20
LTC3445
3445fa
prevent oscillations. The LTC3445 LDOs are micropower
devices and output transient response will be a function of
output capacitance. Larger values of output capacitance
decrease the peak deviations and provide improved tran-
sient response for larger load current changes.
PowerPath CONTROLLER
The PowerPath circuitry in the LTC3445 is used to provide
backup power from V
BACKUP
to the V
CC
BATT pin when
V
CC1
is low or disconnected. When V
CC1
is below 2.8V, the
PowerPath routes V
BACKUP
, typically a coin cell, to the V
CC
BATT pin. While V
BACKUP
is selected there is no current
limiting except for a small (<5&) resistance from the
V
BACKUP
input to the V
CC
BATT output. The LTC3445 sinks
less than 6.5礎 from V
BACKUP
when it is selected and sinks
less than 0.1礎 from V
BACKUP
when it is not selected.
When V
CC1
exceeds 2.8V, V
BACKUP
is disconnected from
V
CC
BATT and an internal LDO regulates the V
CC
BATT
voltage to the minimum of V
CC1
or typically 3V. The
internal LDO is current limited to less than 50mA, but
greater than 10mA. Capacitance on the V
CC
BATT pin
should be at least 2礔 with an ESR less than 3&.
V
BACKUP
will be routed to the V
CC
BATT output when the
main battery voltage falls below 2.4V. As the main battery,
V
CC1
, voltage drops from 3V to 2.4V, the LDO will be in
dropout, V
CC
BATT will follow V
CC1
down, rebounding to
V
BACKUP
when V
CC1
falls below 2.4V. If V
CC1
is removed
quickly, the capacitor on V
CC
BATT will limit the V
CC
BATT
droop until V
BACKUP
is switched in.
The V
TRACK
input offers the capability of the V
CC
BATT
voltage to follow the voltage on V
TRACK
up to V
CC1
. In
effect, V
TRACK
overrides the internal reference of the LDO,
resulting in the LDO output (V
CC
BATT) having a gain of 1
relative to V
TRACK
once V
TRACK
exceeds a typical value of
3V. V
CC
BATT will follow V
TRACK
to within 200mV provid-
ing V
TRACK
does not exceed the dropout voltage of the
LDO, which is powered by V
CC1
.
V
BACKUP
should be present prior to V
CC1
being connected.
V
BACKUP
provides power to the BATTFAULT driver which
APPLICATIO S I FOR ATIO
U
U
U
is used to detect an absent or low V
CC1
. If V
BACKUP
is not
present, the LTC3445 will be unable to pull the BATTFAULT
pin low to signal a V
CC1
fault condition.
Output Capacitance and Transient Response
The LDO used LTC3445 PowerPath is designed to be
stable with a wide range of output capacitors. A minimum
output capacitor of 2.2礔 with an ESR of 3& or less is
recommended to prevent oscillations. The LTC3445
PowerPath LDO is a micropower device and output tran-
sient response will be a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes.
THERMAL CONSIDERATIONS
In most applications the LTC3445 does not dissipate
much heat due to its high efficiency. But, in applications
where the LTC3445 is running at high ambient tempera-
ture with low supply voltage and high duty cycles, such as
in dropout, the heat dissipated may exceed the maximum
junction temperature of the part. If the junction tempera-
ture reaches approximately 150癈, both power switches
will be turned off and the SW node will become high
impedance. The remaining regulators will also turn off.
To ensure the LTC3445 doesnt exceed the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
T
R
= ?/DIV>
JA
" (PD
BUCK
+ PD
LDO1
+ PD
LDO2
+ PD
PowerPath
)
where P
D
is the power dissipated by the regulator and ?/DIV>
JA
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, T
J
, is given by:
T
J
= T
A
+ T
R
where T
A
is the ambient temperature.
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