Datasheet LT1073 (Analog Devices) - 10

LT1073
APPLICATIONS INFORMATION Using the ILIM Pin
Figure 10, the inductor current increases to a high level The LT1073 switch can be programmed to turn off at a set before the comparator turns off the oscillator. This high switch current, a feature not found on competing devices. current can cause excessive output ripple and requires This enables the input to vary over a wide range without oversizing the output capacitor and inductor. With the I exceeding the maximum switch rating or saturating the LIM feature, however, the switch current turns off at a programmed level as shown in Figure 11, keeping output inductor. Consider the case where analysis shows the ripple to a minimum. LT1073 must operate at an 800mA peak switch current with a 2V input. If VIN rises to 4V, the peak switch current
Using the Gain Block
will rise to 1.6A, exceeding the maximum switch current rating. With the proper resistor (see the “Maximum Switch The gain block (GB) on the LT1073 can be used as an er- Current vs R ror amplifier, low-battery detector or linear post-regulator. LIM” characteristic) selected, the switch cur- rent will be limited to 800mA, even if the input voltage The gain block itself is a very simple PNP input op amp increases. The LT1073 does this by sampling a small with an open-collector NPN output. The (–) input of the fraction of the switch current and passing this current gain block is tied internally to the 212mV reference. The (+) input comes out on the SET pin. through the external resistor. When the voltage on the ILIM pin drops a VBE below VIN, the oscillator terminates the Arrangement of the gain block as a low battery detector cycle. Propagation delay through this loop is about 2µs. is straightforward. Figure 12 shows hookup. R1 and R2 need only be low enough in value so that the bias current Another situation where the ILIM feature is useful is when of the SET input does not cause large errors. 100kΩ for the device goes into continuous mode operation. This R2 is adequate. occurs in step-up mode when Output ripple of the LT1073, normally 150mV at 5V V OUT, OUT + VDIODE < 1 can be reduced significantly by placing the gain block in VIN – VSW 1–DC front of the FB input as shown in Figure 13. This effectively reduces the comparator hysteresis by the gain of the gain When the input and output voltages satisfy this relation- block. Output ripple can be reduced to just a few millivolts ship, inductor current does not go to zero during the using this technique. Ripple reduction works with step- switch OFF time. When the switch turns on again, the down or inverting modes as well. current ramp starts from the nonzero current level in the inductor just prior to switch turn-on. As shown in PROGRAMMED CURRENT LIMIT IL IL ON ON SWITCH SWITCH OFF OFF 1073 F10 1073 F11
Figure 10. No Current Limit Causes Figure 11. Current Limit Keeps Inductor Current Under Control Large Inductor Current Build-Up
Rev B 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagrams Operation Applications Information Typical Applications Package Description Typical Application Related Parts