LTC1558CS-5 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1558CS-5' PDF : 20 Pages View PDF
LTC1558-3.3/LTC1558-5
APPLICATIONS INFORMATION
terminate backup operation and remain off until the main
power supply returns. It then runs a fast recharge cycle to
recharge the backup cell. An onboard low-battery com-
parator in the 16-pin GN or SO package provides an early
warning signal when the backup cell drops below 1V.
The UVLO circuit also trips if the LTC1558’s VCC supervi-
sory circuit detects that VCC drops below – 9% of the rated
VCC voltage due to overload or output short-circuit condi-
tions. Once the UVLO circuit trips, the LTC1558 asserts
the RESET pins until the VCC voltage drops below 1V. It will
then remain off until VCC rises to within (VCC – 6%) of the
rated output voltage. During power-up from UVLO, the
LTC1558 asserts the RESET pins until the (VCC – 6%)
threshold. Once VCC exceeds (VCC – 6%), the RESET pins
remain asserted for another 200ms (“hard” reset) before
being released to inform the system to start operating.
Reset Operation
The LTC1558 includes an onboard pushbutton reset switch
controller. If the CTL pin is pulled to ground (< 250mV) by
a pushbutton or an open-drain output, the LTC1558 gen-
erates a pulse at the RESETpins after the trailing edge of
A
CTL
0V
tCTL < 20ms
RESET
20ms
DEBOUNCE
“SOFT” PUSHBUTTON RESET AT CTL
A. CTL < 0.25V FOR LESS THAN 20ms
B. CTL > 0.25V FOR MORE THAN 20ms
0V
CTL
tCTL > 2s
RESET
2s
“HARD” PUSHBUTTON RESET AT CTL
CTL < 0.25V FOR MORE THAN 2s
B
20ms < tCTL < 2s
> 0.25V
100µs
100µs
> 0.25V
200ms
CTL
0V
20ms
RESET
20ms
20ms DEBOUNCE AT FALLING AND RISING RESET EDGE
1558 F05
Figure 5. Pushbutton Resets
the CTL signal. A short (less than 2s) low going signal at
CTL will generate a “soft” reset (100µs) pulse at the RESET
pins. A low CTL signal for more than 2s will generate a
“hard” reset pulse at its RESETpins. During “hard” reset,
the LTC1558 will disable the boost converter if it is in
backup mode. All signals at the CTL pin are debounced for
20ms to prevent multiple resets, allowing the CTL pin to be
connected directly to a pushbutton to ground.
The RESET pin is an open-drain output that requires an
external pull-up resistor. The RESET pin is a TTL-compat-
ible CMOS output.
Shutdown
The 16-pin LTC1558 has a TTL-compatible input, SHDN,
that shuts down the whole chip, asserts the RESET pins
and places the CTL, VBAK and SW pins into high impedance
states. The SHDN pin has an internal pull-up that ensures
the chip will not shut down if the pin is left floating. The
SHDN pin typically draws 8µA when pulled low at VCC = 5V.
The chip consumes less than 50µA during shutdown while
VCC is still alive.
Although there is no SHDN pin for the SO-8 package, the
user can shut down the part by pulling CTL to ground. The
chip enters “hard” reset leaving only the bandgap and
comparators alive. The charger and the boost converter
shut off completely. Note that the backup cell slowly
discharges through REXT in this mode.
Inductor Selection
The LTC1558 is designed to operate with a recommended
inductor value of 22µH (±20%) with < 0.2Ω DC resistance.
Using inductor values higher than 22µH will deliver more
output power but will cause the gas gauge counter to
count inaccurately and under recharge the backup cell. At
the same time, the N-channel transistor timer will limit the
peak current if the charging time becomes overextended
due to the higher inductor value. Using inductor values
lower than 22µH will degrade the boost converter’s maxi-
mum output power and cause the gas gauge counter to
overcharge the backup cell. Table 2 lists the recom-
mended surface mount inductor part numbers.
13
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