LTC3810-5 Datasheet, PDF (15/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
The basic LTC3810-5 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 sense
resistance and power MOSFET switches. The LTC3810-5
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
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 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
LTC3810-5 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.
Connecting the SENSE⁺ and SENSE^– Pins
The LTC3810-5 can be used with or without a sense re-
sistor. 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
defined current limit, but adds cost and reduces efficiency.
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
efficiency, but one must carefully choose the MOSFET
on-resistance, as discussed below.
Power MOSFET Selection
The LTC3810-5 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).
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 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_SENSE
ρ_T
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)
38105 F05
Figure 5. R_DS(ON) vs Temperature
38105fc
15