LTC4053-4.2_15 Datasheet, PDF (9/16 Pages) Linear Technology – USB Compatible Lithium-Ion Battery Charger with Thermal Regulation

LTC4053-4.2
APPLICATIO S I FOR ATIO
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until V_CC
rises above the undervoltage lockout threshold. The UVLO
circuit has a built-in hysteresis of 150mV. Furthermore, to
protect against reverse current in the power MOSFET, the
UVLO circuit keeps the charger in shutdown mode if V_CC
falls to within 35mV of the battery voltage. If the UVLO
comparator is tripped, the charger will not come out of
shutdown until V_CC rises 70mV above the battery voltage.
Trickle Charge And Defective Battery Detection
At the beginning of a charge cycle, if the battery voltage is
low (below 2.48V) the charger goes into trickle charge
reducing the charge current to 10% of the full-scale
current. If the low battery voltage persists for one quarter
of the total charge time, the battery is assumed to be
defective, the charge cycle is terminated, the CHRG pin
output assumes a high impedance state, and the FAULT
pin pulls low. The fault can be cleared by toggling V_CC,
temporarily forcing the SHDN pin to 0V, or temporarily
forcing the BAT pin voltage above 2.48V.
Shutdown
The LTC4053 can be shut down (I_CC = 25µA) by pulling the
SHDN pin to 0V. For normal operation, pull the SHDN pin
above the manual shutdown threshold voltage level. Do
not leave this pin open. In shutdown the internal linear
regulator is turned off, and the internal timer is reset.
Programming Charge Current
The formula for the battery charge current (see Figure 1)
is:
I_CHG = (I_PROG) • 1000
= (1.5V / R_PROG) • 1000 or
R_PROG = 1500V/I_CHG
where R_PROG is the total resistance from the PROG pin to
ground. Under trickle charge conditions, this current is
reduced to 10% of the full-scale value.
For example, if 500mA charge current is required,
calculate:
R_PROG = 1500V/0.5A = 3kΩ
For best stability over temperature and time, 1% metal-
film resistors are recommended.
If the charger is in constant-temperature or constant-
voltage mode, the battery current can be monitored by
measuring the PROG pin voltage as follows:
I_CHG = (V_PROG / R_PROG) • 1000
USB and Wall Adapter Power
Although the LTC4053 allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batteries.
Figure 2 shows an example of how to combine wall adapter
and USB power inputs. A P-channel MOSFET, MP1, is
used to prevent back conducting into the USB port when
a wall adapter is present and Schottky diode, D1, is used
to prevent USB power loss through the 1k pull-down
resistor.
Typically a wall adapter can supply significantly more
current than the 500mA-limited USB port. Therefore, an N-
channel MOSFET, MN1 and an extra 3k program resistor
can be used to increase the charge current to 1A when the
wall adapter is present.
5V WALL
ADAPTER
1A I_CHG
USB
POWER
500mA I_CHG MP1
1k
LTC4053
I_CHG
D1
9
BAT
2
V_CC
7
PROG
+
SYSTEM
LOAD
Li-Ion
BATTERY
3k
MN1
3k
4053 F02
Figure 2. Combining Wall Adapter and USB Power
4053fa
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