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dc2208af
DEMO MANUAL DC2208A
Description
LTC4420
18V Dual Input Micropower PowerPath
Prioritizer with Backup Supply Monitoring
Demonstration circuit DC2208A uses the LTC
®
4420 to
provide backup switchover, keeping the output load pow-
ered during brownout or power loss conditions. Priority is
assigned by supply connection to the V1 and V2 inputs.
The power source connected to the V1 input has a higher
priority, and it is monitored continuously by adjustable
comparator (ADJ) with the proper threshold arranged by
an external resistive divider. If the input voltage V1 ex-
ceeds 1.55V (1.8V worst case) and the ADJ pin is above
1.097V for at least 64ms after V1 has powered up, and
then stays above 1.047V, V1 is considered valid, and the
LTC4420 connects V1 to load (OUT pin). If V1 drops be
-
low 1.52V, or the ADJ pin voltage is lower than 1.047V, V1
is considered invalid, and LTC4420 switches V2 to load,
if the V2DIS pin voltage is above 0.9V. If the V2DIS pin
voltage is below 0.2V, V2 to load connection is activated
only if V2UV is >0.387V.
The DC2208A is populated to provide switchover when V1
is below 9.92V ±0.32V. The upper limit for V1 is the maxi
-
mum operating voltage of 18V; recommended V1 voltage
is 12V. The resistive divider of the auxiliary comparator
CMP1, R1–R2, is adjusted to trigger the comparator when
VCMP1_TOP turret voltage is in the range 9.82V to 10.69V.
The second rail, V2, is monitoring periodically through
the external resistive divider R6, R7 and the V2UV_TEST
jumper head in the ON position. The bottom resistor of
this divider, R7, is periodically connected to GND through
open-drain output GNDSW. V2UV undervoltage range on
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
the DC2208A is 3.26V to 3.53V. If V2DIS is connected
to GND, V2UV_TEST is ON, and V2 drops below the un-
dervoltage level, the LTC4420 disconnects OUT from V2.
If the bottom resistor of the divider,
R7, is connected to
GND, V2 is not monitored for undervoltage condition and
the load can be powered by any voltage from 1.8V to 18V.
Recommended V2 voltage is 5V.
The LTC4420’s internal overcurrent (0.5A to 1.6A) and
thermal protection circuitry allow safe prioritization in
critical operation modes. Overtemperature protection
terminates prioritizer operation when the junction tem-
perature exceeds 125ºC.
Another inherent feature—reverse supply protection—
protects the prioritizer input-output and load when
negative voltage (up to –15V) is applied to any supply
input, V1 or V2.
The LTC4420’s break-before-make switching method pre-
vents cross conduction between input channels during
switch over; connection to any input occurs only if the input
voltage exceeds the output by 50mV. This eliminates reverse
current flow from the output into selected input supply.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2208A
performance summary
Specifications are at T
A
= 25°C
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V1, V2 V1, V2, Operating Voltage Range 1.8 18 V
R
ON
Switch Resistance V1 = V2 = 5V, I
OUT
= –100mA 1 2 5 Ω
t
VALID(V1)
Input Qualification Time V1 Rising, ADJ Rising 34 64 94 ms
V
THA
ADJ Threshold ADJ Falling 1.032 1.047 1.062 V
V
HYSTA
ADJ Comparator Hysteresis ADJ Rising 30 50 70 mV
V
THC
CMP1, V
2UV
Threshold CMP1, V2UV Falling 0.378 0.387 0.396 V
