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LTC4160/LTC4160-1

41601f

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 VBUS. It is possible to

observe voltage overshoot on VBUS 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 VBUS.

Twisting the wires together from the supply to VBUS can

greatly reduce the parasitic inductance of these long leads

and keep VBUS 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 VBUS when On-The-Go is

enabled, the bidirectional switching regulator will not try

to drive VBUS. If USB On-The-Go is enabled and an external

supply is then connected to VBUS, 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

–

RNOM

105k

RNTC

100k

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

–

switching and VBUS 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 VBUS will be regulated to 5.1V. The external supply

will not source current to VBUS.

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 VBUS up to 5.1V. As long as the external supply

holds VBUS to more than VOUT + 70mV, the bidirectional

switching regulator will source up to 680mA into the sup-

ply. If VBUS is held to a voltage that is less than VOUT +

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.

VBUS 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 VBUS when the BUS is not being used.

If the A device has powered down VBUS, the B device can

request the A device to power up VBUS and start a new

session using the session request protocol (SRP). The

SRP consists of data-line pulsing and VBUS pulsing. The

B device must ﬁrst pulse the D+ or D– data lines. The B

device must then pulse VBUS 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 VBUS are not required to

respond to the SRP.

For VBUS pulsing, the limit on the VBUS 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 VBUS 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 VBUS

as soon as the current pulse raises VBUS above 2.1V if the

device is capable of responding to VBUS pulsing.

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