LTC3589_1 Datasheet, PDF (17/46 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589
OPERATION
sequence tie the enable of the first regulator to be powered
up to the WAKE pin. Connect the first regulators output
to the enable pin of the second regulator, and so on. One
or more regulators may be started in any sequence. Each
enable pin has a 200µs (typical) delay between the pin
and the internal enable of the regulator. When the system
controllers are satisfied that power rails are up, the con-
troller must drive PWR_ON HIGH to keep WAKE active. To
ensure correct start-up sequencing, the regulators outputs
are monitored by voltage comparators which require each
output to discharge below 300mV before re-enabling. A
software control command register function is available
which sets the regulators to effectively ignore their enable
pins but respond to I²C register enables. This function
enables software-only control of any combination of pin-
strapped regulators and is useful for implementing system
power saving modes. Keep-alive mode exempts selected
regulators from turning off during normal shutdown. In
keep-alive mode, the LTC3589 powers down normally
and is ready for the next start-up sequence, but selected
regulators are kept on to power memory or other functions
during system standby modes.
The LTC3589 will shut down all regulators and pull down
the WAKE pin under high temperature, V_IN undervoltage,
and extended low regulator output voltage conditions.
Status of a hard shutdown is reported by the IRQ status
pin and the IRQSTAT status register.
The I²C serial port on the LTC3589 contains 13 command
registers for controlling each of the regulators, one read-
only register for monitoring each regulators power good
status, one read-only register for reading the cause of
an IRQ event, and one clear IRQ command register. The
LTC3589 I²C supports random addressing of any register.
ALWAYS-ON LDO
The LTC3589 includes a low quiescent current low dropout
regulator that remains powered whenever a valid supply
is present on V_IN. The always-on LDO will remain active
until V_IN drops below 2.0V (typical). This is below the
2.5V undervoltage threshold in effect for the rest of the
LTC3589 circuits. The always-on LDO is used to provide
power to a standby microcontroller, real-time clock, or
other keep-alive circuits. The LDO is guaranteed to sup-
port a 25mA load. A 1µF low impedance ceramic bypass
capacitor from LDO1_STBY to GND is required for com-
pensation. A power good monitor pulls RSTO LOW for a
minimum of 14ms (typical) whenever LDO1_STBY is 8%
below its regulation target. An LDO1_STBY undervoltage
condition is reported in the PGOOD status register. The
output voltage of LDO1 is set with a resistor divider con-
nected from LDO1_STBY to the feedback pin LDO1_FB,
as shown in Figure 1.
V_{LDO _ STBY}
= 0.8
•

¹⁺

RR21^{(V)}
Typical values for R1 are in the range of 40k to 1M.
0.8V ⁺
–
V_IN
LDO1_STBY
R1
1µF
LDO1_FB
3589 F01
R2
Figure 1. Always-On LDO Application Circuit
LDO1_STBY is protected from short-circuits and over-
loading.
250mA LDO REGULATORS
Three LDO regulators on the LTC3589 will each deliver up to
250mA output. The LDO regulators are enabled by pin input
or I²C command register. Pin EN_LDO2 enables LDO2 and
pin EN_LDO34 enables LDO3 and LDO4 together. An I²C
command register bit is available to decouple LDO4 from
pin EN_LDO34 so that LDO4 is under command register
control only. All the regulators have current limit protection
circuits. When disabled, a 2.5k internal pull-down resistor
is connected to the regulators output.
3589fb
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