Datasheet
I
PEAK
=
1.22V
A x R
S
25 PA
C
RAMP
-
x t
ON
1.22V
A x R
S
,
I
PEAK
=
1.1V
A x R
S
25 PA
C
RAMP
-
x t
ON
1.1V
A x R
S
,
CSG
CS
I
L
LO
R
G
RAMP
1k
R
G
1k
R
S
+
-
10k
10k
0.5V
1.6V
C
RAMP
CURRENT LIMIT
COMPARATOR
g
m
x (VIN - VOUT) + 25 PA
CURRENT SENSE
AMPLIFIER
A =
10k
1k + R
G
-
+
-
+
HO
LM25116
SNVS509D –APRIL 2007–REVISED FEBRUARY 2013
www.ti.com
start cycle once UVLO charges back to 1.215V. This process is repeated until the fault is removed. The hiccup
off-time can be controlled by a capacitor to ground on the UVLO pin. In applications with low output inductance
and high input voltage, the switch current may overshoot due to the propagation delay of the current limit
comparator. If an overshoot should occur, the sample-and-hold circuit will detect the excess recirculating current.
If the sample-and-hold DC level exceeds the internal current limit threshold, the buck switch will be disabled and
skip pulses until the current has decayed below the current limit threshold. This approach prevents current
runaway conditions due to propagation delays or inductor saturation since the inductor current is forced to decay
following any current overshoot.
Figure 33. Current Limit and Ramp Circuit
Using a current sense resistor in the source of the low-side MOSFET provides superior current limit accuracy
compared to R
DS(ON)
sensing. R
DS(ON)
sensing is far less accurate due to the large variation of MOSFET R
DS(ON)
with temperature and part-to-part variation. The CS and CSG pins should be Kelvin connected to the current
sense resistor or MOSFET drain and source.
The peak current which triggers the current limit comparator is:
where
• t
ON
is the on-time of the high-side MOSFET. The 1.1V threshold is the difference between the 1.6V reference
at the current limit comparator and the 0.5V offset at the current sense amplifier. This offset at the current
sense amplifier allows the inductor ripple current to go negative by 0.5V / (A x R
S
) when running full
synchronous operation. (5)
Current limit hysteresis prevents chatter around the threshold when VCCX is powered from VOUT. When 4.5V <
VCC < 5.8V, the 1.6V reference is increased to 1.72V. The peak current which triggers the current limit
comparator becomes:
(6)
This has the effect of a 10% fold-back of the peak current during a short circuit when VCCX is powered from a
5V output.
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