LTC3577-4_15 Datasheet, PDF (22/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
Ideal Diode from BAT to V_OUT
The LTC3577-3/LTC3577-4 have an internal ideal diode as
well as a controller for an optional external ideal diode. Both
the internal and the external ideal diodes respond quickly
whenever V_OUT drops below BAT. If the load increases
beyond the input current limit, additional current will be
pulled from the battery via the ideal diodes. Furthermore, if
power to V_BUS (USB) or V_OUT (external wall power or high
voltage regulator) is removed, then all of the application
power will be provided by the battery via the ideal diodes.
The ideal diodes are fast enough to keep V_OUT from drop-
ping significantly below V_BAT with just the recommended
output capacitor (see Figure 2). The ideal diode consists
of a precision amplifier that enables an on-chip P-channel
MOSFET whenever the voltage at V_OUT is approximately
15mV (V_FWD) below the voltage at BAT. The resistance of
the internal ideal diode is approximately 200mΩ. If this is
sufficient for the application, then no external components
are necessary. However, if lower resistance is needed, an
external P-channel MOSFET can be added from BAT to
V_OUT. The IDGATE pin of the LTC3577-3/LTC3577-4 drives
the gate of the external P-channel MOSFET for automatic
ideal diode control. The source of the MOSFET should be
connected to V_OUT and the drain should be connected to
BAT. Capable of driving a 1nF load, the IDGATE pin can
control an external P-channel MOSFET having extremely
low on-resistance.
4.0V
V_OUT 3.8V
3.6V
500mA
I_BAT 0
–500mA
CHARGE
DISCHARGE
I_VOUT
LOAD
1A
0A
V_BAT = 3.8V
V_BUS = 5V
5x MODE
C_OUT = 10μF
10μs/DIV
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Figure 2. Ideal Diode Transient Response
Using the WALL Pin to Detect the Presence of an
External Power Source
The WALL input pin can be used to identify the presence
of an external power source (particularly one that is not
subject to a fixed current limit like the USB V_BUS input).
Typically, such a power supply would be a 5V wall adapter
output or the low voltage output of a high voltage buck
regulator. When the wall adapter output (or buck regulator
output) is connected directly to the WALL pin, and the volt-
age exceeds the WALL pin threshold, the USB power path
(from V_BUS to V_OUT) will be disconnected. Furthermore,
the ACPR pin will be pulled low. In order for the presence
of an external power supply to be acknowledged, both of
the following conditions must be satisfied:
1. The WALL pin voltage must exceed approximately
4.3V.
2. The WALL pin voltage must be greater than 75mV above
the BAT pin voltage.
The input power path (between V_BUS and V_OUT) is re-
enabled and the ACPR pin is pulled high when either of
the following conditions is met:
1. The WALL pin voltage falls to within 25mV of the BAT
pin voltage.
2. The WALL pin voltage falls below 3.2V.
Each of these thresholds is suitably filtered in time to
prevent transient glitches on the WALL pin from falsely
triggering an event.
Suspend Mode
When I_LIM0 is pulled high and I_LIM1 is pulled low the
LTC3577-3/LTC3577-4 enters suspend mode to comply
with the USB specification. In this mode, the power path
between V_BUS and V_OUT is put in a high impedance state
to reduce the V_BUS input current to 50μA. If no other
power source is available to drive WALL and V_OUT, the
system load connected to V_OUT is supplied through the
ideal diodes connected to BAT.
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