LTC3836
21
3836fb
APPLICATIONS INFORMATION
Using a Sense Resistor
A sense resistor R
SENSE
can be connected between V
IN
and SW to sense the output load current. In this case, the
drain of the topside N-channel MOSFET is connected to
SENSE
–
pin and the source is connected to the SW pin of
the LTC3836. Therefore, the current comparator monitors
the voltage developed across R
SENSE
, not the V
DS
of the
top MOSFET. The output current that the LTC3836 can
provide in this case is given by:
I
OUT(MAX)
=
V
SENSE(MAX)
R
DS(ON)
–
I
RIPPLE
2
Setting ripple current as 40% of I
OUT(MAX)
and using
Figure 1 to choose SF, the value of R
SENSE
is:
R
SENSE
=
5
6
•SF•
V
SENSE(MAX)
I
OUT(MAX)
Variation in the resistance of a sense resistor is much
smaller than the variation in on-resistance of an external
MOSFET. Therefore the load current is well controlled with
a sense resistor. However the sense resistor causes extra
I
2
R losses in addition to those of the MOSFET. Therefore,
using a sense resistor lowers the effi ciency of LTC3836,
especially at high load currents.
Low Supply Operation
Although the LTC3836 can function down to below 2.4V,
the maximum allowable output current is reduced as
V
IN
decreases below 3V. Figure 12 shows the amount of
change as the supply is reduced down to 2.4V. Also
shown is the effect on V
REF
.
Minimum On-Time Considerations
Minimum on-time, t
ON(MIN)
, is the smallest amount of time
that the LTC3836 is capable of turning the main N-chan-
nel MOSFET on and then off. It is determined by internal
timing delays and the gate charge required to turn on the
top MOSFET. Low duty cycle and high frequency applica-
tions may approach the minimum on-time limit and care
should be taken to ensure that:
t
ON(MIN)
<
V
OUT
f
OSC
•V
IN
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3836 will begin to skip
cycles (unless forced continuous mode is selected). The
output voltage will continue to be regulated, but the ripple
current and ripple voltage will increase. The minimum on-
time for the LTC3836 is typically about 200ns. However,
as the peak sense voltage (I
L(PEAK)
• R
DS(ON)
) decreases,
the minimum on-time gradually increases up to about
250ns. This is of particular concern in forced continuous
applications with low ripple current at light loads. If forced
continuous mode is selected and the duty cycle falls below
the minimum on-time requirement, the output will be
regulated by overvoltage protection.
Figure 12. Line Regulation of V
REF
and
Maximum Sense Voltage for Low Input Supply
INPUT VOLTAGE (V)
75
NORMALIZED VOLTAGE OR CURRENT (%)
85
95
105
80
90
100
2.2 2.4 2.6 2.8
3836 F12
3.02.12.0 2.3 2.5 2.7 2.9
V
REF
MAXIMUM
SENSE VOLTAGE
