ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 20

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3455
Soft-Start for each Switcher
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 is battery-powered, an internal 0.15Ω
PMOS switch connects the battery (V
pin to provide power for both switchers and other internal
circuitry. This PMOS switch is turned off in shutdown, and
the V
disconnect for all outputs. At startup, this PMOS must first
charge up any capacitance present on the V
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
voltage is ramping up. Once V
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, which stays well-
controlled while V
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
20
V
V
500mA/DIV
OUT1
OUT2
MAX
2V/DIV
2V/DIV
2V/DIV
(1.8V)
(3.3V)
V
I
MAX
BAT
pin discharges to ground, providing output
Figure 10. In-Rush Current at Startup
U
MAX
is ramping up and while both
U
100µs/DIV
MAX
W
reaches the battery
BAT
pin) to the V
MAX
3455 F10
U
pin to the
MAX
MAX
USB and WALLFB pins). This ensures that the battery can
be charged and that the microcontroller is alive whenever
external power is available. For some applications, it may
be undesirable for the charger to become active immedi-
ately when external power is applied. For such applica-
tions, an NMOS switch can be used to disconnect the
R
turning off the charger. In this manner, charging occurs
only when allowed by the microcontroller.
The LTC3455 battery charger is a constant-current, con-
stant-voltage charger. In constant-current mode, the maxi-
mum 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 battery
voltage is below 4.05V at the end of this cycle, the
LTC3455 will start a new charge cycle. This action will
continue until the battery voltage exceeds the 4.05V thresh-
old. This operation is typically seen only when charging
from USB power. Because the charge current can vary
dramatically when the LTC3455 is 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 tempo-
rarily, or the SUSPEND pin is temporarily pulled high (if the
LTC3455 is under USB power).
PROG
resistor and allow the PROG pin to float high,
3455fb