Datasheet LTC1265, LTC1265-3.3, LTC1265-5 (Analog Devices) - 7

LTC1265/LTC1265-3.3/LTC1265-5
U U W U APPLICATIO S I FOR ATIO
Under short-circuit condition, the peak inductor current is 2V, the LTC1265 reduces tOFF by increasing the discharge determined by: current in CT. This prevents audible operation prior to dropout. (See shelving effect shown in the Operating I Frequency curve under Typical Performance Character- SC(PK) = 150mV (Amps) RSENSE istics.) In this condition, the LTC1265 automatically extends the To maintain continuous inductor current at light load, the off time of the P-channel MOSFET to allow the inductor inductor must be chosen to provide no more than 25mV/ current to decay far enough to prevent any current build- RSENSE of peak-to-peak ripple current. This results in the up. The resulting ripple current causes the average short- following expression for L: circuit current to be approximately IOUT(MAX). L ≥ 5.2(105)RSENSE(CT)VREG
CT and L Selection for Operating Frequency
Using an inductance smaller than the above value will The LTC1265 uses a constant off-time architecture with result in the inductor current being discontinuous. A t consequence of this is that the LTC1265 will delay entering OFF determined by an external capacitor CT. Each time the P-channel MOSFET turns on, the voltage on C Burst Mode operation and efficiency will be degraded at T is reset to approximately 3.3V. During the off time, C low currents. T is discharged by a current that is proportional to VOUT. The voltage on CT
Inductor Core Selection
is analogous to the current in inductor L, which likewise, decays at a rate proportional to VOUT. Thus the inductor With the value of L selected, the type of inductor must be value must track the timing capacitor value. chosen. Basically, there are two kinds of losses in an inductor; core and copper losses. The value of CT is calculated from the desired continuous mode operating frequency: Core losses are dependent on the peak-to-peak ripple current and core material. However it is independent of V C IN – VOUT ) the physical size of the core. By increasing the induc- T = 1 (Farads) 1.3(104)f VIN + VD tance, the peak-to-peak inductor ripple current will de- crease, therefore reducing core loss. Utilizing low core where VD is the drop across the Schottky diode. loss material, such as molypermalloy or Kool Mµ® will As the operating frequency is increased, the gate charge allow user to concentrate on reducing copper loss and losses will reduce efficiency. The complete expression for preventing saturation. operating frequency is given by: Although higher inductance reduces core loss, it in- creases copper loss as it requires more windings. When V f ≈ 1 IN – VOUT (Hz) space is not at a premium, larger wire can be used to tOFF VIN + VD reduce the wire resistance. This also prevents excessive heat dissipation. where: V t REG )
CATCH DIODE SELECTION
OFF = 1.3(104)CT (sec) VOUT Losses in the catch diode depend on forward drop and VREG is the desired output voltage (i.e. 5V, 3.3V). VOUT is switching times. Therefore Schottky diodes are a good the measured output voltage. Thus VREG/VOUT = 1 choice for low drop and fast switching times. in regulation. The catch diode carries load current during the off time. Note that as VIN decreases, the frequency decreases. The average diode current is therefore dependent on the When the input-to-output voltage differential drops below Kool Mµ is a registered trademark of Magnetics, Inc. 7