Datasheet

LTC3862
26
3862fc
For more information www.linear.com/LTC3862
applicaTions inForMaTion
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
n
I
D
L SAT
OMAX
MAX
()
()
••
.•
–
≥+






1
1
2
13
1
χ
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
bur
ying the thermocouple within the windings themselves.
Remember that a single-ended boost converter is not
short-cir
cuit 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
voltage is 5V for the LTC3862 under normal operating con-
ditions. 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
junction-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 voltage
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.
The GATE of MOSFET Q1 could experience transient
voltage spikes during turn-on and turn-off of the MOS
-
FET, due to parasitic lead inductance and improper PCB
layout. These voltage spikes could exceed the absolute
m
axi
mum voltage rating of LTC3862’s GATE pin. The GATE
pins are rated for an absolute maximum voltage of –0.3V
minimum and 6V maximum. Hence it is recommended to
add an external buffer close to the GATE of the MOSFET
as shown in Figure 19.
Figure 19. External Buffer Circuit
LTC3862
GATE1, 2
V
OUT
R
SENSE
C
OUT
3862 F19
L
Q2A
PBS4140DPN
Q1
Q2B
10Ω
PGND
SGND
INTV
CC
V
IN