LTC3455_15 Datasheet, PDF (20/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Soft-start is accomplished by gradually increasing the
peak inductor current for each switcher. This allows each
output to rise slowly, helping minimize the battery in-rush
current. Figure 10 shows the battery current during startup.
A soft-start cycle occurs whenever each switcher first
turns on, or after a fault condition has occurred (thermal
shutdown or UVLO).
In-Rush Current Limiting
When the LTC3455/LTC3455-1 are battery-powered, an
internal 0.15Ω PMOS switch connects the battery (V_BAT
pin) to the V_MAX pin to provide power for both switchers
and other internal circuitry. This PMOS switch is turned
off in shutdown, and the V_MAX pin discharges to ground,
providing output disconnect for all outputs. At startup,
this PMOS must first charge up any capacitance present
on the V_MAX pin to the battery voltage. To minimize the in-
rush current needed from the battery, the PMOS switch is
current-limited to 900mA and both switchers are disabled
while the V_MAX voltage is ramping up. Once V_MAX reaches
the battery voltage, the PMOS current-limit increases to 4A
and both switchers are allowed to turn on. Figure 10 shows
the startup battery current for the LTC3455/LTC3455-1,
which stays well-controlled while V_MAX is ramping up and
while both switchers outputs are rising.
Battery Charger General Information
The battery charger and Switcher 1 will always be enabled
whenever USB or wall power is present (as sensed by the
USB and WALLFB pins). This ensures that the battery can
be charged and that the microcontroller is alive whenever
V_MAX
2V/DIV
V_OUT1 (1.8V)
2V/DIV
V_OUT2 (3.3V)
2V/DIV
I_BAT
500mA/DIV
100μs/DIV
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Figure 10. In-Rush Current at Start-Up
external power is available. For some applications, it may be
undesirable for the charger to become active immediately
when external power is applied. For such applications,
an NMOS switch can be used to disconnect the R_PROG
resistor and allow the PROG pin to float high, turning off
the charger. In this manner, charging occurs only when
allowed by the microcontroller.
The LTC3455/LTC3455-1 battery chargers are constant-
current, constant-voltage chargers. In constant-current
mode, the maximum charge current is set by a single
external resistor. When the battery approaches the final
float voltage, the charge current begins to decrease as the
charger switches to constant-voltage mode. The charge
cycle is terminated only by the charge timer.
Charge and Recharge Cycles
When external power is first applied, a new charge cycle
is always initiated. The battery will continue charging
until the programmed charge time is reached. If the bat-
tery voltage is below 4.05V at the end of this cycle, the
LTC3455/LTC3455-1 will start a new charge cycle. This
action will continue until the battery voltage exceeds the
4.05V threshold. This operation is typically seen only
when charging from USB power. Because the charge cur-
rent can vary dramatically when the LTC3455/LTC3455-1
are USB powered, it takes considerably longer to charge
a battery using the USB supply (as compared to a wall
adapter). If the timer capacitor is chosen correctly, the
battery should be fully charged on one cycle when wall
power is available.
If the battery is above the 4.05V threshold when a charge
cycle has expired, charging will stop. At this point, a
recharge cycle is initiated if any of the following occurs:
The battery voltage drops below 4.05V, external power is
removed and reapplied, the PROG pin is floated temporar-
ily, or the SUSPEND pin is temporarily pulled high (if the
LTC3455/LTC3455-1 are under USB power).
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