Datasheet LTC3035 (Analog Devices) - 6

LTC3035
U U W U APPLICATIO S I FOR ATIO (Refer to Block Diagram)
The LTC3035 is a VLDO (very low dropout) linear regulator 5 which operates from input voltages between 1.7V and 1.9 • V 5.5V. The LDO uses an internal NMOS transistor as the IN pass device in a source-follower configuration. The inter- 3.23 nal charge pump generator provides the high supply BIAS (V) necessary for the LDO circuitry while the output current comes directly from the IN input for high efficiency 1.7 2.63 5.5 3035 F01 regulation. VIN (V)
Charge Pump Operation Figure 1. LTC3035 BIAS Voltage vs VIN Voltage
The LTC3035 contains a charge pump to produce the before the LDO disables. When the LDO is disabled, OUT necessary bias voltage supply for the LDO. The charge is pulled to GND through the external divider and an pump utilizes Burst Mode operation to achieve high internal 2.5k resistor. efficiency for the relatively low current levels needed for the LDO circuitry. The charge pump requires only a small The LDO provides a high accuracy output capable of 0.1µF flying capacitor between the CP and CM pins and a supplying 300mA of output current with a typical dropout 1µF bypass capacitor at BIAS. voltage of only 45mV. A single ceramic capacitor as small as 1µF is all that is required for output bypassing. The low An internal oscillator centered at 800kHz controls the reference voltage allows the LTC3035 output to be two-phase switching cycle of the charge pump. During the programmed from 0.4V to 3.6V. first phase a current source charges the flying capacitor between VIN and GND. During the second phase, the As shown in the Block Diagram, the charge pump output capacitor’s positive terminal connects to BIAS and the at BIAS supplies the LDO circuitry while the output current current source drives the capacitor’s minus terminal, comes directly from the IN input for high efficiency delivering charge to the BIAS bypass capacitor and in- regulation. The low quiescent supply current, IIN = 100µA, creasing its voltage. drops to IIN = 1µA typical in shutdown making the LTC3035 an ideal choice for use in battery-powered systems. A burst comparator with hysteresis monitors the voltage on the BIAS pin. When BIAS is above the upper threshold The device also includes current limit, thermal overload of the comparator, the oscillator is disabled and no switch- protection, and reverse output current protection. The fast ing occurs. When BIAS falls below the comparator’s lower transient response of the follower output stage overcomes threshold, the oscillator is enabled and the BIAS pin gets the traditional tradeoff between dropout voltage, quies- charged. The thresholds of the burst comparator are cent current and load transient response inherent in most dynamically adjusted to maintain a DC level shown by LDO regulator architectures. The LTC3035 also includes Figure 1. BIAS regulates to 1.9 • VIN or 5V, whichever overshoot detection circuitry which brings the output back voltage is lower. The voltage ripple at BIAS is controlled to into regulation when going from heavy to light output approximately 1% of its DC value. loads (see Figure 2). The LTC3035 also includes a soft-start feature to prevent
LDO Operation
excessive current flow during start-up. After the BIAS An undervoltage lockout comparator (UVLO) senses the voltage reaches regulation, the soft-start circuitry gradu- BIAS voltage to ensure that the BIAS supply for the LDO is ally increases the LDO reference voltage from 0V to 0.4V greater than 90% of its regulation value before over a period of about 600µs. There is a short 700µs delay enabling the LDO. Once the LDO gets enabled, the UVLO from the time BIAS reaches regulation until the LDO threshold switches to 50% of its regulation value. Thus the output starts to rise (see Figure 3). BIAS voltage must fall below 50% of its regulation voltage 3035f 6