LTC4040_15 Datasheet, PDF (15/26 Pages) Linear Technology – 2.5A Battery Backup Power Manager

LTC4040
Operation
Differential Undervoltage Lockout
An undervoltage lockout circuit monitors the differential volt-
age between V_SYS and BAT and shuts off the charger if the BAT
voltage reaches within 50mV of the V_SYS voltage. Charging
does not resume until this difference increases to 145mV.
V_BAT Undervoltage Lockout
To prevent the battery from discharging too deeply, the
LTC4040 incorporates an undervoltage lockout circuit
which shuts down the boost regulator when V_BAT drops
below 2.45V.
BACKUP BOOST CONVERTER
To supply the system load from the battery in backup mode,
the LTC4040 contains a 1.125MHz constant-frequency
current-mode synchronous boost switching regulator with
output disconnect and automatic Burst Mode features.
The regulator can provide a maximum load of 2.5A from
a battery as low as 3.2V and the system output voltage
(V_SYS) can be programmed up to a maximum of 5V via
the BSTFB pin. See the Applications Information section
for details. The converter can be disabled by pulling the
BSTOFF pin high. The boost regulator includes safety
features like short-circuit current protection, input un-
dervoltage lockout, and output overvoltage protection.
Zero Current Comparator
The LTC4040 boost converter includes a zero current
comparator which monitors the inductor current and
shuts off the PMOS synchronous rectifier once the current
drops to approximately 250mA. This prevents the induc-
tor current from reversing in polarity thereby improving
efficiency at light loads.
PMOS Synchronous Rectifier
To prevent the inductor current from running away,
the PMOS synchronous rectifier is only enabled when
V_SYS > (V_BAT – 200mV). Additionally, if the current through
the synchronous FET (PMOS) ever exceeds 8A, the con-
verter skips the next two clock cycles so that the inductor
current has a chance to discharge safely below this level.
Short-Circuit Protection
The output disconnect feature enables the LTC4040 boost
converter to survive a short circuit at its output. It incorpo-
rates internal features such as current limit foldback and
thermal shutdown for protection from excessive power
dissipation during short circuit.
Boost Overvoltage Protection
If the BSTFB node were inadvertently shorted to ground,
then the boost converter output would increase indefinitely
with the maximum current that could be sourced from BAT.
The LTC4040 protects against this by shutting off both
switches if the output voltage exceeds 5.5V.
Burst Mode Operation
To improve battery life during backup, the LTC4040 boost
converter provides automatic Burst Mode operation which
increases the efficiency of power conversion at very light
loads. Burst Mode operation is initiated if the output load
current falls below an internally set threshold. Once Burst
Mode operation is initiated, only the circuitry required
to monitor the output is kept alive. This is referred to
as the sleep state in which the backup boost consumes
only 40µA from the battery. When the V_SYS pin voltage
drops by about 1% from its nominal value, the part wakes
up and commences normal PWM operation. The output
capacitor recharges and causes the part to re-enter the
sleep state if the output load remains less than the Burst
Mode threshold. The frequency of this intermittent PWM
or Burst Mode operation depends on the load current;
that is, as the load current drops further below the burst
threshold, the boost converter turns on less frequently.
When the load current increases above the burst thresh-
old, the converter seamlessly resumes continuous PWM
operation. Thus, Burst Mode operation maximizes the
efficiency at very light loads by minimizing switching and
quiescent losses. However, the output ripple typically
increases to about 2% peak-to-peak. Burst Mode ripple
can be reduced, in some circumstances, by placing a small
phase-lead capacitor (C_PL) between the V_SYS and BSTFB
pins. However, this may adversely affect the efficiency
and the quiescent current at light loads. Typical values of
C_PL range from 15pF to 100pF.
For more information www.linear.com/LTC4040
4040fa
15