Datasheet

LTC3708
13
3708fb
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
DS(ON)
of the synchronous
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
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 specifi cation.
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 resistor that appears between the SENSE
+
and
SENSE
–
pins. The maximum sense voltage is set by the
voltage applied to the V
RNG
pin and is equal to approximately
V
RNG
/7. The current mode control loop will not allow the
inductor current valleys to exceed V
RNG
/(7 • R
SENSE
). In
practice, one should allow some margin for variations in
the LTC3708 and external component values. A good guide
for selecting the sense resistance is:
R
V
I
SENSE
RNG
OUT MAX
=
10 •
()
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. Additionally,
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 voltage is about
1.4 times this nominal value.
Connecting the SENSE
+
and SENSE
–
Pins
The LTC3708 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 syn-
chronous MOSFET and ground. To measure the voltage
across this resistor, connect the SENSE
+
pin to the source
of the synchronous MOSFET and the SENSE
–
pin to the
other end of the resistor. The SENSE
+
and SENSE
–
pins
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
+
pin to the switch node SW and
the SENSE
–
pin to the source of the synchronous MOSFET,
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
(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 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
DS(ON)(MAX)
at 25°C. Additional
margin is required to accommodate the rise in MOSFET
on-resistance with temperature:
R
R
DS ON MAX
SENSE
T
()( )
=
ρ
The ρ
T
term is a normalization factor (unity at 25°C) ac-
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
100°C
=
1.3 is reasonable (see Figure 3).