9
LTC3717-1
sn37171 37171fs
APPLICATIO S I FOR ATIO
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the source of the bottom MOSFET M2 and ground. Con-
nect the
SENSE
+
and SENSE
–
pins as a Kelvin connection
to the sense resistor with SENSE
+
at the source of the
bottom MOSFET and the SENSE
–
pin to PGND. 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 MOSFET
as the current sense element by simply connecting the
SE
NSE
+
pin to the drain and the SENSE
–
pin to the source
of the bottom MOSFET. This improves efficiency, but one
must carefully choose the MOSFET on-resistance as dis-
cussed in a later section.
Power MOSFET Selection
The LTC3717-1 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 LTC3717-1 applications.
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 speci-
fied 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
R
DS ON MAX
SENSE
T
()( )
=
ρ
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 junction 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. During normal operation, the duty cycles
for the MOSFETs are:
A typical LTC3717-1 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 LTC3717-1 uses the on-resistance of the
synchronous power MOSFET for determining the induc-
tor current. 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 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
SENSE
+
and SENSE
–
pins. The maximum sense voltage
is set by the voltage applied to the V
RNG
pin and is equal
to approximately (0.13)V
RNG
for sourcing current and
(0.17)V
RNG
for sinking current. The current mode control
loop will not allow the inductor current valleys to exceed
(0.13)V
RNG
/R
SENSE
for sourcing current and (0.17)V
RNG
for sinking current. In practice, one should allow some
margin for variations in the LTC3717-1 and external
component values and a good guide for selecting the
sense resistance is:
R
V
I
SENSE
RNG
OUT MAX
=
10•
when V
RNG
= 0.5 – 2V.
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 1.3 times this nominal value for positive
output current and 1.7 times the nominal value for nega-
tive output current.
Connecting the SENSE
+
and SENSE
–
Pins
The LTC3717-1 can be used with or without a sense
resistor. When using a sense resistor, it is placed between
