LTC3812-5_15 Datasheet, PDF (14/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
The basic LTC3812-5 application circuit is shown on the
first 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 specification. Finally, loop compensation
components are selected to meet the required transient/
phase margin specifications.
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 specified 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<sub>SENSE

T</sub>
The ρ_T term is a normalization factor (unity at 25°C)
accounting for the significant 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.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
38125 F05
Figure 5. R_DS(ON) vs Temperature
38125fc
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