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LTC1142HVCG Datasheet(PDF) 10 Page - Linear Technology |
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LTC1142HVCG Datasheet(HTML) 10 Page - Linear Technology |
10 / 20 page 10 LTC1142/LTC1142L/LTC1142HV APPLICATIO S I FOR ATIO 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 section). The complete expression for operating frequency of the circuit in Figure 1 is given by: f t V V OFF OUT IN =− 1 1 where: tC V V OFF T REG OUT =× × × 13 10 4 . VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT is the measured output voltage. Thus VREG/VOUT = 1 in regulation. Note that as VIN decreases, the frequency decreases. When the input-to-output voltage differential drops below 1.5V for a particular section, the LTC1142 reduces tOFF in that section by increasing the discharge current in CT. This prevents audible operation prior to dropout. Figure 2. Selecting RSENSE MAXIMUM OUTPUT CURRENT (A) 0 0.20 0.15 0.10 0.05 0 4 1142 F02 1 2 3 5 L and CT Selection for Operating Frequency Each regulator section of the LTC1142 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 proportional 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: C f T = ×× 1 26 10 4 . Assumes VIN = 2VOUT, Figure 1 circuit. Figure 3. Timing Capacitor Value FREQUENCY (kHz) 0 50 100 150 200 1142 F03 250 1000 800 600 400 200 0 300 VIN = 12V VIN = 10V VIN = 7V VSENSE = VOUT = 5V 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 both track IMAX. Once RSENSE has been chosen, IBURST and ISC(PK) can be predicted from the following: IBURST SENSE SC(PK) SENSE 15mV R I= 150mV R ≈ The LTC1142 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. |
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