Datasheet

'I =
(V
IN
- V
OUT
) x t
ON
L1
VIN
STOP
RUN
R
ON
Input
Voltage
LM5010A
RON/SD
t
ON(min)
=
V
OUT
x 300 ns
(V
IN(min)
± V
OUT
)
LM5010A
LM5010A-Q1
SNVS376E –OCTOBER 2005–REVISED FEBRUARY 2013 www.ti.com
(8)
Shutdown
The LM5010A can be remotely shut down by forcing the RON/SD pin below 0.7V with a switch or open drain
device. See Figure 9. In the shutdown mode the SS pin is internally grounded, the on-time one-shot is disabled,
the input current at VIN is reduced, and the V
CC
bypass switch is turned off. The V
CC
regulator is not disabled in
the shutdown mode. Releasing the RON/SD pin allows normal operation to resume. The nominal voltage at
RON/SD is shown in the Typical Performance Characteristics. When switching the RON/SD pin, the transition
time should be faster than one to two cycles of the regulator’s nominal switching frequency.
Figure 9. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the internal
current sense resistor (R
SENSE
). The detection threshold is 1.25A, ±0.25A. Referring to the Block Diagram, if the
current into SGND during the off-time exceeds the threshold level the current limit comparator delays the start of
the next on-time period. The next on-time starts when the current into SGND is below the threshold and the
voltage at FB is below 2.5V. Figure 10 illustrates the inductor current waveform during normal operation and
during current limit. The output current I
O
is the average of the inductor ripple current waveform. The Low Load
Current waveform illustrates continuous conduction mode operation with peak and valley inductor currents below
the current limit threshold. When the load current is increased (High Load Current), the ripple waveform
maintains the same amplitude and frequency since the current falls below the current limit threshold at the valley
of the ripple waveform. Note the average current in the High Load Current portion of Figure 10 is above the
current limit threshold. Since the current reduces below the threshold in the normal off-time each cycle, the start
of each on-time is not delayed, and the circuit’s output voltage is regulated at the correct value. When the load
current is further increased such that the lower peak would be above the threshold, the off-time is lengthened to
allow the current to decrease to the threshold before the next on-time begins (Current Limited portion of
Figure 10). Both V
OUT
and the switching frequency are reduced as the circuit operates in a constant current
mode. The load current (I
OCL
) is equal to the current limit threshold plus half the ripple current (ΔI/2). The ripple
amplitude (ΔI) is calculated from:
(9)
The current limit threshold can be increased by connecting an external resistor (R
CL
) between SGND and ISEN.
R
CL
typically is less than 1Ω, and the calculation of its value is explained in Applications Information. If the
current limit threshold is increased by adding R
CL
, the maximum continuous load current should not exceed 1.5A,
and the peak current out of the SW pin should not exceed 2A.
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