Datasheet

PD
L
LIMFtyppeak
t
L
V
II ×+=
)(
( )
L)on(DSOUT(min)OUT(min)IN
RRIVV ++=
TPS62140, TPS62140A
TPS62141, TPS62142, TPS62143
SLVSAJ0B –NOVEMBER 2011–REVISED JUNE 2013 www.ti.com
100% Duty-Cycle Operation
The duty cycle of the buck converter is given by D = Vout/Vin and increases as the input voltage comes close to
the output voltage. In this case, the device starts 100% duty-cycle operation, turning on the high-side switch
100% of the time. The high-side switch stays turned on as long as the output voltage is below the internal
setpoint. This allows the conversion of small input-to-output voltage differences, e.g., for longest operation time
of battery-powered applications. In 100% duty-cycle mode, the low-side FET is switched off.
The minimum input voltage to maintain output voltage regulation, depending on the load current and the output
voltage level, can be calculated as:
(3)
where
I
OUT
is the output current.
R
DS(on)
is the R
DS(on)
of the high-side FET.
R
L
is the dc resistance of the inductor used.
Enable / Shutdown (EN)
When the enable pin (EN) is set High, the device starts operation.
Shutdown is forced if EN is pulled Low with a shutdown current of typically 1.5 µA. During shutdown, the internal
power MOSFETs as well as the entire control circuitry are turned off. The internal resistive divider pulls down the
output voltage smoothly. An internal pulldown resistor of about 400 kΩ is connected and keeps EN logic Low if
the pin is floating. It is disconnected if the pin is High.
Connecting the EN pin to an appropriate output signal of another power rail provides sequencing of multiple
power rails.
Soft-Start / Tracking (SS/TR)
The internal soft-start circuitry controls the output voltage slope during start-up. This avoids excessive inrush
current and ensures a controlled output-voltage rise time. It also prevents unwanted voltage drops from high-
impedance power sources or batteries. When EN is set to start device operation, the device starts switching after
a delay of about 50 µs, and VOUT rises with a slope controlled by an external capacitor connected to the SS/TR
pin. See Figure 33 and Figure 34 for typical start-up operation.
The slope can be controlled by an external capacitor connected to the SS/TR pin. Connecting SS/TR directly to
AVIN provides fastest start-up behavior. The TPS6214X can start into a pre-biased output. During monotonic
pre-biased startup, both the power MOSFETs are not allowed to turn on until the device internal ramp sets an
output voltage above the pre-bias voltage. As long as the output is below about 0.5 V, a reduced current limit of
typically 1.6 A is set internally. If the device is set to shutdown (EN = GND), undervoltage lockout, or thermal
shutdown, an internal resistor pulls the SS/TR pin down to ensure a proper low level. Returning from those states
causes a new start-up sequence as set by the SS/TR connection.
A voltage supplied to SS/TR can be used for tracking a master voltage. The output voltage follows this voltage in
both directions, up and down (see APPLICATION INFORMATION).
Current Limit and Short Circuit Protection
The TPS6214X devices are protected against heavy load and short-circuit events. If a short circuit is detected
(VOUT drops below 0.5 V), the current limit is reduced to 1.6 A, typically. If the output voltage rises above 0.5 V,
the device runs in normal operation again. At heavy loads, the current limit determines the maximum output
current. If the current limit is reached, the high-side FET is turned off. Avoiding shoot-through current, the low-
side FET is switched on to sink the inductor current. The high-side FET turns on again only if the current in the
low-side FET has decreased below the low-side current-limit threshold.
The output current of the device is limited by the current limit (see ELECTRICAL CHARACTERISTICS). Due to
internal propagation delay, the actual current can exceed the static current limit during that time. The dynamic
current limit can be calculated as follows:
(4)
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