LTC3709 Linear Technology, LTC3709 Datasheet - Page 12

• Manufacturer Part Number: LTC3709
• Description: No RSENSE Synchronous DC/DC Controller
• Manufacturer: Linear Technology

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LTC3709
APPLICATIO S I FOR ATIO
12
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
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
ability to handle the large RMS current into the converter
and C
output voltage ripple specification.
Maximum Sense Voltage and V
Inductor current is determined by measuring the voltage
across the R
a sense resistance that appears between the SENSE
the SENSE
voltage applied to the V
mately V
allow the inductor current valleys to exceed V
R
variations in the LTC3709 and external component values.
A good guide for selecting the sense resistance for each
channel is:
The voltage of the V
resistive divider from V
in nominal sense voltages of 50mV to 200mV. Addition-
ally, the V
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
The LTC3709 provides the user with an optional method to
sense current through a sense resistor instead of using the
R
resistor, it is placed between the source of the synchro-
nous MOSFET and ground. To measure the voltage across
SENSE
DS(ON)
R
SENSE
OUT
). In practice, one should allow some margin for
of the synchronous MOSFET. When using a sense
RNG
RNG
is chosen with low enough ESR to meet the
+
=
pins. The maximum sense voltage is set by the
DS(ON)
/7.5. The current mode control loop will not
10
pin can be tied to ground or V
2
•
I
•
of the synchronous MOSFET or through
OUT MAX
U
V
RNG
DS(ON)
RNG
(
CC
+
RNG
pin can be set using an external
and SENSE
U
between 0.5V and 2V resulting
)
of the synchronous MOSFET
pin and is equal to approxi-
RNG
W
IN
–
Pin
Pins
is selected for its
CC
U
RNG
, in which
/(7.5 •
–
and
this resistor, connect the SENSE
the resistor and the SENSE
resistor. The SENSE
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
SENSE
eliminating the sense resistor. This improves efficiency,
but one must carefully choose the MOSFET on-resistance
as discussed in the Power MOSFET Selection section.
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
reverse transfer capacitance C
I
The gate drive voltage is set by the 5V DRV
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
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
The ρ
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
DS(MAX)
(BR)DSS
100°C
R
DS ON MAX
T
(
–
= 1.3 is reasonable (Figure 1).
.
term is a normalization factor (unity at 25°C)
, threshold voltage V
pin to the source of the synchronous MOSFET,
)(
)
=
R
+
SENSE
ρ
+
and SENSE
T
pin to the switch node SW and the
DS(ON)(MAX)
–
(GS)TH
pin to the other end of the
RSS
+
pin to the source end of
–
, on-resistance R
and maximum current
pin connections pro-
at 25°C. In this case
CC
supply.
DS(ON)
3709f
,