Testing Virtual ECUs - Power Systems Design

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SPECIAL REPORT : AUTOMOTIVE ELECTRONICS
Figure 3: LTC3859A Efficiency vs. Load Current for Different Converter
Sections
buck converters on. With all three
channels on and in sleep mode
the LTC3859A draws only 100µA
which significantly extends battery
run times when in idle mode. This
is done by configuring the part to
enter high efficiency Burst Mode®
operation, were the LTC3859A
delivers short bursts of current to
the output capacitor followed by
a sleep period where the output
power is delivered to the load by
the output capacitor only. Figure 2
shows the conceptual timing diagram
of how this works.
The Burst Mode output ripple is
load independent, only the length
of the sleep intervals will change.
In sleep mode, much of the internal
circuitry is turned off except
for the critical circuitry needed to
respond quickly, further reducing
its quiescent current. When
the output voltage drops enough,
the sleep signal goes low and the
controller resumes normal Burst
Mode operation by turning on the
top external MOSFET. Alternatively,
there are instances when the
user will want to operate in forced
Figure 4: Size & Layout of LTC3859A Demo Board (a) Top and (b) Bottom
sides
continuous or constant frequency
pulse skipping mode at light load
currents. Both of these modes
are easily configurable, will have
a higher quiescent current and a
lower peak to peak output ripple.
Load-Dump/Efficiency/Solution
Size
Load dump is a term that refers
to the inductive kick that happens
after the starter motor is turned
off. This surge voltage is normally
clamped to 36 volts maximum for
a 12 volt lead acid automotive type
battery system. This surge requires
the controller, MOSFETs and associated
components being capable
of operating at the clamped
voltage. These higher voltage
devices (such as 40V MOSFETs)
can degrade efficiency and care
must be taken to minimize this effect.
Based on the circuit in Figure
1, the efficiency is above 92% for
each rail as shown in Figure 3. For
clarity the efficiency of each buck
and boost section is show separately.
In addition, the layout and
circuit size for this circuit is shown
in Figure 4 with the tallest part being
4.8mm high.
Soft Start or Tracking
The TRACK/SS1 and TRACK/SS2
pins of the two buck controllers
can be used for adjusting the soft
start turn-on time or to track two
(or more) supplies with Coincident
or Ratiometric tracking during
start up. These associated curves
are shown in Figure 5 and is accomplished
by putting a resistor
divider from the master supply,
to the TRACK/SS pin of the slave
At higher temperatures, or in cases
where the internal power dissipation
causes excessive self heating
on chip, the over temperature
shutdown circuitry will shut down
the LTC3859A. When the junction
temperature exceeds approximately
170°C, the over temperature
circuitry disables the on-board
bias LDO, causing the bias supply
POWER SYSTEMS DESIGN JULY/AUGUST 2011
Figure 5: LTC3859A Output Voltage Tracking: (a) Coincident Tracking and (b) Ratiometric Tracking
supply.
to drop to zero volts and effective ler. Combined with two synchro-
shutting down the entire LTC3859A nous step-down controllers, ideal
in an orderly manner. Once the for powering many automotive
junction temperature drops back electronic devices, the LTC3859A
to approximately 155°C, the LDO maintains regulation for all output
turns back on.
voltages during an engine restart.
Conclusion
The LTC3859A provides a solution
by boosting the battery voltage to
a safe operating level with is onboard
synchronous boost control-
Author: Bruce Haug
Senior Product Marketing Engineer
Linear Technology Corporation
www.linear.com
WWW.POWERSYSTEMSDESIGN.COM WWW.POWERSYSTEMSDESIGN.COM
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