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LT1173CN8 Datasheet(PDF) 5 Page - Linear Technology |
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LT1173CN8 Datasheet(HTML) 5 Page - Linear Technology |
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5 / 16 page  LT1173 5 The LT1173 is a gated oscillator switcher. This type archi- tecture has very low supply current because the switch is cycled only when the feedback pin voltage drops below the reference voltage. Circuit operation can best be under- stood by referring to the LT1173 block diagram. Compara- tor A1 compares the feedback pin voltage with the 1.245V reference voltage. When feedback drops below 1.245V, A1 switches on the 24kHz oscillator. The driver amplifier boosts the signal level to drive the output NPN power switch. An adaptive base drive circuit senses switch current and provides just enough base drive to ensure switch saturation without overdriving the switch, resulting in higher efficiency. The switch cycling action raises the output voltage and feedback pin voltage. When the feed- back voltage is sufficient to trip A1, the oscillator is gated off. A small amount of hysteresis built into A1 ensures loop stability without external frequency compensation. When the comparator is low the oscillator and all high current circuitry is turned off, lowering device quiescent current to just 110 µA, for the reference, A1 and A2. The oscillator is set internally for 23 µs ON time and 19µs OFF time, optimizing the device for circuits where VOUT and VIN differ by roughly a factor of 2. Examples include a 3V to 5V step-up converter or a 9V to 5V step-down converter. LT1173 OPERATI A2 is a versatile gain block that can serve as a low battery detector, a linear post regulator, or drive an under voltage lockout circuit. The negative input of A2 is internally connected to the 1.245V reference. A resistor divider from VIN to GND, with the mid-point connected to the SET pin provides the trip voltage in a low battery detector applica- tion. The gain block output (AO) can sink 100 µA(usea47k resistor pull-up to + 5V). This line can signal a microcon- troller that the battery voltage has dropped below the preset level. A resistor connected between the ILIM pin and VIN sets maximum switch current. When the switch current ex- ceeds the set value, the switch cycle is prematurely terminated. If current limit is not used, ILIM should be tied directly to VIN. Propagation delay through the current limit circuitry is approximately 2 µs. In step-up mode the switch emitter (SW2) is connected to ground and the switch collector (SW1) drives the induc- tor; in step-down mode the collector is connected to VIN and the emitter drives the inductor. The LT1173-5 and LT1173-12 are functionally identical to the LT1173. The -5 and -12 versions have on-chip voltage setting resistors for fixed 5V or 12V outputs. Pin 8 on the fixed versions should be connected to the output. No external resistors are needed. S APPLICATI I FOR ATIO Measuring Input Current at Zero or Light Load Obtaining meaningful numbers for quiescent current and efficiency at low output current involves understanding how the LT1173 operates. At very low or zero load current, the device is idling for seconds at a time. When the output voltage falls enough to trip the comparator, the power switch comes on for a few cycles until the output voltage rises sufficiently to overcome the comparator hysteresis. When the power switch is on, inductor current builds up to hundreds of milliamperes. Ordinary digital multimeters are not capable of measuring average current because of bandwidth and dynamic range limitations. A different approach is required to measure the 100 µA off-state and 500mA on-state currents of the circuit. Quiescent current can be accurately measured using the circuit in Figure 1. VSET is set to the input voltage of the LT1173. The circuit must be “booted” by shorting V2 to VSET. After the LT1173 output voltage has settled, discon- nect the short. Input voltage is V2, and average input current can be calculated by this formula: I VV IN = − () 21 100 01 Ω |
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