ltc3713 Linear Technology Corporation, ltc3713 Datasheet - Page 10

• Manufacturer Part Number: ltc3713
• Description: Low Input Voltage, High Power, No Rsense Synchronous Buck Dc/dc Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
switch cycle, turning on the switch. When the summation
of a signal representing switch current and a ramp gen-
erator (introduced to avoid subharmonic oscillations at
duty factors greater than 50%) exceeds the V
comparator A2 changes state, resetting the flip-flop and
turning off the switch. More power is delivered to the
output as switch current is increased. The output voltage,
attenuated by external resistor divider R7 and R8, appears
at the V
pensation is provided internally by R
APPLICATIO S I FOR ATIO
LTC3713
A typical LTC3713 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 LTC3713 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 (0.133)V
loop will not allow the inductor current valleys to exceed
(0.133)V
margin for variations in the LTC3713 and external com-
ponent values and a good guide for selecting the sense
resistance is:
An external resistive divider from INTV
set the voltage of the V
10
R
SENSE
FB2
+
and SENSE
RNG
IN
pin, closing the overall loop. Frequency com-
is selected for its ability to handle the large
/R
10 •
SENSE
U
I
V
OUT MAX
U
RNG
–
. In practice, one should allow some
pins. The maximum sense voltage
(
RNG
U
RNG
)
pin between 0.5V and 2V
. The current mode control
RNG
W
RNG
C
Pin
OUT
and C
CC
pin and is equal
can be used to
is chosen with
C
. Transient
U
C
signal,
response can be optimized by the addition of a phase lead
capacitor C
large value or low ESR output capacitors are used.
As the load current is decreased, the switch turns on for
a shorter period each cycle. If the load current is further
decreased, the boost converter will skip cycles to main-
tain output voltage regulation. If the V
increased significantly above 1.23V, the boost converter
will enter a low power state.
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 1.33 times this nominal value.
Connecting the SENSE
The LTC3713 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. Connect
the SENSE
sense resistor with SENSE
MOSFET and the 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
drain and the SENSE
MOSFET. This improves efficiency, but one must carefully
choose the MOSFET on-resistance as discussed in a later
section.
Applications Requiring Symmetric Current Limit
The I
corresponding to 0A. In applications in which the output
will only be sourcing current, this allows the output to sink
one third of the maximum source current. For applications
in which the output will be sourcing and sinking current,
it might be desirable to have a symmetrical output current
TH
voltage has a range of 0V to 2.4V with 0.8V
+
PL
and SENSE
in parallel with R7 in applications where
RNG
–
pin can be tied to SGND or INTV
–
+
pin to the source of the bottom
pins as a Kelvin connection to the
and SENSE
–
pin to PGND1. Using a sense
+
at the source of the bottom
–
Pins
FB2
pin voltage is
+
pin to the
3713fa
CC