Datasheet

LTC3814-5
11
38145fc
The basic LTC3814-5 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 power
MOSFET switches. The LTC3814-5 uses the on-resistance
of the synchronous power MOSFET for determining the
inductor 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 is chosen 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 LTC3814-5 is inversely
proportional to the minimum desired operating frequency
and minimum off-time:
V
OUT(MAX)
=
V
IN(MIN)
f
MIN
•t
OFF(MIN)
≤60V
Maximum Sense Voltage and the V
RNG
Pin
The control circuit in the LTC3814-5 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
OPERATION
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 LTC3814-
5 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.
IC/Driver Supply Power
The LTC3814-5’s internal control circuitry and top and bot-
tom MOSFET drivers operate from a supply voltage (INTV
CC
pin) in the range of 4.5V to 14V. If the input supply voltage
or another available supply is within this voltage range it
can be used to supply IC/driver power. If a supply in this
range is not available, two internal regulators are available
to generate a 5.5V supply from the input or output. An
internal low dropout regulator is good for voltages up to
15V, and the second, a linear regulator controller, controls
the gate of an external NMOS to generate the 5.5V supply.
Since the NMOS is external, the user has the fl exibility to
choose a BV
DSS
as high as necessary.
APPLICATIONS INFORMATION