LTC3441 Linear Technology, LTC3441 Datasheet - Page 18

• Manufacturer Part Number: LTC3441
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

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 9 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).
18
AC COUPLED
AC COUPLED
V
V
100mA/DIV
100mA/DIV
500mA/DIV
OUT1
OUT2
50mV/DIV
10mV/DIV
Figure 8. Burst Mode and PWM Mode Waveforms
2V/DIV
2V/DIV
2V/DIV
2V/DIV
2V/DIV
(1.8V)
(3.3V)
V
V
V
V
V
OUT1
OUT1
I
SW1
SW1
MAX
BAT
I
I
L1
L1
Figure 9. In-Rush Current at Startup
U
PWM Mode
Burst Mode
U
100 s/DIV
5 s/DIV
1 s/DIV
W
3455 F08a
3455 F08b
3455 F09
U
In-Rush Current Limiting
When the LTC3455 is battery-powered, an internal 0.15
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 9 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
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.
PROG
MAX
resistor and allow the PROG pin to float high,
pin discharges to ground, providing output
MAX
is ramping up and while both
MAX
reaches the battery
BAT
pin) to the V
MAX
pin to the
MAX
MAX
3455f