STOP

RUN

RON/SD

Input

Voltage

LM34917A

LM34917A

www.ti.com

SNOSAX7C – DECEMBER 2007 – REVISED MAY 2008

Figure 4. Remote Shutdown

Input Over-Voltage Shutdown

If the input voltage at VIN increases above 34.8V an internal comparator disables the buck switch and the on-

when the soft-start voltage (at the SS pin) has reduced below 0.18V.

Current Limit

Current limit detection occurs during the off-time by monitoring the recirculating current flowing out of the ISEN

pin. Referring to the Block Diagram, during the off-time the inductor current flows through the load, into SGND,

through the internal sense resistor, out of ISEN and through D1 to the inductor. If that current exceeds the

current limit threshold the current limit comparator output delays the start of the next on-time period. The next on-

time starts when the current out of ISEN is below the threshold and the voltage at FB falls below 2.5V. The

operating frequency is typically lower due to longer-than-normal off-times.

peak value at high line and load. To further reduce the inductor’s peak current, the next cycle’s on-time is

reduced by approximately 50% if the voltage at FB is below its threshold when the inductor current reduces to

Figure 3 illustrates the inductor current waveform during normal operation and in current limit. During the first

reduced, the inductor current increases until it exceeds the current limit threshold. During the “Current Limited”

FB) to reduce. The on-time is reduced by approximately 50%, resulting in lower ripple amplitude for the inductor’s

current. During this time the LM34917A is in a constant current mode, with an average load current equal to the

current limit threshold +

second period of “Normal Operation”, even though the inductor’s peak current exceeds the current limit threshold

during part of each cycle, the circuit is not in current limit since the current falls below the threshold before the

feedback voltage reduces to its threshold to initiate the next on-time.

The peak current allowed through the buck switch, and the ISEN pin, is 2A, and the maximum allowed average

current is 1.5A.

Copyright © 2007–2008, Texas Instruments Incorporated

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