LTC3455 Datasheet, PDF (12/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
Operation in USB Low Power Mode
Most applications that draw power from the USB bus
should be in low power mode only for a brief amount of
time. All devices must be in low power mode (draw no
more than 100mA of current from the USB bus) upon
power-up, and can transition to high power mode (draw
up to 500mA from the USB bus) only after the device has
been given permission to do so by the USB host controller.
The change to high power mode is usually very quick, so
the full 500mA of current is available shortly after connect-
ing to the USB bus. While the LTC3455 will operate when
in low power mode, the amount of power available is so
small that it is difficult or impossible to charge a battery or
even provide enough current to power the rest of the
system. For this reason, USB high power operation should
always be used with the LTC3455.
Handling Large Transient Loads when USB Powered
Many portable devices have nominal loads that can easily
be supported by the USB supply, but they have brief
transient loads that can exceed the maximum available
USB power. The LTC3455 is designed to handle these
overloads while drawing as much power as possible from
the USB port. If the USB bus is providing power but the
LTC3455 (or any other devices connected to the V_MAX pin)
needs more total power than the USB bus can supply, the
battery charger turns off completely and the USB power
controller becomes a 500mA (or 100mA) current source
and the V_MAX voltage begins to decrease. At this point, the
capacitance connected to the V_MAX pin provides the addi-
tional current needed by the system. As long as the USB
pin stays above 3.9V, the USB bus will continue to provide
as much current as possible. Once the V_MAX pin drops just
below the V_BAT voltage, the battery will provide the addi-
tional current needed. This operation allows the LTC3455
to withstand load transients that briefly demand more
power than can be provided by the USB bus.
The oscilloscope photographs in Figure 1 show how the
LTC3455 handles load transients when USB powered. The
top photo shows a brief transient load that turns off the
charger but does not dip the V_MAX voltage. The bottom
photo shows a prolonged transient condition that turns off
the charger and completely dips the V_MAX voltage to the
point where the battery must provide current. For both
cases, normal operation resumes as soon as the transient
passes.
Extra capacitance can be connected to the V_MAX pin to act
as a reservoir to help support large transient currents. For
most systems this is not necessary, as the LTC3455
cleanly handles heavy transients. For some designs, how-
ever, it may be desirable to use a larger capacitor con-
nected to V_MAX to act as a larger reservoir. Up to 50µF of
V_MAX
2V/DIV
I_MAX
500mA/DIV
I_USB
500mA/DIV
I_BAT
500mA/DIV
100µs/DIV
3455 F01a
USB Maximum Current Condition
V_MAX
2V/DIV
I_MAX
500mA/DIV
I_USB
500mA/DIV
I_BAT
500mA/DIV
100µs/DIV
3455 F01b
USB Heavy Over-Current Condition
Figure 1. Handling Load Transients when USB Powered
3455f
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