Datasheet LTC1265, LTC1265-3.3, LTC1265-5 (Analog Devices) - 5

LTC1265/LTC1265-3.3/LTC1265-5
U U W FU CTIO AL DIAGRA (Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
1, 13 PWR VIN SENSE+ SENSE– 8 7 14 SW 12 PGND VFB – 9 ADJUSTABLE VERSION V + – SLEEP R 25mV TO 150mV C + Q + 5pF VOS S S – – VTH2 13k ITH 6 G V + 100k TH1 – T 2 V + IN 3 LB0UT + REFERENCE 10 5 CT OFF-TIME A3 SENSE– SHDN CONTROL – VFB 4 LBIN 11 SGND 1265 FD
U OPERATIO (Refer to Functional Diagram)
The LTC1265 uses a constant off-time architecture to the voltage across the shunt reaches the comparator’s switch its internal P-channel power MOSFET. The off time threshold value, its output signal will change state, setting is set by an external timing capacitor at CT (Pin 5). The the flip flop and turning the internal P-channel MOSFET off. operating frequency is then determined by the off time and The timing capacitor connected to Pin 5 is now allowed to the difference between VIN and VOUT. discharge at a rate determined by the off-time controller. The output voltage is set by an internal resistive divider When the voltage on the timing capacitor has discharged (LTC1265-3.3 and LTC1265-5) connected to SENSE– past VTH1, comparator T trips, sets the flip flop and causes (Pin 7) or an external divider returned to VFB (Pin 9 for the switch to turn on. Also, the timing capacitor is re- LTC1265). A voltage comparator V, and a gain block G, charged. The inductor current will again ramp up until the compare the divided output voltage with a reference current comparator C trips. The cycle then repeats. voltage of 1.25V. When the load current increases, the output voltage de- To optimize efficiency, the LTC1265 automatically switches creases slightly. This causes the output of the gain stage between continuous and Burst Mode operation. The volt- (Pin 6) to increase the current comparator threshold, thus age comparator is the primary control element when the tracking the load current. device is in Burst Mode operation, while the gain block When the load is relatively light, the LTC1265 automati- controls the output voltage in continuous mode. cally goes into Burst Mode operation. The current loop is When the load is heavy, the LTC1265 is in continuous interrupted when the output voltage exceeds the desired operation. During the switch ON time, current comparator regulated value. The hysteretic voltage comparator V trips C monitors the voltage between Pins 7 and 8 connected when VOUT is above the desired output voltage, shutting across an external shunt in series with the inductor. When off the switch and causing the capacitor to discharge. This 5