Design Once; Use Twice: Monolithic SEPIC/Boost Regulators with Wide VIN Range Satisfy Requirements of Both Consumer and Commercial Vehicles

Design Once; Use Twice: Monolithic SEPIC/Boost Regulators
with Wide VIN Range Satisfy Requirements of Both Consumer
and Commercial Vehicles
Molly Zhu
Automobile manufacturers continually add electronic control units (ECUs) to support
increasing numbers of performance, comfort and safety features. ECU power either
comes from a single lead-acid battery in consumer vehicles, or from two batteries in
commercial vehicles. Ideally, an ECU can run off either, enabling a single design for
both consumer and commercial vehicles. This requires that ECU power ICs support
an input range covering both configurations—namely 3.5V to 60V. Furthermore,
the power ICs should feature ultralow quiescent current, preserving the vehicle’s
battery run time when the engine is off, but always-on systems remain engaged.
The LT8495 and LT8494 are high voltage
switching regulators that meet these
requirements when configured as
SEPIC or boost converters. Both parts
operate over 2.5V to 60V input, and
have low quiescent current to extend
the battery life. The quiescent current
of the LT8495 is 9µ A, and is 7µ A for the
LT8494. The parts are available in 20-lead
QFN and 20-lead TSSOP packages.
supply voltage is monitored by poweron reset, and the software/hardware
activities are supervised by watchdog
timers. These functions are integrated
in the LT8495, simplifying designs with
enhanced safety and reliability.
DUAL SUPPLY PINS
The input voltage of the LT8494/LT8495
can be as high as 60V for SEPIC topologies, and 32V for boost circuits with the
60V ride-through voltage. The internal
power switch driver must be in the
2.4V~34V (typical) range to enable the
LT8494/LT8495, but the minimum operating VIN range can be reduced to 1V. The
The LT8494 and LT8495 are similar, but
the LT8495 adds power-on reset and
watchdog timers. It is designed specifically
for microcontrolled applications, where
reliability and safety are critical. The
Figure 1. The LT8494 in a 750kHz, 48V boost converter
100
SW
VIN
SWEN
PG
RT
93.1k
SS
10pF
VOUT
48V
0.5A
FB
LT8494
GND BIAS
0.2µF
25.5k
C2
4.7µF
×2
95
EFFICIENCY (%)
D1
1M
C1
2.2µF
A typical application of a boost converter
using the LT8494 is shown in Figure 1.
The BIAS pin is connected to ground
instead of the output since the input
Figure 2. Efficiency of the circuit in Figure 1
L1
22µH
VIN
16V TO 32V
integrated power switch drivers can
operate from either of two supplies: VIN
or BIAS. This allows the part to optimize
efficiency and reduces the minimum
input voltage requirement. The LT8494/
LT8495 automatically chooses the lower
supply of the two, provided it is in the
operation range. This selection is made
on-the-fly as VIN or BIAS voltages change.
After initial start-up, the part can draw
current from BIAS if it is lower than VIN .
90
85
80
75
C1: 2.2µF, 50V, X5R, 1206
C2: 4.7µF, 100V, X7R, 1210
D1: ONSEMI MBRA2H100
L1: WÜRTH LHMI 74437349220
28 | May 2016 : LT Journal of Analog Innovation
70
0
100
200
300
400
LOAD CURRENT (mA)
500
design ideas
RSTIN
1.1V
WDI
WDO
RST
tRST
tUV
tWDL< t < tWDU
t < tWDL
t > tWDU
tRST
tRST
tWDU = WATCHDOG UPPER BOUNDARY PERIOD, APPROXIMATELY 31 RAMPING CYCLES ON CWDT PIN
tWDL = WATCHDOG LOWER BOUNDARY PERIOD, APPROXIMATELY 1 RAMPING CYCLE ON CWDT PIN
tUV = TIME REQUIRED TO ASSERT RST LOW AFTER RSTIN
GOES BELOW ITS THRESHOLD, APPROXIMATELY 23µs
tRST = PROGRAMMED RESET PERIOD
(a)
(b)
Figure 3. POR (a) and watchdog (b) timing
voltage is always lower than the output.
The efficiency is given in Figure 2. At
very light load, the efficiency of the
LT8494 is slightly higher than that of
the LT8495 because the LT8494 is not
supporting a watchdog function.
WATCHDOG TIMER AND POR
FUNCTIONS
The LT8495 is similar to the LT8494, but
it adds integrated power-on reset (POR)
and watchdog timer functions to enhance
system safety in automotive applications.
The POR monitors the supply voltages,
while the watchdog timer monitors the
software and hardware functions.
by the RSTIN pin, and the watchdog
timer supervises the microcontroller.
The LT8495’s watchdog timer includes
an independent enable pin (WDE), and
can operate without the VIN supply. If
the time between the negative edges
on the WDI is too long or too short,
the WDO pin is pulled low for the reset
delay time, tRST, before it is released.
The window time of WDI can be
programmed through the cap on CWDT
pin. The timing diagrams of the POR and
watchdog timer are shown in Figure 3.
The LT8494 and LT8495 are monolithic
boost/SEPIC switching regulators with
input voltage ranges of 1V to 60V after
start-up. Both parts can automatically
select the lower supply pins, VIN or BIAS,
to improve efficiency. The LT8495 features
an integrated power-on reset and a watchdog timer to monitor the microcontroller’s
activity. Their wide input voltage ranges,
high efficiency, low quiescent current and
programmable timing make them ideal for
industrial and automotive applications. n
Figure 4 shows the LT8495 configured as
a SEPIC converter with a 3V–60V input
voltage and 5V output. The max load
current increases with the input voltage
until reaching the full load current of 1A at
12V input. The output voltage is monitored
The LT8495 monitors the output via the
RSTIN pin voltage. During normal operation, if the voltage of the RSTIN is below its
threshold, the RST pin is asserted low. Once
the RSTIN rises above its threshold, the RST
Figure 4. The LT8495 in a 450kHz, 5V
output SEPIC converter with POR
and watchdog timer
pin is released after the reset delay time.
The reset delay time, tRST, is programmable through the cap on the CPOR pin.
L1
15µH
VIN
3V TO 60V
•
C3
2.2µF
C1
2.2µF
•
SW
SWEN
1nF
SS
RT
4.7nF
1µF
169k
1M
4.7pF
BIAS
RSTIN
FB
VIN
CPOR
VOUT
5V
0.4A (VIN = 3V)
0.6A (VIN = 5V)
1.0A (VIN > 12V)
D1
L2
15µH
CWDT
CONCLUSION
C2
47µF ×2
8.87k
316k
GND
LT8495
WDO
WDE
RST
WDI
µC
C1, C3: 2.2µF, 100V, X5R, 1206
C2: TAIYO YUDEN, EMK325BJ476MM-T
D1: ONSEMI MBRA2H100
L1, L2: COILTRONICS DRQ125-150-R
May 2016 : LT Journal of Analog Innovation | 29
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