Datasheet

TPS2500, TPS2501
www.ti.com SLVS886C –OCTOBER 2008–REVISED AUGUST 2010
Figure 11. Eco-mode Control Scheme Operation, V
IN
= 3.3 V, I
AUX
= 10 mA
Overvoltage Protection
The TPS2500 provides overvoltage protection on V
AUX
to protect downstream devices. Overvoltage protection is
provided by disabling the gate drivers and power MOSFETs when an overvoltage condition is detected. The
TPS2500 uses a single AUX-high comparator to monitor the AUX voltage by sensing the voltage on the internal
feedback node fed into the error amplifier. The AUX-high comparator disables the gate driver whenever the
voltage on AUX exceeds the regulation point by 5% (typical). The gate driver remains disabled until the AUX
voltage falls below the 5% high OVP threshold. The overvoltage protection feature is disabled when V
IN
> V
NFM
(5.05 V typical) to prevent unwanted shutdown.
Overload Conditions
The TPS2500 boost converter uses multiple overcurrent protection features to limit current in the event of an
overload or short-circuit condition. The first feature is the lower current-limit comparator that works on a
cycle-by-cycle basis. This comparator turns off the low-side MOSFET by resetting the PWM latch whenever the
current through the low-side MOSFET exceeds 4.5 A (typical). The low-side MOSFET remains off until the next
switching cycle. The second feature is the upper current-limit comparator that disables switching for eight
switching cycles whenever the current in the low-side MOSFET exceeds 6.7 A (typical). After eight switching
cycles, the boost converter resumes normal operation. The third feature is the constant-current start-up I
START
comparator that disables switching and regulates the current through the high-side MOSFET whenever the
voltage on V
AUX
drops below the input voltage by V
EXIT
(700 mV typ). This feature protects the boost converter in
the event of an output short circuit on V
AUX
. I
START
also current-limit protects the synchronous MOSFET in
no-frequency mode when V
IN
> V
NFM
(5.05 V typical). The converter goes through normal start-up operation once
the short-circuit condition is removed. A fourth feature is the 85% (typical) maximum-duty-cycle clamp that
prevents excessive current from building in the inductor.
Determining the Maximum Allowable AUX and USB Current
The maximum output current of the boost converter out of AUX depends on several system-level factors
including input voltage, inductor value, switching frequency, and ambient temperature. The limiting factor for the
TPS2500 is the peak inductor current, which cannot exceed I
SW
(3 A minimum). The cycle-by-cycle current-limit
turns off the low-side NMOS as a protection mechanism whenever the inductor current exceeds I
SW
. The graph
in Figure 12 can be used as a guideline for determining the maximum total current at different input voltages. The
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