Datasheet

'I =
(V
IN
- V
OUT
) x t
ON
L1
V
IN
STOP
R
ON
/SD
R
ON
Input
Voltage
LM5010
RUN
LM5010
SNVS307F –SEPTEMBER 2004–REVISED FEBRUARY 2013 www.ti.com
The inverse relationship of t
ON
vs. V
IN
results in a nearly constant frequency as V
IN
is varied. If the application
requires a high frequency the minimum value for t
ON
, and consequently R
ON
, is limited by the off-time (265 ns,
±15%) which limits the maximum duty cycle at minimum V
IN
. The tolerance for Equation 5 is ±25%. Frequencies
in excess of 1 MHz are possible with the LM5010.
Shutdown
The LM5010 can be remotely shut down by taking the R
ON
/SD pin below 0.65V. See Figure 11. In this mode the
soft start pin is internally grounded, the on-timer is disabled, and the input current at V
IN
is reduced (Figure 7).
Releasing the R
ON
/SD pin allows normal operation to resume. When the switch is open, the nominal voltage at
R
ON
/SD is shown in Figure 6.
Figure 11. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling
diode (D1). The detection threshold is 1.25A, ±0.25A. Referring to Typical Application Circuit and Block Diagram,
when the buck switch is off the inductor current flows through the load, into S
GND
, through the sense resistor, out
of I
SEN
and through D1. If that current exceeds the threshold the current limit comparator output switches to delay
the start of the next on-time period. The next on-time starts when the current out of I
SEN
is below the threshold
and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed the
threshold during each on-time, that is detected at the beginning of each off-time. The operating frequency is
lower due to longer-than-normal off-times.
Figure 12 illustrates the inductor current waveform. During normal operation the load current is I
O
, the average of
the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak attempts to
exceed the threshold. During the Current Limited portion of Figure 12, the current ramps down to the threshold
during each off-time, initiating the next on-time (assuming the voltage at FB is < 2.5V). During each on-time the
current ramps up an amount equal to:
(6)
During this time the LM5010 is in a constant current mode, with an average load current (I
OCL
) equal to the
threshold + ΔI/2.
The “valley current limit” technique allows the load current to exceed the current limit threshold as long as the
lower peak of the inductor current is less than the threshold.
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