7V SERIES

REGULATOR

ON TIMER

START

COMPLETE

STOP

RUN

SM74304

SM74304

SNVS720A – OCTOBER 2011 – REVISED APRIL 2013

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Figure 4. Shutdown Implementation

Current Limit

The SM74304 contains an intelligent current limit off timer intended to reduce the foldback characteristic inherent

with fixed off-time over-current protection. If the current in the Buck switch exceeds 725mA the present cycle on

time is immediately terminated (cycle by cycle current limit). Following the termination of the cycle a non-

and the FB pin voltage. When the FB pin voltage equals zero, the current limit off time is internally preset to

17uS. This condition occurs in short circuit operation when a maximum amount of off time is required.

In cases of overload (not complete short circuit) the current limit off time can be reduced as a function of the

output voltage (measured at the FB pin). Reducing the off time with smaller overloads reduces the amount of

resistor can be calculated by the following equation:

(5)

Applications utilizing low resistance inductors and/or a low voltage drop rectifier may require special evaluation at

high line, short circuit conditions. In this special case the preset 17uS (FB = 0V) off time may be insufficient to

balance the inductor volt*time product. Additional inductor resistance, output resistance or a larger voltage drop

rectifier may be necessary to balance the inductor cycle volt*time product and limit the short circuit current.

N - Channel Buck Switch and Driver

The SM74304 integrates an N-Channel Buck switch and associated floating high voltage gate driver. This gate

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

connected between the BST pin and SW pin is recommended.

During each cycle when the Buck switch turns off, the SW pin is approximately 0V. When the SW pin voltage is

low, the bootstrap capacitor will be charged from Vcc through the internal diode. The minimum off timer, set to

300ns, ensures that there will be a minimum interval every cycle to recharge the bootstrap capacitor.

An external re-circulating diode from the SW pin to ground is necessary to carry the inductor current after the

internal Buck switch turns off. This external diode must be of the Ultra-fast or Schottky type to reduce turn-on

losses and current over-shoot. The reverse voltage rating of the re-circulating diode must be greater than the

maximum line input voltage.

8

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