LTC3577_15 Datasheet, PDF (22/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
Ideal Diode from BAT to V_OUT
The LTC3577 has an internal ideal diode as well as a con-
troller 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 dropping
significantly with just the recommended output capacitor
(see Figure 2). The ideal diode consists of a precision ampli-
fier 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 neces-
sary. 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 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
22
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 (specifically, LT3480, LT3653 or LT3505). When
the wall adapter output (or buck regulator output) is con-
nected directly to the WALL pin, and the voltage 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.
External HV Buck Control Through the V_C Pin
The WALL, ACPR and V_C pins can be used in conjunction
with an external high voltage buck regulator such as the
LT3480, LT3505 or LT3653 to provide power directly to the
V_OUT pin as shown in Figures 3 to 5 (Consult factory for
complete list of approved high voltage buck regulators).
When the WALL pin voltage exceeds 4.3V, V_C pin control
circuitry is enabled and drives the V_C pin of the LT3480,
LT3505 or LT3653. The V_C pin control circuitry is designed
so that no compensation components are required on the
V_C node. The voltage at the V_OUT pin is regulated to the
larger of (BAT + 300mV) or 3.6V as shown in Figure 6.
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