LTC3709 Datasheet, PDF (12/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

LTC3709
APPLICATIO S I FOR ATIO
The basic LTC3709 application circuit is shown on the
first page of this data sheet. External component selec-
tion is primarily determined by the maximum load cur-
rent and begins with the selection of the power MOSFET
switches and/or sense resistor. The inductor current is
determined by the R_DS(ON) of the synchronous MOSFET
while the user has the option to use a sense resistor for
a more accurate current limiting. The desired amount of
ripple current and operating frequency largely deter-
mines the inductor value. Finally, 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 specification.
this resistor, connect the SENSE⁺ pin to the source end of
the resistor and the SENSE^– pin to the other end of the
resistor. The SENSE⁺ and SENSE^– pin connections pro-
vide the Kelvin connections, ensuring accurate voltage
measurement across the resistor. Using a sense resistor
provides a well-defined current limit, but adds cost and
reduces efficiency. Alternatively, one can use the synchro-
nous MOSFET as the current sense element by simply
connecting the SENSE⁺ pin to the switch node SW and the
SENSE^– pin to the source of the synchronous MOSFET,
eliminating the sense resistor. This improves efficiency,
but one must carefully choose the MOSFET on-resistance
as discussed in the Power MOSFET Selection section.
Maximum Sense Voltage and V_RNG Pin
Inductor current is determined by measuring the voltage
across the R_DS(ON) of the synchronous MOSFET or through
a sense resistance that appears between the SENSE^– and
the SENSE⁺ pins. The maximum sense voltage is set by the
voltage applied to the V_RNG pin and is equal to approxi-
mately V_RNG/7.5. The current mode control loop will not
allow the inductor current valleys to exceed V_RNG/(7.5 •
R_SENSE). In practice, one should allow some margin for
variations in the LTC3709 and external component values.
A good guide for selecting the sense resistance for each
channel is:
R_SENSE
=
2 • V_RNG
10 • I_OUT(MAX)
The voltage of the V_RNG pin can be set using an external
resistive divider from V_CC between 0.5V and 2V resulting
in nominal sense voltages of 50mV to 200mV. Addition-
ally, the V_RNG pin can be tied to ground or V_CC, in which
case the nominal sense voltage defaults to 70mV or
140mV, respectively. The maximum allowed sense volt-
age is about 1.3 times this nominal value.
Connecting the SENSE⁺ and SENSE^– Pins
The LTC3709 provides the user with an optional method to
sense current through a sense resistor instead of using the
R_DS(ON) of the synchronous MOSFET. When using a sense
resistor, it is placed between the source of the synchro-
nous MOSFET and ground. To measure the voltage across
Power MOSFET Selection
The LTC3709 requires four external N-channel power
MOSFETs, two for the top (main) switches and two for the
bottom (synchronous) switches. Important parameters
for the power MOSFETs are the breakdown voltage
V_(BR)DSS, threshold voltage V_(GS)TH, on-resistance R_DS(ON),
reverse transfer capacitance C_RSS and maximum current
I_DS(MAX).
The gate drive voltage is set by the 5V DRV_CC supply.
Consequently, logic-level threshold MOSFETs must be
used in LTC3709 applications. If the driver’s voltage is
expected to drop below 5V, then sub-logic level threshold
MOSFETs should be used.
When the bottom MOSFETs are used as the current sense
elements, particular attention must be paid to their on-
resistance. 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, typically about 0.4%/°C. Junction-to-
case temperature is about 20°C in most applications. For
a maximum junction temperature of 100°C, using a value
ρ_100°C = 1.3 is reasonable (Figure 1).
3709f
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