TPS2500, TPS2501

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SLVS886C – OCTOBER 2008 – REVISED AUGUST 2010

Figure 11. Eco-mode Control Scheme Operation, VIN = 3.3 V, IAUX = 10 mA

Overvoltage Protection

The TPS2500 provides overvoltage protection on VAUX 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 VIN > VNFM

(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 ISTART

comparator that disables switching and regulates the current through the high-side MOSFET whenever the

voltage on VAUX drops below the input voltage by VEXIT (700 mV typ). This feature protects the boost converter in

the event of an output short circuit on VAUX. ISTART also current-limit protects the synchronous MOSFET in

no-frequency mode when VIN > VNFM (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 ISW (3 A minimum). The cycle-by-cycle current-limit

turns off the low-side NMOS as a protection mechanism whenever the inductor current exceeds ISW. 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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