LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 15

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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
is selected for its ability to handle the large RMS current
into the converter and C
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
Inductor current is determined by measuring the volt-
age across a sense resistance that appears between the
SENSE
is set by the voltage applied to the V
to approximately:
The current mode control loop will not allow the inductor
current valleys to exceed V
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:
An external resistive divider from INTV
to set the voltage of the V
resulting in nominal sense voltages of 60mV to 320mV.
Additionally, the V
in which case the nominal sense voltage defaults to 95mV
or 215mV, respectively.
Connecting the SENSE
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
V
R
SENSE(MAX)
SENSE
–
and SENSE
=
+
and SENSE
1.3 •I
V
= 0.173V
SENSE(MAX)
RNG
OUT(MAX)
+
pins. The maximum sense voltage
pin can be tied to SGND or INTV
+
–
RNG
OUT
and SENSE
pins to the top and bottom of
RNG
SENSE(MAX)
is chosen with low enough
– 0.026
pin between 0.5V and 2V
RNG
RNG
–
Pin
Pins
/R
CC
pin and is equal
SENSE
can be used
. In prac-
CC
IN
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
and SENSE
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
threshold voltage V
capacitance and maximum current I
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
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
The ρ
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.
R
DS(ON)(MAX)
T
term is a normalization factor (unity at 25°C)
0.5
2.0
1.5
1.0
–
0
–50
pin to the MOSFET source. This improves
Figure 5. R
=
R
JUNCTION TEMPERATURE (°C)
(GS)TH
SENSE
0
DS(ON)(MAX)
T
+
DS(ON)
pin to the lower MOSFET drain
, on-resistance R
50
vs Temperature
at 25°C. In this case,
100
DS(MAX)
LTC3810
3810 F05
.
150
DS(ON)
15
, input
DSS
3810fb
,