LTC3521
13
3521fb
For more information www.linear.com/LTC3521
Burst Mode Operation
With the PWM pin held low, the buck-boost converter
operates utilizing a variable frequency switching algorithm
designed to improve efficiency at light load and reduce
the standby current at zero load. In Burst Mode operation,
the inductor is charged with fixed peak amplitude current
pulses. These current pulses are repeated as often as
necessary to maintain the output regulation voltage. The
maximum output current which can be supplied in Burst
Mode operation is dependent upon the input and output
voltage as given by the following formula:
OUT(MAX),BURST
=
IN
V
+ V
A
( )
In Burst Mode operation, the error amplifier is not used
but is instead placed in a low current standby mode to
reduce supply current and improve light load efficiency.
Soft-Start
The buck-boost converter has an internal voltage mode
soft-start circuit with a nominal duration of 600μs. The
converter remains in regulation during soft-start and will
therefore respond to output load transients that occur
during this
time. In addition, the output voltage rise time
has
minimal dependency on the size of the output capaci-
tor or
load. During soft-start, the buck-boost converter is
for
ced into PWM operation regardless of the state of the
PWM pin.
PGOOD Comparator
The PGOOD1 pin is an open-drain output which indicates
the status of the buck-boost converter. In Burst Mode
operation (PWM = Low), the PGOOD1 open-drain output
will pull low when the feedback voltage falls 9% below the
regulation voltage. There is approximately 3% hysteresis in
this threshold when the output voltage is returning good.
In addition, there is a 60μs typical deglitching delay to
prevent false trips due to short duration voltage transients
in response to load steps.
In PWM mode, operation of the PGOOD1 comparator is
complicated
by the fact that the feedback pin voltage is
driven to the reference voltage independent of the output
operaTion
voltage through the action of the voltage mode error am-
plifier. Since the soft-start is voltage mode, the feedback
voltage
will track the output voltage correctly during
soft-start, and the PGOOD1 output will correctly indicate
the point at which the buck-boost attains regulation at the
end of soft-start. Therefore, the PGOOD1 output can be
utilized for sequencing purposes. Once in regulation, the
feedback voltage will no longer track the output voltage,
and the PGOOD1 pin will not directly respond to a loss
of regulation in the output. However, the only means by
which a loss of regulation can occur is if the current limit
has been reached, thereby preventing the buck-boost
converter from delivering the required output current.
In such cases, the occurrence of current limit will cause
the PGOOD1 flag to fall indicating a fault state. There can
be cases, however, when the buck-boost converter is
continuously in current limit, causing the PGOOD1 output
to pull low, while the output voltage still remains slightly
above the PGOOD1 comparator trip point.
The PGOOD1
output also pulls low during overtemperature
shutdown, undervoltage lockout or if the SHDN1 pin is
pulled low.
COMMON FUNCTIONS
Thermal Shutdown
If the die temperature exceeds 150°C, all three converters
will be disabled. All power devices will be turned off and
all switch nodes will be high impedance. The soft-start
circuits for all three converters are reset during thermal
shutdown to provide a smooth recovery once the over
-
temperature condition
is eliminated. All three converters
will restart (if enabled) when the die temperature drops
to approximately 140°C.
Undervoltage Lockout
If the supply voltage decreases below 1.7V (typical) then
all three converters will be disabled and all power devices
will be turned off. The soft-start circuits for all three con
-
verters are reset during undervoltage lockout to provide
a
smooth restart once the input voltage rises above the
undervoltage lockout threshold.
