LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 27

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
The inductor saturation current rating needs to be higher
than the worst-case peak inductor current during an
overload condition. If I
current, then the maximum current limit value (I
would normally be chosen to be some factor (e.g., 30%)
greater than I
Reflecting this back to the input, where the current is be-
ing measured, and accounting for the ripple current, gives
a minimum saturation current rating for the inductor of:
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
commercially 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 fol-
lowing conditions: 1) worst-case operating conditions,
2) maximum allowable ambient temperature and 3) with
the power supply mounted in the final enclosure. Thermal
characterization 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
voltage is 10V for the LTC3862-2 under normal operat-
ing conditions. Selection criteria for the power MOSFETs
include the R
breakdown voltage BV
I
I
O(CL)
L(SAT)
= 1.3 • I
≥
n
1
O(MAX)
• 1+
DS(ON)

 
O(MAX)
.
χ
2
, gate charge Q

 
O(MAX)
DSS
•
1.3 •I
, maximum continuous drain
1– D
is the maximum rated load
O(MAX)
MAX
G
, drain-to-source
O(CL)
CC
)
current I
R
The gate driver for the LTC3862-2 consists of PMOS pull-
up and NMOS pull-down devices, allowing the full INTV
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
is specified for a voltage less than or equal to the nominal
INTV
MOSFET rated at 5V, please refer to the LTC3862 data sheet.
Also pay close attention to the BV
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
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.
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
a fraction of a volt. Avalanche-rated MOSFETs are better
able to sustain high frequency drain-to-source ringing near
the device BV
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
result, some iterative calculation is normally required to
determine a reasonably accurate value.
TH(JC)
CC
—both junction-to-ambient and junction-to-case.
voltage of 10V. For applications that require a power
D(MAX)
DS
DSS
exceeds the device BV
, and thermal resistances R
during the turn-off transition.
DS(ON)
DSS
LTC3862-2
DSS
specifications for
, even if only by
of the MOSFET
DS(ON)
TH(JA)
DSS
27
). As a
of the
38622f
and
CC