TPS2521 datasheet(20/47 Pages) TI

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The fastest output slew rate is achieved by leaving the dVdt pin open.

Note

For CdVdt > 10 nF, it's recommended to add a 100-Ω resistor in series with the capacitor on the dVdt

pin.

8.3.4.2 Active Current Limiting

The TPS2521xx responds to output overcurrent conditions by actively limiting the current after a user adjustable

transient fault blanking interval. When the load current exceeds the set overcurrent threshold (ILIM) set by the

ILM pin resistor (RILM), but stays lower than the short-circuit threshold (2 x ILIM), the device starts discharging the

ITIMER pin capacitor using an internal 1.8-μA pull-down current. If the load current drops below the overcurrent

threshold before the ITIMER capacitor (CITIMER) discharges by ΔVITIMER, the ITIMER is reset by pulling it up to

VINT internally and the current limit action is not engaged. This allows short load transient pulses to pass through

the device without getting current limited. If the overcurrent condition persists, the CITIMER continues to discharge

and once it discharges by ΔVITIMER, the current limit starts regulating the HFET to actively limit the current to

the set overcurrent threshold (ILIM). At the same time, the CITIMER is charged up to VINT again so that it is at its

default state before the next overcurrent event. This ensures the full blanking timer interval is provided for every

overcurrent event. Equation 4 can be used to calculate the RILM value for a desired overcurrent threshold.

RILM

:À; =

3334

ILIM

:A;

(4)

Note

1. Leaving the ILM pin Open sets the current limit to nearly zero and results in the part entering

current limit with the slightest amount of loading at the output.

2. The current limit circuit employs a foldback mechanism. The current limit threshold in the foldback

region (0 V < VOUT < VFB) is lower than the steady state current limit threshold (ILIM).

3. Shorting the ILM pin to ground at any point during normal operation is detected as a fault and the

part shuts down. There’s a minimum current (IFLT) which the part allows in this condition before

the pin short condition is detected.

The duration for which transients are allowed can be adjusted using an appropriate capacitor value from ITIMER

pin to ground. The CITIMER value needed to set the desired transient overcurrent blanking interval can be

calculated using Equation 5 below.

tITIMER (ms) =

¿VITIMER (V) x CITIMER (nF)

IITIMER (µA)

(5)

TPS2521

SLVSFX8 – MARCH 2021

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