LTC3576-1_15 Datasheet, PDF (22/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
proaches this limit CLPROG servos to 1.15V and V_BUS falls
rapidly to V_OUT. When V_BUS is close to V_OUT there may not
be sufficient negative slope on the inductor current when
the PMOS switch is on to balance the rise in the inductor
current when the NMOS switch is on. This will cause the
inductor current to run away and the voltage on CLPROG
to rise. When CLPROG reaches 1.2V the switching of the
synchronous PMOS is terminated and V_OUT is applied
statically to its gate. This ensures that the inductor current
will have sufficient negative slope during the time current
is flowing to the output. The PMOS will resume switching
when CLPROG drops down to 1.15V.
The LTC3576/LTC3576-1 maintain voltage regulation even
if V_OUT is above V_BUS. This is achieved by disabling the
PMOS switch. The PMOS switch is enabled when V_BUS
rises above V_OUT + 180mV and is disabled when it falls
below V_OUT + 70mV to prevent the inductor current from
running away when not in current limit. Since the PMOS
no longer acts as a low impedance switch in this mode,
there will be more power dissipation within the IC. This
will cause a sharp drop in efficiency.
If V_BUS is less than 4V and the PMOS switch is disabled
for more than 7.2ms a short-circuit fault will be declared
and the part will shut off. The CHRG pin will blink at 35kHz
with a duty cycle that varies between 12% and 88% at a
4Hz rate. See Table 2. To re-enable step-up mode, the
ENOTG pin or, with ENOTG grounded, the B0 bit in the
I²C port must be cycled low and then high.
Bat-Track Auxiliary High Voltage Switching Regulator
Control
The WALL, ACPR and V_C pins can be used in conjunction
with an external high voltage step-down switching regula-
tor such as the LT^®3480 or the LT3653 to minimize heat
production when operating from higher voltage sources,
as shown in Figures 1 and 3. Bat-Track control circuitry
regulates the external switching regulator’s output voltage
to the larger of (BAT + 300mV) or 3.6V. This maximizes
battery charger efficiency while still allowing instant-on
operation when the battery is deeply discharged.
The feedback network of the high voltage regulator should
be set to generate an output voltage between 4.5V and
5.5V. When high voltage is applied to the external regulator,
22
WALL will rise toward this programmed output voltage.
When WALL exceeds approximately 4.3V, ACPR is brought
low and the Bat-Track control of the LTC3576/LTC3576-1
overdrives the local V_C control of the external high volt-
age step-down switching regulator. Therefore, once the
Bat-Track control is enabled, the output voltage is set in-
dependent of the switching regulator feedback network.
Bat-Track control provides a significant efficiency advantage
over the simple use of a 5V switching regulator output to
drive the battery charger. With a 5V output driving V_OUT,
battery charger efficiency is approximately:
η_TOTAL
=
η_BUCK
•
V_BAT
5V
where η_BUCK is the efficiency of the high voltage switching
regulator and 5V is the output voltage of the switching
regulator. With a typical switching regulator efficiency of
87% and a typical battery voltage of 3.8V, the total bat-
tery charger efficiency is approximately 66%. Assuming
a 1A charge current, 1.7W of power is dissipated just to
charge the battery!
With Bat-Track, battery charger efficiency is approxi-
mately:
η_TOTAL
=
η_BUCK
•
V_BAT
V_BAT + 0.3V
With the same assumptions as above, the total battery
charger efficiency is approximately 81%. This example
works out to less than 1W of power dissipation, or almost
60% less heat.
See the Typical Applications section for complete circuits
using the LT3480 and the LT3653 with Bat-Track control.
Ideal Diode(s) from BAT to V_OUT
The LTC3576/LTC3576-1 each have an internal ideal diode as
well as a controller for an optional external ideal diode. Both
the internal and the external ideal diodes are always on and
will respond quickly whenever V_OUT drops below BAT.
If the load current increases beyond the power allowed
from the switching regulator, additional power will be
pulled from the battery via the ideal diode(s). Further-
more, if power to V_BUS (USB or wall adapter) is removed,
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