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LM34925

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SNVS846E – JUNE 2012 – REVISED DECEMBER 2013

Current Limit

The LM34925 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 270 mA the

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

maximum off-time is set to 16

μs. This condition occurs when the output is shorted, and during the initial part of

start-up. This amount of time ensures safe short circuit operation even up to the maximum input voltage of 100

V.

In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is

reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and

start-up time. The off-time is calculated from the following equation:

(5)

The current limit protection feature is peak limited, the maximum average output will be less than the peak.

N-Channel Buck Switch and Driver

The LM34925 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

uF ceramic capacitor connected between the BST pin and SW pin provides the voltage to the driver during the

through the internal diode. The minimum off-timer, set to 144 ns, ensures a minimum time each cycle to recharge

the bootstrap capacitor.

Synchronous Rectifier

The LM34925 provides an internal synchronous N-Channel MOSFET rectifier. This MOSFET provides a path for

the inductor current to flow when the high-side MOSFET is turned off.

The synchronous rectifier has no diode emulation mode, and is designed to keep the regulator in continuous

conduction mode even during light loads which would otherwise result in discontinuous operation. This feature

specifically allows the user to design a secondary regulator using a transformer winding off the main inductor to

generate the alternate regulated output voltage.

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