/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001
APPLICATIO S I FOR ATIO
Insert the highest power dissipation figure into the follow-
ing equation to determine maximum junction temperature:
T_J = T_A + (P_D • 37°C/W)
The LTC4001 includes chip overtemperature protection. If
junction temperature exceeds 160°C (typical), the chip
will stop battery charging until chip temperature drops
below 150°C.
Using the LTC4001 in Applications Without a Battery
The LTC4001 is normally used in end products that only
operate with the battery attached (Figure 6). Under these
conditions the battery is available to supply load transient
currents. For indefinite operation with a powered wall
adapter there are only two requirements—that the aver-
age current drawn by the load is less than the high rate
charge current, and that V_BAT stays above the trickle
charge threshold when the load is initially turned on and
during other load transients. When making this determi-
nation take into account battery impedance. If battery
voltage is less than the trickle charge threshold, the
system load may be turned off until V_BAT is high enough
to meet these conditions.
The situation changes dramatically with the battery re-
moved (Figure 7). Since the battery is absent, V_BAT begins
at zero when a powered wall adapter is first connected to
the battery charger. With a maximum load less than the
LTC4001 trickle charge current, battery voltage will ramp
up until V_BAT crosses the trickle charge threshold. When
this occurs, the LTC4001 switches over from trickle charge
to high rate (PWM) charge mode but initially delivers zero
current (because the soft-start pin is at zero). Battery
voltage drops as a result of the system load, crossing
below the trickle charge threshold. The charger re-enters
trickle charge mode and the battery voltage ramps up
again until the battery charger re-enters high rate mode.
The soft-start voltage is slightly higher this time around
(than in the previous PWM cycle). Every successive time
that the charger enters high rate (PWM) charge mode, the
soft-start pin is at a slightly higher voltage. Eventually high
rate charge mode begins with a soft-start voltage that
causes the PWM charger to provide more current than the
system load demands, and V_BAT rapidly rises until the float
voltage is reached.
For battery-less operation, system load current should be
restricted to less than the worst case trickle charge current
(preferably less than 30mA) when V_BAT is less than 3.15V
(through an undervoltage lockout or other means). Above
V_BAT = 3.15V, system load current less than or equal to the
high rate charge current is allowed. If operation without a
battery is required, additional low-ESR output filtering
improves start-up and other load transients. Battery-less
start-up is also improved if a 10k resistor is placed in
series with the soft-start capacitor.
WALL
ADAPTER
LTC4001
BATTERY
CHARGER
+
SYSTEM
LOAD
4001 F06
Li-Ion
BATTERY
Figure 6. Typical Application
4001f
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