LTC4160/LTC4160-1
25
41601fa
APPLICATIONS INFORMATION
Hot Plugging and USB Inrush Current Limiting
The overvoltage protection circuit provides inrush current
limiting due to the long time it takes for OVGATE to fully
enhance the N-channel MOSFET. This prevents the current
from building up in the cable too quickly and dampens
out any resonant overshoot on V
BUS
. It is possible to
observe voltage overshoot on V
BUS
when connecting the
LTC4160/LTC4160-1 to a lab supply if the overvoltage
protection circuit is not used. This overshoot is caused by
the inductance of the long leads from the supply to V
BUS
.
Twisting the wires together from the supply to V
BUS
can
greatly reduce the parasitic inductance of these long leads
and keep V
BUS
at a safe level. USB cables are generally
manufactured with the power leads in close proximity and
thus have fairly low parasitic inductance.
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
Figure 8. Modified NTC Configuration
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
NTCBIAS
0.1V
NTC_ENABLE
41601 F08
LTC4160/LTC4160-1
NTC BLOCK
TOO_COLD
TOO_HOT
0.765 • NTCBIAS
0.349 • NTCBIAS
–
+
3
4
T
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 regulation
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 V
OUT
+ 70mV, the bidirectional
switching regulator will source up to 680mA into the sup-
ply. If V
BUS
is held to a voltage that is less than V
OUT
+
70mV then the short circuit timer will shut off the switching
regulator after 7.2ms. The FAULT pin will then go low to
indicate 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 using 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 lines. 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. USB 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 2.1 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.
