LTC3455 Datasheet, PDF (20/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
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 from excessive tem-
perature, and allows the user to push the limits of the
power handling capability of a given circuit board without
risk of damaging the LTC3455. 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 auto-
matically 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 has several special features that help make
the most of the power available from the USB power
supply. The internal USB power controller automatically
throttles back the battery charge current to help keep the
total system current under the strict 500mA/100mA USB
limit. The graph in Figure 11 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.
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)
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Figure 11. Charge Current vs Total System Current
20
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 12 shows
this reduction in charge 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
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Figure 12. Charge Current vs USB Voltage
Because the charge current can vary dramatically when
the LTC3455 is 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,
connect 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 pin to GND. Since the charge cycle is termi-
nated only by the charge timer, external charge termina-
tion is required when using either of these methods. Use
an external NMOS to float the PROG pin and disable
charging.
Constant-current-only mode is a good choice for systems
that are always powered by a USB supply or wall adapter,
and the charger can be used to charge a super-cap or
backup battery. Disabling the voltage amplifier allows the
super-cap/backup battery to charge up fully to the avail-
able USB or wall adapter voltage.
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