The ADP3000 is a versatile step-up/step-down switching regulator. It operates from 2 V to 12 V in the step-up mode, and up to 30 V in the step-down mode. ADP3000 has a low 500-µA operating current (with the oscillator shut off) and operates with a gated-oscillator (up to 400 kHz) to make it possible to use very small inductors and capacitors.
The functional block diagram shows the main sections of the device: The 1.245-V reference is connected to one input of the comparator; the other comparator input is from the feedback-sense terminal. The output of the comparator gates, the oscillator on/off. Hysteresis stabilizes the comparator's switching.
When the voltage at the FB pin drops below the reference voltage, the oscillator is turned on. This oscillator is set to a 2:1 mark/space ratio, with 1.7 µs ON and 0.8 µs OFF ideal for applications that need a factor of about 2 between the input voltage and the output voltage, such as +3 V to +5 V or +5 V to +12 V. All other conversion factors can be achieved, too, like +5 V to +3.3 V. The oscillator is turned OFF when the voltage Fb is higher than the reference. The fixed-voltage versions of the ADP3000 contain a precision voltage divider, as shown, so that the Fb pin can be directly connected to the output voltage; the adjustable version (used in the example of Figure 2) requires an external voltage divider to set the correct output voltage.
Another control that can shut off the output from the oscillator is, the current limit, ILIM. In this circuit, without an external resistor, the current limit is set by an internal 80-ohms resistor to 1.5-A peak current in the switch. The current in Q2 is 0.5% of the current in Q1, the power switch. When the voltage at the base of Q3 crosses 0.6 V, Q3 opens and shuts off the oscillator. Resistance added externally (R3) reduces the value of IPEAK. R3, in conjunction with the internal 80-ohms resistor, limits the current in the switch: R3 = (120 V/IPEAK A) 80 ohms. If a small inductor is used, it is necessary to limit the current to prevent saturation of the core.
As shown, the inductor is connected between the positive input voltage and the collector of the switch. Note that for an output of 3.3 V and 150 mA only 5 µH inductance is needed (currently, the smallest-physical-size inductor from Sumida is type CDH53). This is made possible by the high operating frequency of the ADP3000. The boost function is achieved by periodically connecting the inductor to ground (switch is ON), then disconnecting it again. When the switch is ON, current starts flowing from VIN to ground via the inductor. Before current through the transistor can build up substantially, the inductor is disconnected by switching the transistor OFF. Since inductor current seeks to continue flowing, it flows through the Schottky (for low dissipation) diode and gets rid of the magnetic field's stored energy by building up an increment of charge on the capacitor during the second half of each cycle. The voltage on the capacitor will be higher than the input voltage. Fb monitors this output voltage and, as described above, controls the oscillator/switching, skipping cycles when the output is too high.
ADP3000 IN BUCK-MODE
When switch is ON, current starts flowing through the switch transistor via the diode into the inductor. When set to OFF, the transistor current stops flowing, but the inductor current keeps flowing, thus bringing the pin SW2 below ground. (As this point can only accept a maximum of 0.5 V, diode D1 has to be a Schottky type.) The capacitor is charged, but this time with a voltage lower than VIN. The ADP3000 used here is an adjustable type; the resistors R1 and R2 are the external voltage divider; Fb is connected at the divider point, and the nominal value of controlled output voltage is (1 + R2/R1) x 1.245 V.
A second diode, D2, not shown in the boost regulator, is needed for output voltage above 6.2 V, the maximum the switch can tolerate. D2 simply isolates high voltages from the switch.
As shown, only a few small external components are needed to use the ADP3000. Due to the high operating frequency, only very small inductors and capacitors are used, saving significant space and some cost. ADP3000 is available in an 8-lead plastic DIP or SOIC package. Pricing starts at $2.14 in 1000s.
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