LTC3708 LINER [Linear Technology], LTC3708 Datasheet - Page 13

• Manufacturer Part Number: LTC3708
• Description: Fast 2-Phase, No RSENSE Buck Controller with Output Tracking
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The basic LTC3708 application circuit is shown on the fi rst
page of this data sheet. External component selection is
primarily determined by the maximum load current and
begins with the selection of the power MOSFET switches
and/or sense resistor. For the LTC3708, the inductor cur-
rent is determined by the R
MOSFET or by a sense resistor when the user opts for
more accurate current sensing. The desired amount of
ripple current and operating frequency largely determines
the inductor value. Finally, C
handle the large RMS current into the converter and C
is chosen with low enough ESR to meet the output voltage
ripple specifi cation.
Maximum Sense Voltage and V
Inductor current is determined by measuring the voltage
across the R
a sense resistor that appears between the SENSE
SENSE
voltage applied to the V
V
inductor current valleys to exceed V
practice, one should allow some margin for variations in
the LTC3708 and external component values. A good guide
for selecting the sense resistance is:
The voltage of the V
resistive divider from V
in nominal sense voltages of 50mV to 200mV. Additionally,
the V
the nominal sense voltage defaults to 70mV or 140mV,
respectively. The maximum allowed sense voltage is about
1.4 times this nominal value.
Connecting the SENSE
The LTC3708 provides the user with an optional method
to sense current through a sense resistor instead of using
the R
sense resistor, it is placed between the source of the syn-
chronous MOSFET and ground. To measure the voltage
RNG
R
SENSE
/7. The current mode control loop will not allow the
RNG
DS(ON)
–
pins. The maximum sense voltage is set by the
pin can be tied to ground or V
=
DS(ON)
of the synchronous MOSFET. When using a
10 •
I
V
OUT MAX
of the synchronous MOSFET or through
RNG
RNG
RNG
(
CC
+
pin can be set using an external
and SENSE
pin and is equal to approximately
between 0.5V and 2V, resulting
)
IN
DS(ON)
is selected for its ability to
RNG
of the synchronous
RNG
–
Pin
Pins
CC
/(7 • R
, in which case
SENSE
+
). In
and
OUT
across this resistor, connect the SENSE
of the synchronous MOSFET and the SENSE
other end of the resistor. The SENSE
provide the Kelvin connections, ensuring accurate voltage
measurement across the resistor. Using a sense resistor
provides a well-defi ned current limit, but adds cost and
reduces effi ciency. Alternatively, one can use the synchro-
nous MOSFET as the current sense element by simply
connecting the SENSE
the SENSE
eliminating the sense resistor. This improves effi ciency,
but one must carefully choose the MOSFET on-resistance
as discussed below.
Power MOSFET Selection
Each output stage of the LTC3708 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
R
current, I
The gate drive voltage is set by the 5V DRV
Consequently, logic-level threshold MOSFETs must be
used in LTC3708 applications. If the driver’s 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-re-
sistance. MOSFET on-resistance is typically specifi ed
with a maximum value R
margin is required to accommodate the rise in MOSFET
on-resistance with temperature:
The ρ
counting for the signifi cant variation in on-resistance with
temperature, typically about 0.4%/°C. For a maximum
junction temperature of 100°C, using a value ρ
1.3 is reasonable (see Figure 3).
DS(ON)
R
DS ON MAX
T
(
term is a normalization factor (unity at 25°C) ac-
, reverse transfer capacitance, C
(BR)DSS
DS(MAX)
)(
–
pin to the source of the synchronous MOSFET,
)
, threshold voltage V
.
=
R
SENSE
ρ
+
T
pin to the switch node SW and
DS(ON)(MAX)
GS(TH)
+
at 25°C. Additional
RSS
+
and SENSE
LTC3708
pin to the source
, and maximum
, on-resistance
–
CC
pin to the
13
100°C
supply.
–
pins
3708fb
=