LTC3026
9
3026ff
The LTC3026 is a VLDO (very low dropout) linear regulator
which operates from input voltages as low as 1.14V. The
LDO uses an internal NMOS transistor as the pass device
in a source-follower configuration. The BST pin provides
the higher supply necessary for the LDO circuitry while the
output current comes directly from the IN input for high
efficiency regulation. The BST pin can either be supplied
off-chip by an external 5V source or it can be generated
through the internal boost converter of the LTC3026.
Boost Converter Operation
For applications where an external 5V supply is not avail-
able, the LTC3026 contains an internal boost converter to
produce the necessary 5V supply for the LDO. The boost
converter utilizes Burst Mode
®
operation to achieve high
efficiency for the relatively low current levels needed for
the LDO circuitry. The boost converter requires only a
small chip inductor between the IN and SW pins and a
small 4.7µF capacitor at BST.
The operation of the boost converter is described as fol-
lows. During the first half of the switching cycle, an internal
NMOS switch between SW and GND turns on, ramping
the inductor current. A peak comparator senses when the
inductor current reaches 100mA, at which point the NMOS
is turned off and an internal PMOS between SW and BST
turns on, transferring the inductor current to the BST pin.
The PMOS switch continues to deliver power to BST until
the inductor current approaches zero, at which point the
PMOS turns off and the NMOS turns back on, repeating
the switching cycle.
A burst comparator with hysteresis monitors the voltage
on the BST pin. When BST is above the upper threshold
of the comparator, no switching occurs. When BST falls
below the comparator’s lower threshold, switching com-
mences and the BST pin gets charged. The upper and lower
thresholds of the burst comparator are set to maintain a 5V
supply at BST with approximately 40mV to 50mV of ripple.
Care must be taken not to short the BST pin to GND, since
the body diode of the internal PMOS transistor connects
the BST and SW pins. Shorting BST to GND with an induc-
tor connected between IN and SW can ramp the inductor
current to destructive levels, potentially destroying the
inductor and/or the part.
Operating with Boost Converter Disabled
The LTC3026 has an option to disable the internal boost
converter. With the boost converter disabled, the LTC3026
becomes a bootstrapped device and the BST pin must be
driven by an external 5V supply, or driven by the BST pin
of a second LTC3026 with the boost converter enabled. The
recommended method for disabling the boost converter
is to simply float the SW pin. With the SW pin floating no
energy can be transferred to BST which effectively disables
the boost converter.
A single LTC3026 boost converter can be used to drive
multiple bootstrapped LTC3026s with the internal boost
converters disabled. Thus a single inductor can be used
to power two (or possibly more) functioning LTC3026s.
In cases where all LTC3026s have the same input supply
(IN) the internal boost converters of the bootstrapped
LTC3026s can be disabled by floating the SW pin. If the
LTC3026s are not all connected to the same input supply
then the internal boost converters of the bootstrapped
LTC3026s are disabled by floating the SW pin.
LDO Operation
An undervoltage lockout comparator (UVLO) senses the
BST pin voltage to ensure that the bias supply for the LDO
is greater than 4.2V before enabling the LDO. If BST is
below 4.2V, the UVLO shuts down the LDO, and OUT is
pulled to GND through the external divider.
operaTion
