ltc3717-1 Linear Technology Corporation, ltc3717-1 Datasheet - Page 9

• Manufacturer Part Number: ltc3717-1
• Description: Wide Operating Range, No Rsense Step-down Controller For Ddr/qdr Memory Termination
• Manufacturer: Linear Technology Corporation

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, C
RMS current into the converter and C
low enough ESR to meet the output voltage ripple and
transient specification.
Maximum Sense Voltage and V
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the
SENSE
is set by the voltage applied to the V
to approximately (0.13)V
(0.17)V
loop will not allow the inductor current valleys to exceed
(0.13)V
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:
when V
An external resistive divider from INTV
set the voltage of the V
resulting in nominal sense voltages of 50mV to 200mV.
Additionally, the V
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
The LTC3717-1 can be used with or without a sense
resistor. When using a sense resistor, it is placed between
R
SENSE
+
RNG
RNG
RNG
and SENSE
IN
/R
is selected for its ability to handle the large
for sinking current. The current mode control
= 0.5 – 2V.
10 •
SENSE
I
V
OUT MAX
RNG
RNG
U
for sourcing current and (0.17)V
–
(
pins. The maximum sense voltage
pin can be tied to SGND or INTV
+
RNG
and SENSE
U
RNG
)
pin between 0.5V and 2V
for sourcing current and
RNG
W
RNG
–
Pin
OUT
Pins
CC
pin and is equal
can be used to
is chosen with
U
RNG
CC
the source of the bottom MOSFET M2 and ground. Con-
nect the SENSE
to the sense resistor with SENSE
bottom MOSFET and the SENSE
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
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
threshold voltage V
transfer capacitance C
The gate drive voltage is set by the 5V INTV
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
case, additional margin is required to accommodate the
rise in MOSFET on-resistance with temperature:
The
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
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:
R
DS ON MAX
T
(
+
term is a normalization factor (unity at 25 C)
pin to the drain and the SENSE
)(
T
+
)
= 1.3 is reasonable.
and SENSE
(GS)TH
R
SENSE
RSS
T
, on-resistance R
and maximum current I
–
DS(ON)(MAX)
pins as a Kelvin connection
–
+
pin to PGND. Using a
at the source of the
LTC3717-1
–
pin to the source
at 25 C. In this
DS(ON)
CC
sn37171 37171fs
, reverse
DS(MAX)
(BR)DSS
supply.
9
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.