LTC3445 Datasheet, PDF (20/24 Pages) Linear Technology – I2C Controllable Buck Regulator with Two LDOs in a 4mm × 4mm QFN

LTC3445
APPLICATIO S I FOR ATIO
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 (<5Ω) resistance from the
V_BACKUP input to the V_CC BATT output. The LTC3445 sinks
less than 6.5µA from V_BACKUP when it is selected and sinks
less than 0.1µA 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 2µF with an ESR less than 3Ω.
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
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 2.2µF with an ESR of 3Ω 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°C, 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 doesn’t 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 = θ_JA • (PD_BUCK + PD_LDO1 + PD_LDO2 + PD_PowerPath)
where P_D is the power dissipated by the regulator and θ_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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