Datasheet

LTC3810
15
3810fc
The basic LTC3810 application circuit is shown on the fi rst
page of this data sheet. External component selection is
primarily determined by the maximum input voltage and
load current and begins with the selection of the sense
resistance and power MOSFET switches. The LTC3810
uses either a sense resistor or the on-resistance of the
synchronous power MOSFET for determining the inductor
current. The desired amount of ripple current and operating
frequency largely determines the inductor value. Next, C
IN
is selected for its ability to handle the large RMS current
into the converter and C
OUT
is chosen with low enough
ESR to meet the output voltage ripple and transient
specifi cation. Finally, loop compensation components
are selected to meet the required transient/phase margin
specifi cations.
Maximum Sense Voltage and V
RNG
Pin
Inductor current is determined by measuring the volt-
age across a sense resistance that appears between the
SENSE
–
and SENSE
+
pins. The maximum sense voltage
is set by the voltage applied to the V
RNG
pin and is equal
to approximately:
V
SENSE(MAX)
= 0.173V
RNG
– 0.026
The current mode control loop will not allow the inductor
current valleys to exceed V
SENSE(MAX)
/R
SENSE
. In prac-
tice, one should allow some margin for variations in the
LTC3810 and external component values and a good guide
for selecting the sense resistance is:
R
SENSE
=
V
SENSE(MAX)
1.3 •I
OUT(MAX)
An external resistive divider from INTV
CC
can be used
to set the voltage of the V
RNG
pin between 0.5V and 2V
resulting in nominal sense voltages of 60mV to 320mV.
Additionally, the V
RNG
pin can be tied to SGND or INTV
CC
in which case the nominal sense voltage defaults to 95mV
or 215mV, respectively.
Connecting the SENSE
+
and SENSE
–
Pins
The LTC3810 can be used with or without a sense resis-
tor. When using a sense resistor, place it between the
source of the bottom MOSFET, M2, and PGND. Connect
the SENSE
+
and SENSE
–
pins to the top and bottom of
the sense resistor. Using a sense resistor provides a well
defi ned current limit, but adds cost and reduces effi ciency.
Alternatively, one can eliminate the sense resistor and use
the bottom MOSFET as the current sense element by simply
connecting the SENSE
+
pin to the lower MOSFET drain
and SENSE
–
pin to the MOSFET source. This improves
effi ciency, but one must carefully choose the MOSFET
on-resistance, as discussed below.
Power MOSFET Selection
The LTC3810 requires two external N-channel power
MOSFETs, one for the top (main) switch and one for the
bottom (synchronous) switch. Important parameters for
the power MOSFETs are the breakdown voltage BV
DSS
,
threshold voltage V
(GS)TH
, on-resistance R
DS(ON)
, input
capacitance and maximum current I
DS(MAX)
.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-resis-
tance. MOSFET on-resistance is typically specifi ed with
a maximum value R
DS(ON)(MAX)
at 25°C. In this case,
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
R
DS(ON)(MAX)
=
R
SENSE

T
The ρ
T
term is a normalization factor (unity at 25°C)
accounting for the signifi cant variation in on-resistance
with
temperature (see Figure 5) and typically varies
from 0.4%/
° C to 1.0%/ ° C depending on the particular
MOSFET used.
Figure 5. R
DS(ON)
vs Temperature
APPLICATIONS INFORMATION
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
3810 F05
0.5
0
0
50
100
2.0
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