STOP

RUN

SM74301

VIN

Input

Voltage

SM74301

www.ti.com

SNVS719A – JULY 2011

Figure 11. Shutdown Implementation

Current Limit

The SM74301 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.51A the

present cycle is immediately terminated, and a non-resetable OFF timer is initiated. The length of off-time is

the output is shorted, and during the initial part of start-up. This amount of time ensures safe short circuit

operation up to the maximum input voltage of 95V. In cases of overload where the FB voltage is above zero volts

(not a short circuit) the current limit off-time will be less than 35µs. Reducing the off-time during less severe

overloads reduces the amount of foldback, recovery time, and the start-up time. The off-time is calculated from

the following equation:

(5)

The current limit sensing circuit is blanked for the first 50-70ns of each on-time so it is not falsely tripped by the

current surge which occurs at turn-on. The current surge is required by the re-circulating diode (D1) for its turn-

off recovery.

N - Channel Buck Switch and Driver

The SM74301 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate

driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01

µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during

the on-time.

During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from Vcc through

the internal diode. The minimum OFF timer, set to 300ns, ensures a minimum time each cycle to recharge the

bootstrap capacitor.

The internal pre-charge switch at the SW pin is turned on for

≊150 ns during the minimum off-time period,

ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. This feature helps prevent

operating problems which can occur during very light load conditions, involving a long off-time, during which the

voltage across the bootstrap capacitor could otherwise reduce below the Gate Drive UVLO threshold. The pre-

charge switch also helps prevent startup problems which can occur if the output voltage is pre-charged prior to

turn-on. After current limit detection, the pre-charge switch is turned on for the entire duration of the forced off-

time .

Copyright © 2011, Texas Instruments Incorporated

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