LTC3455 Datasheet, PDF (18/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
Burst Mode
V_SW1
2V/DIV
V_OUT1
50mV/DIV
AC COUPLED
I_L1
100mA/DIV
5µs/DIV
PWM Mode
3455 F08a
V_SW1
2V/DIV
V_OUT1
10mV/DIV
AC COUPLED
I_L1
100mA/DIV
1µs/DIV
3455 F08b
Figure 8. Burst Mode and PWM Mode Waveforms
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).
V_MAX
2V/DIV
V_OUT1 (1.8V)
2V/DIV
V_OUT2 (3.3V)
2V/DIV
I_BAT
500mA/DIV
100µs/DIV
3455 F09
Figure 9. In-Rush Current at Startup
In-Rush Current Limiting
When the LTC3455 is 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 9 shows the
startup battery current for the LTC3455, 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
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_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 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.
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