LTC3455_15 Datasheet, PDF (22/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Battery Charger Thermal Limit
An internal thermal limit reduces the charge current if
the die temperature attempts to rise above approximately
105°C. This protects the LTC3455/LTC3455-1 from ex-
cessive temperature, and allows the user to push the
limits of the power handling capability of a given circuit
board without risk of damaging the LTC3455/LTC3455-1.
Another benefit of the thermal limit is that charge current
can be set according to typical, not worst-case, ambient
temperatures for a given application with the assurance
that the charger will automatically reduce the current under
worst-case conditions.
CHRG Status Output
The CHRG pin is pulled low with an internal N-channel
MOSFET whenever the battery charger is enabled, and is
forced into a high impedance state whenever it is disabled.
This NMOS device is capable of driving an external LED.
This pin does not provide any C/10 information.
Special Charger Features while USB Powered
The LTC3455/LTC3455-1 have several special features that
help make the most of the power available from the USB
power supply. The internal USB power controller automati-
cally throttles back the battery charge current to help keep
the total system current under the strict 500mA/100mA USB
limit. The graph in Figure 12 shows how charge current,
I_BAT, decreases as the current needed for the rest of the
system increases (both switchers and all other external
devices pull current from the V_MAX pin). The total USB
current, I_USB, always stays below 500mA.
As the USB voltage drops below 4.5V, the charge current
gradually reduces (and eventually shuts off around 4V).
This helps prevent “chattering” and stability problems
when using long, resistive USB cables. Figure 13 shows
this reduction in charge current.
500
400
I_USB
300
I_BAT
200
100
0 USB HIGH POWER MODE
V_USB = 5V
–100 ^{VBAT = 3.6V}
0 100 200 300 400
500 600
TOTAL SYSTEM CURRENT (mA)
3455 F12
Figure 12. Charge Current vs Total System Current
500
USB HIGH POWER MODE
V_BAT = 3.6V
I_BAT
400
300
200
100
0
3.75 4.00 4.25 4.50 4.75
V_USB (V)
5.00 5.25
3455 F13
Figure 13. Charge Current vs USB Voltage
Because the charge current can vary dramatically when the
LTC3455/LTC3455-1 are USB powered, battery charging
can take considerably longer using the USB supply (as
compared to a wall adapter).
Constant-Current-Only Charger/Disabling the
Charger Timer
To use the charger in a constant-current-only mode, con-
nect the TIMER pin to V_MAX to disable the timer, voltage
amplifier, and trickle charge function. To disable only the
timer function and leave all others intact, connect the TIMER
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