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LTC1149CS Datasheet(PDF) 8 Page - Linear Technology |
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LTC1149CS Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1149 LTC1149-3.3/LTC1149-5 APPLICATIO S I FOR ATIO Figure 3. Timing Capacitor Selection L and CT Selection for Operating Frequency The LTC1149 series uses a constant off-time architecture with tOFF determined by an external timing capacitor CT. Each time the P-channel MOSFET switch turns on, the voltage on CT is reset to approximately 3.3V. During the off-time, CT is discharged by a current which is propor- tional to VOUT. The voltage on CT is analogous to the current in inductor L, which likewise decays at a rate proportional to VOUT. Thus the inductor value must track the timing capacitor value. The value of CT is calculated from the desired continuous mode operating frequency, f: CT = (7.8)(10–5) f ) )1– VOUT VIN A graph for selecting CT versus frequency including the effects of input voltage is given in Figure 3. As the operating frequency is increased the gate charge losses will be higher, reducing efficiency (see Efficiency Considerations). The complete expression for operating frequency is given by: f = 1 tOFF ) )1– VOUT VIN where: tOFF = (1.3)(104)(CT) ) )VREG VOUT VREG is the desired output voltage (i.e., 5V, 3.3V), while VOUT is the actual output voltage. Thus VREG/VOUT = 1 when in regulation. Note that as VIN decreases, the frequency decreases. When the input to output voltage differential drops below 1.5V, the LTC1149 series reduces tOFF by increasing the discharge current in CT. This prevents audible operation prior to dropout. Once the frequency has been set by CT, the inductor L must be chosen to provide no more than 25mV/RSENSE of peak- to-peak inductor ripple current. This results in a minimum required inductor value of: LMIN =( 5.1)(105)(RSENSE)(CT)(VREG) As the inductor value is increased from the minimum value, the ESR requirements for the output capacitor are eased at the expense of efficiency. If too small an inductor is used, the inductor current will decrease past zero and change polarity. A consequence of this is that the LTC1149 series may not enter Burst Mode operation and efficiency will be severely degraded at low currents. FREQUENCY (kHz) 0 0 200 400 600 1400 1000 50 100 1200 800 150 200 250 VOUT = 5V VIN = 48V VIN=12V VIN = 24V 1149 F03 The LTC1149 series automatically extends tOFF during a short circuit to allow sufficient time for the inductor current to decay between switch cycles. The resulting ripple current causes the average short-circuit current ISC(AVG) to be reduced to approximately IMAX. Figure 2. RSENSE vs Maximum Output Current MAXIMUM OUTPUT CURRENT (A) 0 0.12 0.16 0.20 4 1149 F02 0.08 0.04 0 1 2 3 5 0.10 0.14 0.18 0.06 0.02 |
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