LTC3862 Datasheet, PDF (26/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
APPLICATIONS INFORMATION
The inductor saturation current rating needs to be higher
than the worst-case peak inductor current during an
overload condition. If I_O(MAX) is the maximum rated load
current, then the maximum current limit value (I_O(CL))
would normally be chosen to be some factor (e.g., 30%)
greater than I_O(MAX).
I_O(CL) = 1.3 • I_O(MAX)
Reflecting this back to the input, where the current is being
measured, and accounting for the ripple current, gives a
minimum saturation current rating for the inductor of:
I_L(SAT)
≥
1
n
•
⎛
⎝⎜
1+
χ
2
⎞
⎠⎟
•
1.3 • I_O(MAX)
1– D_MAX
The saturation current rating for the inductor should be
determined at the minimum input voltage (which results
in the highest duty cycle and maximum input current),
maximum output current and the maximum expected core
temperature. The saturation current ratings for most com-
mercially available inductors drop at high temperature. To
verify safe operation, it is a good idea to characterize the
inductor’s core/winding temperature under the following
conditions: 1) worst-case operating conditions, 2) maxi-
mum allowable ambient temperature and 3) with the power
supply mounted in the final enclosure. Thermal character-
ization can be done by placing a thermocouple in intimate
contact with the winding/core structure, or by burying the
thermocouple within the windings themselves.
Remember that a single-ended boost converter is not
short-circuit protected, and that under a shorted output
condition, the output current is limited only by the input
supply capability. For applications requiring a step-up
converter that is short-circuit protected, consider using
a SEPIC or forward converter topology.
Power MOSFET Selection
The peak-to-peak gate drive level is set by the INTV_CC volt-
age is 5V for the LTC3862 under normal operating condi-
tions. Selection criteria for the power MOSFETs include
the R_DS(ON), gate charge Q_G, drain-to-source breakdown
voltage BV_DSS, maximum continuous drain current I_D(MAX),
and thermal resistances R_TH(JA) and R_TH(JC)—both junc-
tion-to-ambient and junction-to-case.
The gate driver for the LTC3862 consists of PMOS pull-up
and NMOS pull-down devices, allowing the full INTV_CC
voltage to be applied to the gates during power MOSFET
switching. Nonetheless, care must be taken to ensure
that the minimum gate drive voltage is still sufficient to
full enhance the power MOSFET. Check the MOSFET data
sheet carefully to verify that the R_DS(ON) of the MOSFET
is specified for a voltage less than or equal to the nominal
INTV_CC voltage of 5V. For applications that require a power
MOSFET rated at 6V or 10V, please refer to the LTC3862-1
data sheet.
Also pay close attention to the BV_DSS specifications for
the MOSFETs relative to the maximum actual switch volt-
age in the application. Check the switching waveforms of
the MOSFET directly on the drain terminal using a single
probe and a high bandwidth oscilloscope. Ensure that the
drain voltage ringing does not approach the BV_DSS of the
MOSFET. Excessive ringing at high frequency is normally
an indicator of too much series inductance in the high di/dt
current path that includes the MOSFET, the boost diode,
the output capacitor, the sense resistor and the PCB traces
connecting these components. In some challenging ap-
plications it may be necessary to use a snubber in order
to limit the switch node dV/dt.
Finally, check the MOSFET manufacturer’s data sheet for
an avalanche energy rating (EAS). Some MOSFETs are not
rated for body diode avalanche and will fail catastrophi-
cally if the V_DS exceeds the device BV_DSS, even if only by
a fraction of a volt. Avalanche-rated MOSFETs are better
able to sustain high frequency drain-to-source ringing near
the device BV_DSS during the turn-off transition.
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be known.
This power dissipation is a function of the duty cycle, the
load current and the junction temperature itself (due to
the positive temperature coefficient of its R_DS(ON)). As a
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