LTC3711 Datasheet, PDF (10/24 Pages) Linear Technology – 5-Bit Adjustable, Wide Operating Range, No RSENSE

LTC3711
APPLICATIO S I FOR ATIO
The basic LTC3711 application circuit is shown in
Figure 1. External component selection is primarily de-
termined by the maximum load current and begins with
the selection of the sense resistance and power MOSFET
switches. The LTC3711 can use 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 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 and transient specification.
Maximum Sense Voltage and V_RNG Pin
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the PGND
and SENSE⁺ pins. The maximum sense voltage is set by
the voltage applied to the V_RNG pin and is equal to
approximately (0.133)V_RNG. The current mode control
loop will not allow the inductor current valleys to exceed
(0.133)V_RNG/R_SENSE. In practice, one should allow some
margin for variations in the LTC3711 and external compo-
nent values and a good guide for selecting the sense
resistance is:
R_SENSE
=
10
V_RNG
• 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 50mV to 200mV.
Additionally, the V_RNG pin can be tied to SGND or INTV_CC
in which case the nominal sense voltage defaults to 70mV
or 140mV, respectively. The maximum allowed sense
voltage is about 1.33 times this nominal value.
Connecting the SENSE⁺ Pin
The LTC3711 can be used with or without a sense resistor.
When using a sense resistor, it is placed between the
source of the bottom MOSFET M2 and ground. Connect
the SENSE⁺ pin to the source of the bottom MOSFET so
that the resistor appears between the SENSE⁺ and PGND
pins. Using a sense resistor provides a well defined
current limit, but adds cost and reduces efficiency. Alter-
natively, one can eliminate the sense resistor and use the
10
bottom MOSFET as the current sense element by simply
connecting the SENSE⁺ pin to the switch node SW at the
drain of the bottom MOSFET. This improves efficiency, but
one must carefully choose the MOSFET on-resistance as
discussed below.
Power MOSFET Selection
The LTC3711 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 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 INTV_CC supply.
Consequently, logic-level threshold MOSFETs must be
used in LTC3711 applications. If the input voltage is
expected to drop below 5V, then sub-logic level threshold
MOSFETs should be considered.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its 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 as shown in
Figure 2. For a maximum temperature of 100°C, using a
value ρ_T = 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC3711 is operating in
continuous mode, the duty cycles for the MOSFETs are:
D_TOP
=
V_OUT
V_IN
D_BOT
=
V_IN
– V_OUT
V_IN
3711f