Datasheet

LTC3812-5
14
38125fc
The basic LTC3812-5 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 power
MOSFET switches. The LTC3812-5 uses 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 voltage
across a sense resistance (the on-resistance of the bottom
MOSFET) that appears between the PGND and SW 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
LTC3812-5 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.
POWER MOSFET SELECTION
The LTC3812-5 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)
.
Since 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
38125 F05
0.5
0
0
50
100
2.0