LTC3814-5_15 Datasheet, PDF (11/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
OPERATION
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 flexibility to
choose a BV_DSS as high as necessary.
APPLICATIONS INFORMATION
The basic LTC3814-5 application circuit is shown on the
first 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 specification. Finally,
loop compensation components are selected to meet the
required transient/phase margin specifications.
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 )}
=
f
V_IN(MIN)
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 reflected 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 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.
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