Datasheet

'I =
(V
OUT
+ V
FD
+ V
ESR
) x t
OFF
L
'I =
(V
IN
- V
OUT
) x t
ON
L
t
OFF(CL)
=
(V
FB
x 0.93) + 0.28V
4.1 x 10
-6
x ((V
IN
/31) + 0.15)
LM5085, LM5085-Q1
SNVS565G –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com
CURRENT LIMITING
The LM5085 current limiting operates by sensing the voltage across either the R
DS(ON)
of Q1, or a sense resistor,
during the on-time and comparing it to the voltage across the resistor R
ADJ
(see Figure 26). 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.
If the voltage across R
DS(ON)
of Q1, or the sense resistor, is greater than the voltage across R
ADJ
, the current limit
comparator switches to turn off Q1. Current sensing is disabled for a blanking time of ≊100ns at the beginning of
the on-time to prevent false triggering of the current limit comparator due to leading edge current spikes.
Because of the blanking time and the turn-on and turn-off delays created by the PFET, the on-time at the PGATE
pin should not be set less than 150ns. An on-time shorter than that may prevent the current limit detection circuit
from properly detecting an over-current condition. The duration of the subsequent forced off-time is a function of
the input voltage and the voltage at the FB pin, as shown in the graph “Off-time vs. V
IN
and V
FB
”. 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 forced off-time, provides effective protection from output load faults over
a wide range of operating conditions.
The voltage across the R
ADJ
resistor is set by an internal 40µA current sink at the ADJ pin. When using Q1’s
R
DS(ON)
for sensing, the current at which the current limit comparator switches is calculated from:
I
CL
= 40µA x R
ADJ
/R
DS(ON)
(8)
When using a sense resistor (R
SEN
) the threshold of the current limit comparator is calculated from:
I
CL
= 40µA x R
ADJ
/R
SEN
(9)
When using Equation 8 or Equation 9, the tolerances for the ADJ pin current sink and the offset of the current
limit comparator should be included to ensure the resulting minimum current limit is not less than the required
maximum switch current. Simultaneously increasing the values of R
ADJ
and R
SEN
decreases the effects of the
current limit comparator offset, but at the expense of higher power dissipation. When using a sense resistor, the
R
SEN
resistor value should be chosen within the practical limitations of power dissipation and physical size. For
example, for a 10A current limit, setting R
SEN
= 0.005Ω results in a power dissipation as high as 0.5W. Current
sense connections to the R
SEN
resistor, or to Q1, must be Kelvin connections to ensure accuracy.
The C
ADJ
capacitor filters noise from the ADJ pin, and helps prevent unintended switching of the current limit
comparator due to input voltage transients. The recommended value for C
ADJ
is 1000pF.
CURRENT LIMIT OFF-TIME
When the current through Q1 exceeds the current limit threshold, the LM5085 forces an off-time longer than the
normal off-time defined by Equation 1. See the graph “Off-Time vs. V
IN
and V
FB
”, or calculate the current limit off-
time from the following equation:
(10)
where V
IN
is the input voltage, and V
FB
is the voltage at the FB pin at the time current limit was detected. This
feature is necessary to allow the inductor current to decrease sufficiently to offset the current increase which
occurred during the on-time. During the on-time, the inductor current increases an amount equal to:
(11)
During the off-time the inductor current decreases due to the reverse voltage applied across the inductor by the
output voltage, the freewheeling diode’s forward voltage (V
FD
), and the voltage drop due to the inductor’s series
resistance (V
ESR
). The current decrease is equal to:
(12)
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