LTC3576-1_15 Datasheet, PDF (23/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

LTC3576/LTC3576-1
OPERATION
2200
2000
1800
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
1600
1400
1200
1000
LTC3576/
LTC3576-1
IDEAL DIODE
800
600
ON
400
SEMICONDUCTOR
MBRM120LT3
200
0
0 60 120 180 240 300 360 420 480
FORWARD VOLTAGE (mV) (BAT – V_OUT)
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Figure 4. Ideal Diode V-I Characteristics
then all of the application power will be provided by the
battery via the ideal diodes. The ideal diode(s) will be fast
enough to keep V_OUT from drooping with only the stor-
age capacitance required for the switching regulator. The
internal ideal diode consists of a precision amplifier that
activates a large on-chip P-channel MOSFET whenever
the voltage at V_OUT is approximately 15mV (V_FWD) below
the voltage at BAT. Within the amplifier’s linear range, the
small-signal resistance of the ideal diode will be quite low,
keeping the forward drop near 15mV. At higher current
levels, the MOSFET will be in full conduction.
To supplement the internal ideal diode, an external
P-channel MOSFET may be added from BAT to V_OUT. The
IDGATE pin of the LTC3576/LTC3576-1 drives the gate of
the external P-channel MOSFET for automatic ideal diode
control. The source of the external P-channel MOSFET
should be connected to V_OUT and the drain should be con-
nected to BAT. Capable of driving a 1nF load, the IDGATE
pin can control an external P-channel MOSFET transistor
having an on-resistance of 30mΩ or lower.
Suspend LDO
If the LTC3576/LTC3576-1 are configured for USB suspend
mode, the bidirectional switching regulator is disabled and
the suspend LDO provides power to the V_OUT pin (presum-
ing there is power available to V_BUS). This LDO will prevent
the battery from running down when the portable product
has access to a suspended USB port. Regulating at 4.6V,
this LDO only becomes active when the switching converter
is disabled (suspended). The suspend LDO sends a scaled
copy of the V_BUS current to the CLPROG pin, which will
servo to approximately 100mV in this mode. To remain
compliant with the USB specification, the input to the LDO
is current limited so that it will not exceed the low power
or high power suspend specification. If the load on V_OUT
exceeds the suspend current limit, the additional current
will come from the battery via the ideal diode(s).
3.3V Always-On LDO Supply
The LTC3576/LTC3576-1 include a low quiescent current
low dropout regulator that is always powered. This LDO
can be used to provide power to a system pushbutton
controller, standby microcontroller or real time clock. De-
signed to deliver up to 20mA, the always-on LDO requires
at least a 1μF low impedance ceramic bypass capacitor
for compensation. The LDO is powered from V_OUT, and
therefore will enter dropout at loads less than 20mA as
V_OUT falls near 3.3V. If the LDO3V3 output is not used, it
should be disabled by connecting it to V_OUT.
Battery Charger
The LTC3576/LTC3576-1 include a constant-current/con-
stant-voltage battery charger with automatic recharge,
automatic termination by safety timer, low voltage trickle
charging, bad cell detection and thermistor sensor input
for out-of-temperature charge pausing.
Battery Preconditioning
When a battery charge cycle begins, the battery charger
first determines if the battery is deeply discharged. If the
battery voltage is below V_TRKL, typically 2.85V, an automatic
trickle charge feature sets the battery charge current to
10% of the programmed value. If the low voltage persists
for more than 1/2 hour, the battery charger automatically
terminates and indicates via the CHRG pin that the battery
was unresponsive.
Once the battery voltage is above 2.85V, the charger begins
charging in full power constant-current mode. The cur-
rent delivered to the battery will try to reach 1030/R_PROG.
Depending on available input power and external load
conditions, the battery charger may or may not be able
to charge at the full programmed rate. The external load
will always be prioritized over the battery charge current.
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