Datasheet LTC3025-1, LTC3025-2, LTC3025-3, LTC3025-4 (Analog Devices) - 8

LTC3025-1/LTC3025-2/ LTC3025-3/LTC3025-4
applicaTions inForMaTion Operation (Refer to Block Diagram) Adjustable Output Voltage (LTC3025-1)
The LTC3025-X is a micropower, VLDO (very low dropout) The output voltage is set by the ratio of two external resis- linear regulator which operates from input voltages as low tors as shown in Figure 2. The device servos the output as 0.9V. The device provides a highly accurate output that to maintain the ADJ pin voltage at 0.4V (referenced to is capable of supplying 500mA of output current with a ground). Thus, the current in R1 is equal to 0.4V/R1. For typical dropout voltage of only 85mV. A single ceramic good transient response, stability, and accuracy, the current capacitor as small as 1µF is all that is required for output in R1 should be at least 8µA, thus the value of R1 should bypassing. A low reference voltage allows the LTC3025-1 be no greater than 50k. The current in R2 is the current in output to be programmed to much lower voltages than R1 plus the ADJ pin bias current. Since the ADJ pin bias available in common LDOs (range of 0.4V to 3. 6V). The current is typically <10nA, it can be ignored in the output LTC3025-2/LTC3025-3/LTC3025-4 have fixed outputs of voltage calculation. The output voltage can be calculated 1.2V, 1.5V and 1.8V respectively, eliminating the need for using the formula in Figure 2. Note that in shutdown the an external resistor divider. output is turned off and the divider current will be zero As shown in the Block Diagram, the BIAS input supplies once COUT is discharged. the internal reference and LDO circuitry while all output The LTC3025-1 operates at a relatively high gain of –0.7µV/ current comes directly from the IN input for high efficiency mA referred to the ADJ input. Thus a load current change regulation. The low quiescent supply currents IIN = 4µA, of 1mA to 500mA produces a –0.35mV drop at the ADJ IBIAS = 50µA drop to IIN = 1µA, IBIAS = 0.01µA typical in input. To calculate the change referred to the output shutdown making the LTC3025-X an ideal choice for use simply multiply by the gain of the feedback network in battery-powered systems. (i. e. ,1 + R2/R1). For example, to program the output for The device includes current limit and thermal overload 1.2V choose R2/R1 = 2. In this example, an output current protection. The fast transient response of the follower change of 1mA to 500mA produces –0.35mV • (1 + 2) = output stage overcomes the traditional tradeoff between 1.05mV drop at the output. dropout voltage, quiescent current and load transient Because the ADJ pin is relatively high impedance (depend- response inherent in most LDO regulator architectures. ing on the resistor divider used), stray capacitance at this The LTC3025-X also includes overshoot detection circuitry pin should be minimized (<10pF) to prevent phase shift which brings the output back into regulation when going in the error amplifier loop. Additionally, special attention from heavy to light output loads (see Figure 1). should be given to any stray capacitances that can couple external signals onto the ADJ pin producing undesirable output ripple. For optimum performance connect the ADJ 300mA I pin to R1 and R2 with a short PCB trace and minimize all OUT 0mA other stray capacitance to the ADJ pin. OUT VOUT ( R2 = 0.4V 1 + R1) VOUT R2 AC 20mV/DIV ADJ COUT R1 GND VIN = 1.5V 100µs/DIV 30251234 F01 V 30251234 F02 OUT = 1.2V VBIAS = 3.6V COUT = 1µF
Figure 2. Programming the LTC3025-1 Figure 1. LTC3025-X Transient Response
30251234ff 8 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Package Description Revision History Typical Application Related Parts