ltc3576 Linear Technology Corporation, ltc3576 Datasheet - Page 22

• Manufacturer Part Number: ltc3576
• Description: Switching Power Manager With Usb On-the-go + Triple Step-down Dc/dcs
• Manufacturer: Linear Technology Corporation

LTC3576/LTC3576-1
rapidly to V
be suffi cient 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
statically to its gate. This ensures that the inductor current
will have suffi cient negative slope during the time current
is fl owing 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
PMOS switch. The PMOS switch is enabled when V
rises above V
below V
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 effi ciency.
If V
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
I
Bat-Track Auxiliary High Voltage Switching Regulator
Control
The WALL, ACPR and V
with an external high voltage step-down switching regula-
tor such as the LT
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 effi ciency 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, WALL will rise toward this programmed output
voltage. When WALL exceeds approximately 4.3V, ACPR
OPERATION
22
2
C port must be cycled low and then high.
OUT
BUS
is above V
is less than 4V and the PMOS switch is disabled
OUT
OUT
+ 70mV to prevent the inductor current from
OUT
. When V
+ 180mV and is disabled when it falls
®
BUS
3480 or the LT3653 to minimize heat
. This is achieved by disabling the
BUS
C
pins can be used in conjunction
is close to V
OUT
OUT
there may not
is applied
BUS
is brought low and the Bat-Track control of the LTC3576/
LTC3576-1 overdrives the local V
high voltage step-down switching regulator. Therefore,
once the Bat-Track control is enabled, the output voltage
is set independent of the switching regulator feedback
network.
Bat-Track control provides a signifi cant effi ciency advantage
over the simple use of a 5V switching regulator output to
drive the battery charger. With a 5V output driving V
battery charger effi ciency is approximately:
where η
regulator and 5V is the output voltage of the switching
regulator. With a typical switching regulator effi ciency of
87% and a typical battery voltage of 3.8V, the total bat-
tery charger effi ciency is approximately 66%. Assuming
a 1A charge current, 1.7W of power is dissipated just to
charge the battery!
With Bat-Track, battery charger effi ciency is approxi-
mately:
With the same assumptions as above, the total battery
charger effi ciency 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
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
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-
η
η
TOTAL
TOTAL
BUCK
= η
= η
is the effi ciency of the high voltage switching
BUCK
BUCK
•
•
V
V
5V
BAT
BAT
V
BAT
+ 0.3V
OUT
C
control of the external
OUT
drops below
OUT
3576f
,