Datasheet LT1513, LT1513-2 (Analog Devices) - 6

LT1513/LT1513-2
W BLOCK DIAGRAM
VIN SW SHUTDOWN LOW DROPOUT S/S ANTISAT DELAY AND RESET 2.3V REG 500kHz SYNC LOGIC DRIVER SWITCH OSC + IFBA 4k IFB – 50k* COMP – + – VFB EA IA 0.04Ω + – V A C V ≈ 6 1.245V REF *REMOVE ON LT1513-2 LT1513 • BD
Figure 2 U OPERATION
The LT1513 is a current mode switcher. This means that 1.245V bandgap reference biases the noninverting input. switch duty cycle is directly controlled by switch current The first inverting input of the error amplifier is brought out rather than by output voltage or current. Referring to the for positive output voltage sensing. The second inverting Block Diagram, the switch is turned “on” at the start of each input is driven by a “current” amplifier which is sensing oscillator cycle. It is turned “off” when switch current output current via an external current sense resistor. The reaches a predetermined level. Control of output voltage current amplifier is set to a fixed gain of – 12.5 which and current is obtained by using the output of a dual provides a – 100mV current limit sense voltage. feedback voltage sensing error amplifier to set switch The LT1513-2 option removes the feedback resistors current trip level. This technique has the advantage of around the I simplified loop frequency compensation. A low dropout FB amplifier and connects its output to the FB signal. This provides a ground referenced current sense internal regulator provides a 2.3V supply for all internal voltage suitable for external current programming and circuitry on the LT1513. This low dropout design allows makes amplifier input and output available for external input voltage to vary from 2.7V to 25V. A 500kHz oscillator loop compensation. is the basic clock for all internal timing. It turns “on” the output switch via the logic and driver circuitry. Special The error signal developed at the amplifier output is adaptive antisat circuitry detects onset of saturation in the brought out externally and is used for frequency compen- power switch and adjusts driver current instantaneously to sation. During normal regulator operation this pin sits at a limit switch saturation. This minimizes driver dissipation voltage between 1V (low output current) and 1.9V (high and provides very rapid turn-off of the switch. output current). Switch duty cycle goes to zero if the VC pin is pulled below the V A unique error amplifier design has two inverting inputs C pin threshold, placing the LT1513 in an idle mode. which allow for sensing both output voltage and current. A sn1513 1513fas 6