Datasheet

TPS62750, TPS62751
SLVS955A –JULY 2009–REVISED SEPTEMBER 2009 ................................................................................................................................................. www.ti.com
OUTPUT CAPACITOR DISCHARGE
With EN = GND, the device enters shutdown mode and all internal circuits are disabled. The SW pin is
connected to PGND via an internal resistor (typically 235Ω) to discharge the output capacitor.
SOFT START
The TPS62750 has an internal soft start circuit that controls the ramp up of the output voltage. The output
voltage ramps up from 5% to 95% of its nominal value in a controlled manner. This limits the inrush of current in
the converter during start-up and prevents possible voltage drops when a battery or high impedance power
source is used.
During soft start, the target average input current limit is reduced to 1/3 of its nominal value (I
LIM_L
or I
LIM_U
) until
the output voltage reaches 1/3 of its nominal value. Once the output voltage trips this threshold, the device
operates with its set target average input current limit.
The Soft-Start circuit is enabled after the start-up time t
start-up
has expired.
HOT-PLUG PROTECTION
In many applications it may be necessary to remove modules or pc boards while the main unit is still operating.
These are considered hot-plug applications. Such implementations require the control of current surges seen by
the main power supply and the card being inserted. The most effective way to control these surges is to limit and
slowly ramp the current and voltage being applied to the card, similar to the way in which a power supply
normally turns on.
Due to the controlled rise times and fall times and input over-voltage clamping of the TPS62750, these devices
can be used to provide a softer start-up to devices being hot-plugged into a powered system. The UVLO feature
of the TPS62750 also ensures that the switch is off after the card has been removed, and that the switch is off
during the next insertion. The UVLO feature insures a soft start with a controlled rise time for every insertion of
the card or module.
REVERSE CURRENT PROTECTION
The USB specification does not allow an output device to source current back into the USB port. However, the
TPS62750 is designed to safely power non-compliant devices. When disabled, each output is switched to a
high-impedance state, blocking reverse current flow from the output back to the input.
SHORT-CIRCUIT PROTECTION
During normal operation the High Side and Low Side MOSFET switches are protected by its current limits I
LIMF
.
Once the High Side MOSFET switch reaches its current limit, it is turned off and the Low Side MOSFET switch is
turned on. The High Side MOSFET switch can only turn on again, once the current in the Low Side MOSFET
switch decreases below its current limit. The device is capable to provide peak inductor currents up to its internal
current limit I
LIMF
.
As soon as the output voltage falls below 1/3 of the nominal output voltage due to overload or short circuit
condition, the converter current limit is reduced to 1/3 of the nominal value ILIMF. Due to the short-circuit
protection is enabled during start-up, the device does not deliver more than 1/3 of its nominal current limit I
LIMF
until the output voltage exceeds 1/3 of the nominal output voltage. This needs to be considered when a load is
connected to the output of the converter, which acts as a current sink.
THERMAL SHUTDOWN
As soon as the junction temperature, T
J
, exceeds 150°C (typical) the device goes into thermal shutdown. In this
mode, the High Side and Low Side MOSFETs are turned-off. The device continues its operation when the
junction temperature falls below the thermal shutdown hysteresis.
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