LTC3714 Linear, LTC3714 Datasheet - Page 12

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
LTC3714
The basic LTC3714 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 LTC3714 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
ability to handle the large RMS current into the converter
and C
output voltage ripple and transient specification.
Maximum Sense Voltage and V
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
mately (0.133)V
not allow the inductor current valleys to exceed
(0.133)V
margin for variations in the LTC3714 and external compo-
nent values and a good guide for selecting the sense
resistance is:
An external resistive divider from INTV
set the voltage of the V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V
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 LTC3714 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. Alternatively,
one can eliminate the sense resistor and use the bottom
12
R
SENSE
OUT
RNG
is chosen with low enough ESR to meet the
/R
10 •
SENSE
RNG
I
V
RNG
OUT MAX
U
RNG
. The current mode control loop will
. In practice, one should allow some
(
pin can be tied to SGND or INTV
RNG
RNG
U
)
pin and is equal to approxi-
pin between 0.5V and 2V
RNG
W
IN
Pin
is selected for its
CC
can be used to
U
CC
MOSFET as the current sense element by simply connect-
ing 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 LTC3714 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
threshold voltage V
transfer capacitance C
The gate drive voltage is set by the 5V INTV
Consequently, logic-level threshold MOSFETs must be
used in LTC3714 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
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
The
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/ C as shown in
Figure 3. Junction-to-case temperature is about 30 C in
most applications. For a maximum ambient temperature
of 70 C, using a value
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC3714 is operating in
continuous mode, the duty cycles for the MOSFETs are:
R
D
D
DS ON MAX
BOT
TOP
T
(
term is a normalization factor (unity at 25 C)
)(
V
V
V
IN
OUT
IN
–
V
)
IN
V
OUT
(GS)TH
R
SENSE
RSS
T
100 C
DS(ON)(MAX)
, on-resistance R
and maximum current I
= 1.3 is reasonable.
at 25 C. In this case,
DS(ON)
CC
, reverse
DS(MAX)
(BR)DSS
supply.
3714f
,
.