30015643
FIGURE 7. 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 RDS(ON) sensing. RDS(ON) sensing is far less accurate due
to the large variation of MOSFET RDS(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.
connected to the SS pin resulting in a gradual rise of FB and
the output voltage.
The peak current which triggers the current limit comparator
is:
Where tON 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 al-
lows the inductor ripple current to go negative by 0.5V / (A x
RS) when running full synchronous operation.
30015644
FIGURE 8. Diode Emulation Control
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 cur-
rent which triggers the current limit comparator becomes:
During this initial charging of CSS to the internal reference
voltage, the LM25116 will force diode emulation. That is, the
low-side MOSFET will turn off for the remainder of a cycle if
the sensed inductor current becomes negative. The inductor
current is sensed by monitoring the voltage between SW and
DEMB. As the SS capacitor continues to charge beyond
1.215V to 3V, the DEMB bias current will increase from 0µA
up to 40µA. With the use of an external DEMB resistor
(RDEMB), the current sense threshold for diode emulation will
increase resulting in the gradual transition to synchronous
operation. Forcing diode emulation during soft-start allows
the LM25116 to start up into a pre-biased output without un-
necessarily discharging the output capacitor. Full syn-
chronous operation is obtained if the DEMB pin is always
biased to a higher potential than the SW pin when LO is high.
RDEMB = 10kΩ will bias the DEMB pin to 0.45V minimum,
which is adequate for most applications. The DEMB bias po-
tential should always be kept below 2V. When RDEMB = 0Ω,
the LM25116 will always run in diode emulation.
This has the effect of a 10% fold-back of the peak current
during a short circuit when VCCX is powered from a 5V out-
put.
Soft-Start and Diode Emulation
The soft-start feature allows the regulator to gradually reach
the initial steady state operating point, thus reducing start-up
stresses and surges. The LM25116 will regulate the FB pin to
the SS pin voltage or the internal 1.215V reference, whichever
is lower. At the beginning of the soft-start sequence when SS
= 0V, the internal 10µA soft-start current source gradually in-
Once SS charges to 3V the SS latch is set, increasing the
DEMB bias current to 65µA. An amplifier is enabled that reg-
ulates SS to 160mV above the FB voltage. This feature can
prevent overshoot of the output voltage in the event the output
creases the voltage of an external soft-start capacitor (CSS
)
15
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