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LTC1174CN8-5 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1174CN8-5 Datasheet(HTML) 7 Page - Linear Technology |
7 / 20 page 7 LTC1174 LTC1174-3.3/LTC1174-5 1174fe (Refer to Functional Diagram) The LTC1174 uses a constant off-time architecture to switch its internal P-channel power MOSFET. The off-time is set by an internal timing capacitor and the operating frequency is a function of VIN. The output voltage is set by an internal resistive divider (LTC1174-3.3 and LTC1174-5) or an external divider re- turned to VFB Pin 1 (LTC1174). A voltage comparator A1 compares the divided output voltage to a reference voltage of 1.25V. To optimize efficiency, the LTC1174 automatically switches between continuous and Burst Mode ® operation. The volt- age comparator is the primary control element when the device is in Burst Mode operation, while the current com- parator controls the output voltage in continuous mode. During the switch“ON” time, switch current flows through the 0.1 Ω sense resistor. When this current reaches the threshold of the current comparator A2, its output signal will change state, setting the flip-flop and turning the switch off. The timing capacitor, CT, begins to discharge until its voltage goes below VTH1. Comparator A4 will then trip, which resets the flip-flop and causes the switch to turn on again. Also, the timing capacitor is recharged. The inductor current will again ramp up until the current comparator A2 trips. The cycle then repeats. When the load is relatively light, the LTC1174 automatically goes into Burst Mode operation. The current mode loop is interrupted when the output voltage reaches the desired regulated value. The hysteretic voltage comparator A1 trips when VOUT is above the desired output voltage, shutting off the switch and causing the timing capacitor to discharge. This capacitor discharges past VTH1 until its voltage drops below VTH2. Comparator A5 then trips and a sleep signal is generated. In sleep mode, the LTC1174 is in standby and the load current is supplied by the output capacitor. All unused circuitry is shut off, reducing quiescent current from 0.45mA to 0.13mA. When the output capacitor discharges by the amount of the hysteresis of the comparator A1, the P-channel switch turns on again and the process repeats itself. Operating Frequency and Inductor Since the LTC1174 utilizes a constant off-time architecture, its operating frequency is dependent on the value of VIN. The frequency of operation can be expressed as: f t VV VV Hz OFF IN OUT IN D = − + ⎛ ⎝⎜ ⎞ ⎠⎟ () 1 where tOFF =4µsandVDisthevoltagedropacrossthediode. Note that the operating frequency is a function of the input and ouput voltage. Although the size of the inductor does not affect the fre- quency, it does affect the ripple current. The peak-to-peak ripple current is given by: I VV L A RIPPLE OUT D PP = + ⎛ ⎝⎜ ⎞ ⎠⎟ () − − 410 6 • By choosing a smaller inductor, a low ESR output filter capacitor has to be used (see CIN and COUT). Moreover, core loss will also increase (see Inductor Core Selection section) due to higher ripple current. OPERATIO Burst Mode is a registered trademark of Linear Technology Corporation. |
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Similar Description - LTC1174CN8-5 |
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