Datasheet

LTC3813
13
3813fb
The basic LTC3813 application circuit is shown on the fi rst
page of this data sheet. External component selection is
primarily determined by the maximum input voltage and
load current and begins with the selection of the sense
resistance and power MOSFET switches. The LTC3813
uses either a sense resistor or 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. Next, C
OUT
is selected for its ability to handle the
large RMS current and with low enough ESR to meet the
output voltage ripple and transient specifi cation. Finally,
loop compensation components are selected to meet the
required transient/phase margin specifi cations.
Duty Cycle Considerations
For a boost converter, the duty cycle of the main switch is:
D= 1
V
IN
V
OUT
;D
MAX
= 1
V
IN(MIN)
V
OUT
The maximum V
OUT
capability of the LTC3813 is inversely
proportional to the minimum desired operating frequency
and minimum off-time:
V
OUT(MAX)
=
V
IN(MIN)
f
MIN
•t
OFF(MIN)
 100V
Maximum Sense Voltage and the V
RNG
Pin
The control circuit in the LTC3813 measures the input
current by using the R
DS(ON)
of the bottom MOSFET or
by using a sense resistor in the bottom MOSFET source,
so the output current needs to be refl ected back to the
input in order to dimension the power MOSFET properly
and to choose the maximum sense voltage. Based on the
fact that, ideally, the output power is equal to the input
power, the maximum average input current and average
inductor current is:
I
IN(MAX)
= I
L,AVG(MAX)
=
I
O(MAX)
1 D
MAX
The current mode control loop will not allow the induc-
tor peak to exceed V
SENSE(MAX)
/R
SENSE
. In practice, one
should allow some margin for variations in the LTC3813
and external component values, and a good guide for
selecting the maximum sense voltage when V
DS
sensing
is used is:
V
SENSE(MAX)
=
1.7 •R
DS(ON)
•I
O(MAX)
1 D
MAX
V
SENSE
is set by the voltage applied to the V
RNG
pin. Once
V
SENSE
is chosen, the required V
RNG
voltage is calculated
to be:
V
RNG
= 5.78 • (V
SENSE(MAX)
+ 0.026)
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 60mV to 320mV.
Additionally, the V
RNG
pin can be tied to SGND or INTV
CC
in which case the nominal sense voltage defaults to 95mV
or 215mV, respectively.
Connecting the SENSE
+
and SENSE
–
Pins
The LTC3813 can be used with or without a sense resis-
tor. When using a sense resistor, place it between the
source of the bottom MOSFET, M2, and PGND. Connect
the SENSE
+
and SENSE
–
pins to the top and bottom of
the sense resistor. Using a sense resistor provides a well
defi ned current limit, but adds cost and reduces effi ciency.
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 lower MOSFET drain
and SENSE
–
pin to the MOSFET source. This improves
effi ciency, but one must carefully choose the MOSFET
on-resistance, as discussed in the following section.
APPLICATIONS INFORMATION