LTC3576 Datasheet, PDF (39/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
APPLICATIONS INFORMATION
Hot Plugging and USB On-the-Go
If there is more than 4.3V on V_BUS when on-the-go is
enabled, the bidirectional switching regulator will not try
to drive V_BUS. If USB on-the-go is enabled and an external
supply is then connected to V_BUS, one of three things will
happen depending on the properties of the external sup-
ply. If the external supply has a regulation voltage higher
than 5.1V, the bidirectional switching regulator will stop
switching and V_BUS will be held at the regulation voltage
of the external supply. If the external supply has a lower
regulation voltage and is capable of only sourcing current
then V_BUS will be regulated to 5.1V. The external supply
will not source current to V_BUS.
For a supply that can also sink current and has a regula-
tion voltage less than 5.1V, the bidirectional switching
regulator will source current into the external supply in an
attempt to bring V_BUS up to 5.1V. As long as the external
supply holds V_BUS to more than 4V or V_OUT + 70mV, the
bidirectional switching regulator will source up to 680mA
into the supply. If V_BUS is held to a voltage that is less than
4V and V_OUT + 70mV then the short circuit timer will shut
off the switching regulator after 7.2ms. The CHRG pin will
then blink indicating a short circuit current fault.
V_BUS Bypass Capacitance and USB On-The-Go
Session Request Protocol
When two on-the-go devices are connected, one will be the
A device and the other will be the B device depending on
whether the device is connected to a micro A or micro B
plug. The A device provides power to the B device and
starts as the host. To prolong battery life, the A device can
power down V_BUS when the bus is not being used. If the A
device has powered down V_BUS, the B device can request
the A device to power up V_BUS and start a new session us-
ing the session request protocol (SRP). The SRP consists
of data-line pulsing and V_BUS pulsing. The B device must
first pulse the D⁺ or D^– data line. The B device must then
pulse V_BUS only if the A device does not respond to the
data-line pulse. The A device is required to respond to only
one of the pulsing methods. A devices that never power
down V_BUS are not required to respond to the SRP.
For V_BUS pulsing, the limit on the V_BUS capacitance on
the A device allows a B device to differentiate between
a powered down on-the-go device and a powered down
standard host. The B device will send out a pulse of current
that will raise V_BUS to a voltage between 2.1V and 5.25V if
connected to an on-the-go A device which must have no
more than 6.5μF. An on-the-go A device must drive V_BUS
as soon as the current pulse raises V_BUS above 2.1V if the
device is capable of responding to V_BUS pulsing.
This same current pulse must not raise V_BUS any higher
than 2V when connected to a standard host which must
have at least 96μF. The 96μF for a standard host represents
the minimum capacitance with V_BUS between 4.75V and
5.25V. Since the SRP pulse must not drive V_BUS greater
than 2V, the capacitance seen at these voltage levels can be
greater than 96μF, especially if MLCCs are used. Therefore,
the 96μF represents a lower bound on the standard host
bypass capacitance for determining the amplitude and
duration of the current pulse. More capacitance will only
decrease the maximum level that V_BUS will rise to for a
given current pulse.
Figure 11 shows an on-the-go device using the LTC3576/
LTC3576-1 acting as the A device. Additional capacitance
can be placed on the V_BUS pin of the LTC3576/LTC3576-1
when using the overvoltage protection circuit. A B device
may not be able to distinguish between a powered down
LTC3576/LTC3576-1 with overvoltage protection and a
powered down standard host because of this extra ca-
pacitance. In addition, if the SRP pulse raises V_BUS above
its UVLO threshold of 4.3V the LTC3576/LTC3576-1 will
assume input power is available and will not attempt to
drive V_BUS. Therefore, it is recommended that an on-
the-go device using the LTC3576/LTC3576-1 respond to
data-line pulsing.
When an on-the-go device using the LTC3576/LTC3576-1
becomes the B device, as in Figure 12, it must send out
a data line pulse followed by a V_BUS pulse to request a
session from the A device. The on-the-go device designer
can choose how much capacitance will be placed on the
V_BUS pin of the LTC3576/LTC3576-1 and then generate
a V_BUS pulse that can distinguish between a powered
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