LTC3836_15 Datasheet, PDF (21/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

LTC3836
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
I²R losses in addition to those of the MOSFET. Therefore,
using a sense resistor lowers the efficiency 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.
105
V_REF
100
95
MAXIMUM
SENSE VOLTAGE
90
85
80
75
2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0
INPUT VOLTAGE (V)
3836 F12
Figure 12. Line Regulation of V_REF and
Maximum Sense Voltage for Low Input Supply
3836fb
21