Datasheet

LT8705
34
8705fb
For more information www.linear.com/LT8705
applicaTions inForMaTion
Similar calculations can be used to select a resistor divider
connected to SWEN that would stop switching activity dur-
ing an undervoltage condition. Make sure that the divider
d
o
esn’t cause SWEN to exceed 7V (absolute maximum
rating) under maximum V
IN
conditions. Using the FBIN
pin as an undervoltage lockout is discussed in the Input
Voltage Regulation or Undervoltage Lockout section.
Inductor Current Sense Filtering
Certain applications may require filtering of the inductor
current sense signals due to excessive switching noise
that can appear across R
SENSE
. Higher operating voltages,
higher values of R
SENSE
, and more capacitive MOSFETs
will all contribute additional noise across R
SENSE
when
the SW pins transition. The CSP/CSN sense signals can
be filtered by adding one of the RC networks shown in
Figures 12a and 12b. Most PC board layouts can be drawn
to accommodate either network on the same board. The
network should be placed as close as possible to the IC.
The network in Figure 12b can reduce common mode
noise seen by the CSP and CSN pins of the LT8705 at the
expense of some increased ground trace noise as current
passes through the capacitors. A short direct path from the
capacitor grounds to the IC ground should be used on the
PC board. Resistors greater than 10Ω should be avoided
as this can increase offset voltages at the CSP/CSN pins.
The RC product should be kept to less than 30ns.
Junction Temperature Measurement
The duty cycle of the CLKOUT signal is linearly proportional
to the die junction temperature, T
J
. Measure the duty cycle
of the CLKOUT signal and use the following equation to
approximate the junction temperature:
T
J
≅
DC
CLKOUT
–35.9%
0.329%
°C
where DC
CLKOUT
is the CLKOUT duty cycle in % and T
J
is the die junction temperature in °C. The actual die tem-
perature can deviate from the above equation by ±10°C
Thermal Shutdown
I
f t
he die junction temperature reaches approximately
165°C, the part will go into thermal shutdown. The power
switch will be turned off and the INTV
CC
and LDO33
regulators will be turned off (see Figure 2). The part will
be re-enabled when the die temperature has dropped by
~5°C (nominal). After re-enabling, the part will start in
the switcher off state as shown in Figure 2. The part will
then initialize, perform a soft-start, then enter normal
operation as long as the die temperature remains below
approximately 165°C.
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Although all dissipative elements
in the circuit produce losses, four main sources account
for most of the losses in LT8705 circuits:
1. Switching losses. These losses arises from the brief
amount of time switch M1 or switch M3 spends in the
saturated region during switch node transitions. Power
loss depends upon the input voltage, load current, driver
strength and MOSFET capacitance, among other fac-
tors. See the Power MOSFET Selection and Efficiency
Co
nsiderations section for more details.
2. DC I
2
R losses. These arise from the resistances of the
MOSFETs, sensing resistors, inductor and PC board
traces and cause the efficiency to drop at high output
currents.
R
SENSE
1nF
CSP
CSN
LT8705
8705 F12a
10Ω
10Ω
Figure 12. Inductor Current Sense Filter
(12a)
(12b)
R
SENSE
1nF
1nF
CSP
CSN
LT8705
8705 F12b
10Ω
10Ω