LTC3522_15 Datasheet, PDF (12/20 Pages) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

LTC3522
OPERATION
has minimal dependency on the size of the output capaci-
tor or load. During soft-start, the buck-boost converter is
forced into PWM operation regardless of the state of the
PWM pin.
PGOOD1 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 output voltage falls 10% below the
regulation voltage. There is approximately 2.5% 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
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, but the output voltage still remains slightly
above the PGOOD1 comparator trip point.
The PGOOD1 output also pulls low during overtemperature
shutdown and undervoltage lockout. The PGOOD1 output
is only active if the buck-boost converter is enabled.
COMMON FUNCTIONS
Thermal Shutdown
If the die temperature exceeds 150°C (typical) both con-
verters will be disabled. All power devices will be turned
off and all switch nodes will be high impedance. The
soft-start circuits for both converters are reset during
thermal shutdown to provide a smooth recovery once the
overtemperature condition is eliminated. Both converters
will restart (if enabled) when the die temperature drops to
approximately 140°C.
Undervoltage Lockout
If the supply voltage decreases below 2.3V (typical) then
both converters will be disabled and all power devices will
be turned off. The soft-start circuits for both converters
are reset during undervoltage lockout to provide a smooth
restart once the input voltage rises above the undervoltage
lockout threshold.
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