Datasheet LTC1265, LTC1265-3.3, LTC1265-5 (Analog Devices) - 6

LTC1265/LTC1265-3.3/LTC1265-5
U OPERATIO (Refer to Functional Diagram)
capacitor discharges past VTH1 until its voltage drops To avoid the operation of the current loop interfering with below VTH2. Comparator S then trips and a sleep signal is Burst Mode operation, a built-in offset VOS is incorporated generated. The circuit now enters into sleep mode with the in the gain stage. This prevents the current from increas- power MOSFET turned off. In sleep mode, the LTC1265 is ing until the output voltage has dropped below a minimum in standby and the load current is supplied by the output threshold. capacitor. All unused circuitry is shut off, reducing quies- Using constant off-time architecture, the operating fre- cent current from 2mA to 160µA. When the output capaci- quency is a function of the voltage. To minimize the tor discharges by the amount of the hysteresis of the frequency variation as dropout is approached, the off-time comparator V, the P-channel switch turns on again and the controller increases the discharge current as V process repeats itself. During Burst Mode operation the IN drops below V peak inductor current is set at 25mV/R OUT + 2V. In dropout the P-channel MOSFET is SENSE. turned on continuously (100% duty cycle) providing low dropout operation with VOUT ≅ VIN.
U U W U APPLICATIO S I FOR ATIO
The basic LTC1265 application circuit is shown in I 25mV OUT(MAX) = 150mV – (Amps) Figure 1. External component selection is driven by the RSENSE 2 • RSENSE load requirement, and begins with the selection of RSENSE. 137.5mV Once R = SENSE is known, CT and L can be chosen. Next, the (Amps) R Schottky diode D1 is selected followed by C SENSE IN and COUT. Solving for R
R
SENSE and allowing a margin of variations in
SENSE Selection for Output Current
the LTC1265 and extended component values yields: RSENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- RSENSE = 100mV (Ω) oped across RSENSE, the threshold of the comparator IOUT(MAX) determines the peak inductor current. Depending on the load current condition, the threshold of the comparator The LTC1265 is rated with a capability to supply a maximum lies between 25mV/R of 1.2A of output current. Therefore, the minimum value of SENSE and 150mV/RSENSE. The maxi- mum output current of the LTC1265 is: RSENSE that can be used is 0.083Ω. A graph for selecting RSENSE versus maximum output is given in Figure 2. I IRIPPLE OUT(MAX) = 150mV – (Amps) R 0.5 SENSE 2 where IRIPPLE is the peak-to-peak inductor ripple current. 0.4 At a relatively light load, the LTC1265 is in Burst Mode ) Ω 0.3 operation. In this mode the peak inductor current is set at ( 25mV/RSENSE. To fully benefit from Burst Mode operation, SENSER 0.2 the inductor current should be continuous during burst periods. Hence, the peak-to-peak inductor ripple current 0.1 must not exceed 25mV/RSENSE. To account for light and heavy load conditions, the I 0 OUT(MAX) 0 0.2 0.4 0.6 0.8 1 is then given by: MAXIMUM OUTPUT CURRENT (A) 1265 G10
Figure 2. Selecting RSENSE
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