Datasheet LT1300 - 5
| Beschreibung | Micropower High Efficiency 3.3/5V Step-Up DC/DC Converter |
| Seiten / Seite | 8 / 5 — TEST CIRCUITS. Oscillator Test Circuit. OPERATION. Figure 2. Switch Pin … |
| Dateiformat / Größe | PDF / 230 Kb |
| Dokumentensprache | Englisch |
TEST CIRCUITS. Oscillator Test Circuit. OPERATION. Figure 2. Switch Pin Current With ILIM Floating or Grounded
Textversion des Dokuments
LT1300
TEST CIRCUITS Oscillator Test Circuit
5V 2V 100Ω VIN IL SEL SW fOUT 100µF LT1300 SENSE SHDN GND PGND
U OPERATION
Operation of the LT1300 is best understood by referring to the Block Diagram in Figure 1. When A1’s negative input, TRACE A 500mA/DIV related to the Sense pin voltage by the appropriate resis- ILIM PIN OPEN tor-divider ratio, is higher that the 1.25V reference voltage, A1’s output is low. A2, A3 and the oscillator are turned off, drawing no current. Only the reference and A1 consume TRACE B 500mA/DIV current, typically 120µA. When the voltage at A1’s nega- ILIM PIN GROUNDED tive input decreases below 1.25V, overcoming A1’s 6mV hysteresis, A1’s output goes high, enabling the oscillator, 20µs/DIV LT1300 F2 current comparator A2, and driver A3. Quiescent current
Figure 2. Switch Pin Current With ILIM Floating or Grounded
increases to 2mA as the device prepares for high current reduced by tying the ILIM pin to ground, causing 15µA to switching. Q1 then turns on in a controlled saturation for flow through R2 into Q3’s collector. Q3’s current causes (nominally) 5.3µs or until current comparator A2 trips, a 10.4mV drop in R2 so that only an additional 7.6mV is whichever comes first. After a fixed off-time of (nominally) required across R1 to turn off the switch. This corre- 1.2µs, Q1 turns on again. The LT1300’s switching causes sponds to a 400mA switch current as shown in Figure 2, current to alternately build up in L1 and dump into capaci- trace B. The reduced peak switch current reduces I2R tor C2 via D1, increasing the output voltage. When the loses in Q1, L1, C1 and D1. Efficiency can be increased by output is high enough to cause A1’s output to go to low, doing this provided that the accompanying reduction in switching action ceases. C2 is left to supply current to the full load output current is acceptable. Lower peak currents load until VOUT decreases enough to force A1’s output also extend alkaline battery life due to the alkaline cell’s high, and the entire cycle repeats. high internal impedance. Typical operating waveforms are If switch current reaches 1A, causing A2 to trip, switch on- shown in Figure 3. time is reduced and off-time increases slightly. This allows continuous mode operation during bursts. Current com- VOUT 20mV/DIV parator A2 monitors the voltage across 3Ω resistor R1 AC COUPLED which is directly related to inductor L1’s current. Q2’s collector current is set by the emitter-area ratio to 0.6% VSW 5V/DIV of Q1’s collector current. When R1’s voltage drop exceeds 18mV, corresponding to 1A inductor current, A2’s output ISW goes high, truncating the on-time portion of the oscillator IA/DIV cycle and increasing off-time to about 2µs as shown in 20µS/DIV LT1300 F2 Figure 2, trace A. This programmed peak current can be
Figure 3. Burst Mode Operation in Action
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TEST CIRCUITS Oscillator Test Circuit
5V 2V 100Ω VIN IL SEL SW fOUT 100µF LT1300 SENSE SHDN GND PGND
U OPERATION
Operation of the LT1300 is best understood by referring to the Block Diagram in Figure 1. When A1’s negative input, TRACE A 500mA/DIV related to the Sense pin voltage by the appropriate resis- ILIM PIN OPEN tor-divider ratio, is higher that the 1.25V reference voltage, A1’s output is low. A2, A3 and the oscillator are turned off, drawing no current. Only the reference and A1 consume TRACE B 500mA/DIV current, typically 120µA. When the voltage at A1’s nega- ILIM PIN GROUNDED tive input decreases below 1.25V, overcoming A1’s 6mV hysteresis, A1’s output goes high, enabling the oscillator, 20µs/DIV LT1300 F2 current comparator A2, and driver A3. Quiescent current
Figure 2. Switch Pin Current With ILIM Floating or Grounded
increases to 2mA as the device prepares for high current reduced by tying the ILIM pin to ground, causing 15µA to switching. Q1 then turns on in a controlled saturation for flow through R2 into Q3’s collector. Q3’s current causes (nominally) 5.3µs or until current comparator A2 trips, a 10.4mV drop in R2 so that only an additional 7.6mV is whichever comes first. After a fixed off-time of (nominally) required across R1 to turn off the switch. This corre- 1.2µs, Q1 turns on again. The LT1300’s switching causes sponds to a 400mA switch current as shown in Figure 2, current to alternately build up in L1 and dump into capaci- trace B. The reduced peak switch current reduces I2R tor C2 via D1, increasing the output voltage. When the loses in Q1, L1, C1 and D1. Efficiency can be increased by output is high enough to cause A1’s output to go to low, doing this provided that the accompanying reduction in switching action ceases. C2 is left to supply current to the full load output current is acceptable. Lower peak currents load until VOUT decreases enough to force A1’s output also extend alkaline battery life due to the alkaline cell’s high, and the entire cycle repeats. high internal impedance. Typical operating waveforms are If switch current reaches 1A, causing A2 to trip, switch on- shown in Figure 3. time is reduced and off-time increases slightly. This allows continuous mode operation during bursts. Current com- VOUT 20mV/DIV parator A2 monitors the voltage across 3Ω resistor R1 AC COUPLED which is directly related to inductor L1’s current. Q2’s collector current is set by the emitter-area ratio to 0.6% VSW 5V/DIV of Q1’s collector current. When R1’s voltage drop exceeds 18mV, corresponding to 1A inductor current, A2’s output ISW goes high, truncating the on-time portion of the oscillator IA/DIV cycle and increasing off-time to about 2µs as shown in 20µS/DIV LT1300 F2 Figure 2, trace A. This programmed peak current can be
Figure 3. Burst Mode Operation in Action
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