LTC3717-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3717-1
Linear Technology
'LTC3717-1' PDF : 20 Pages View PDF
LTC3717-1
APPLICATIO S I FOR ATIO
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, CIN is selected for its ability to handle the large
RMS current into the converter and COUT is chosen with
low enough ESR to meet the output voltage ripple and
transient specification.
Maximum Sense Voltage and VRNG 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 VRNG pin and is equal
to approximately (0.13)VRNG for sourcing current and
(0.17)VRNG for sinking current. The current mode control
loop will not allow the inductor current valleys to exceed
(0.13)VRNG/RSENSE for sourcing current and (0.17)VRNG
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:
RSENSE
=
10
VRNG
• IOUT(MAX)
when VRNG = 0.5 – 2V.
An external resistive divider from INTVCC can be used to
set the voltage of the VRNG pin between 0.5V and 2V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the VRNG pin can be tied to SGND or INTVCC
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
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
SENSE + 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 RDS(ON), reverse
transfer capacitance CRSS and maximum current IDS(MAX).
The gate drive voltage is set by the 5V INTVCC 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 RDS(ON)(MAX) at 25°C. In this
case, additional margin is required to accommodate the
rise in MOSFET on-resistance with temperature:
RDS(ON)(MAX)
=
RSENSE
ρT
The ρT 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 ρT = 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:
sn37171 37171fs
9
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