Datasheet

I
CL(min)
=
0.01:
(2.05 k: x 32 PA) - 9 mV
= 5.66A
I
CL(max)
=
0.01:
(2.05 k: x 48 PA) + 9 mV
= 10.74A
t
OFF(CL)
=
(V
FB
x 0.93) + 0.56V
8 x 10
-6
x ((V
IN
/31) + 0.15)
www.ti.com Current Limit
5 Current Limit
The LM25085A peak current limit detection operates by sensing the voltage across either the R
DS(ON)
of
Q1, or a sense resistor (R5), during the on-time and comparing it to the voltage across R3 at the ADJ pin.
The current limit threshold is reached when the sensed voltage exceeds the voltage across R3. When
current limit is reached Q1 is immediately switched off. The current limit function is much more accurate
and stable over temperature when a sense resistor is used. The R
DS(ON)
of a MOSFET has a wide process
variation and a large temperature coefficient.
Current sensing is disabled for a blanking time of ≊100 ns at the beginning of each on-time to prevent
false triggering of the current limit comparator due to leading edge current spikes. After Q1 is turned off
due to current limit detection, Q1 is held off for a longer-than-normal off-time. The extended off-time is a
function of the input voltage and the voltage at the FB pin, as shown below in the graph “Current Limit Off-
time vs. V
IN
and V
FB
”. The current limit off-time can be calculated from the following:
(2)
The longer-than-normal forced off-time allows the inductor current to decrease to a low level before the
next on-time. This cycle-by-cycle monitoring, followed by a long forced off-time, provides effective
protection from output load faults over a wide range of operating conditions.
Figure 2. Current Limit Off-time vs. V
IN
and V
FB
A) Sense resistor method – This evaluation board is supplied configured for the sense resistor method
of current limit detection. Jumpers A-B are in place at both jumper locations (JP1, JP2), which connects
the ADJ pin resistor (R3) and the ISEN pin across the sense resistor (R5). If the voltage across R5
exceeds the voltage across R3 during the on-time, the current limit comparator switches to turn off Q1.
The voltage across R3 is set by an internal 40 µA current sink at the ADJ pin. The current at which the
current limit comparator switches is calculated from:
I
CL
= 40 µA x R3/R5 (3)
With R5 = 10 mΩ and R3 = 2.05 kΩ, the nominal current limit threshold calculates to 8.2A. Since that is
the peak of the inductor current waveform, the load current is equal to that peak value minus one half the
ripple current amplitude. At Vin = 4.5V, the ripple amplitude is ≊622 mAp-p, and the load current at current
limit is equal to 7.89A. At Vin = 24V, the ripple amplitude is ≊851 mAp-p, and the load current at current
limit is equal to ≊7.77A.
Using the tolerances for the ADJ pin current and the current limit comparator offset, the maximum current
limit threshold calculates to:
(4)
and the load current at current limit calculates to 10.43A at 4.5V, and 10.32A at 24V. The minimum
current limit thresholds calculate to:
(5)
3
SNVA384B–February 2009–Revised April 2013 AN-1933 LM25085A Evaluation Board
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