Datasheet LTC3026 (Analog Devices) - 9

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