Datasheet

STOP
RUN
RON/SD
Input
Voltage
LM34914
V
IN
R
ON
LM34914
SNVS453B –MAY 2006–REVISED MARCH 2013 www.ti.com
Shutdown
The LM34914 can be remotely shut down by taking the RON/SD pin below 0.8V. See Figure 9. In this mode the
SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD pin
allows the circuit to resume operation. The voltage at the RON/SD pin is normally between 1.5V and 3.0V,
depending on V
IN
and the R
ON
resistor.
Figure 9. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current flowing out of the ISEN
pin. Referring to the Typical Application Circuit and 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.
The valley current limit threshold is a function of the input voltage (V
IN
) and the output voltage sensed at FB, as
shown in the graph “Valley Current Limit Threshold vs. V
FB
and V
IN
”. This feature reduces the inductor current’s
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
the current limit threshold (V
OUT
is low due to current limiting).
Figure 10 illustrates the inductor current waveform during normal operation and in current limit. During the first
“Normal Operation” the load current is I
OUT1
, the average of the ripple waveform. As the load resistance is
reduced, the inductor current increases until it exceeds the current limit threshold. During the “Current Limited”
portion of Figure 10, the current limit threshold lowers since the high load current causes V
OUT
(and the voltage
at 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 LM34914 is in a constant current mode, with an average load current
equal to the current limit threshold + ΔI/2 (I
OUT2
). Normal operation resumes when the load current is reduced to
I
OUT3
, allowing V
OUT
, the current limit threshold, and the on-time to return to their normal values. Note that in the
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.
The peak current allowed through the buck switch, and the ISEN pin, is 2A, and the maximum allowed average
current is 1.5A.
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