Datasheet LTC1174, LTC1174-3.3, LTC1174-5 (Analog Devices) - 7

LTC1174 LTC1174-3.3/LTC1174-5
U OPERATIO (Refer to Functional Diagram)
The LTC1174 uses a constant off-time architecture to In sleep mode, the LTC1174 is in standby and the load switch its internal P-channel power MOSFET. The off-time current is supplied by the output capacitor. All unused is set by an internal timing capacitor and the operating circuitry is shut off, reducing quiescent current from frequency is a function of VIN. 0.45mA to 0.13mA. When the output capacitor discharges by the amount of the hysteresis of the comparator A1, the The output voltage is set by an internal resistive divider P-channel switch turns on again and the process repeats (LTC1174-3.3 and LTC1174-5) or an external divider re- itself. turned to VFB Pin 1 (LTC1174). A voltage comparator A1 compares the divided output voltage to a reference voltage
Operating Frequency and Inductor
of 1.25V. Since the LTC1174 utilizes a constant off-time architecture, To optimize efficiency, the LTC1174 automatically switches its operating frequency is dependent on the value of V between continuous and Burst Mode® operation. The volt- IN. The frequency of operation can be expressed as: age comparator is the primary control element when the device is in Burst Mode operation, while the current com- 1 ⎛ V − V IN OUT ⎞ parator controls the output voltage in continuous mode. f = Hz t ⎝⎜ V + V OFF IN D ⎠ ⎟ ( ) During the switch“ON” time, switch current flows through the 0.1Ω sense resistor. When this current reaches the where tOFF = 4µs and VD is the voltage drop across the diode. threshold of the current comparator A2, its output signal will Note that the operating frequency is a function of the input change state, setting the flip-flop and turning the switch off. and ouput voltage. The timing capacitor, CT, begins to discharge until its Although the size of the inductor does not affect the fre- voltage goes below VTH1. Comparator A4 will then trip, quency, it does affect the ripple current. The peak-to-peak which resets the flip-flop and causes the switch to turn on ripple current is given by: again. Also, the timing capacitor is recharged. The inductor current will again ramp up until the current comparator A2 − ⎛ V + V OUT D = ⎞ trips. The cycle then repeats. I 4 10 6 • A RIPPLE P−P ⎝⎜ L ⎠⎟ ( ) When the load is relatively light, the LTC1174 automatically By choosing a smaller inductor, a low ESR output filter goes into Burst Mode operation. The current mode loop is capacitor has to be used (see C interrupted when the output voltage reaches the desired IN and COUT). Moreover, core loss will also increase (see Inductor Core Selection section) regulated value. The hysteretic voltage comparator A1 trips due to higher ripple current. when VOUT is above the desired output voltage, shutting off the switch and causing the timing capacitor to discharge. Burst Mode is a registered trademark of Linear Technology Corporation. This capacitor discharges past VTH1 until its voltage drops below VTH2. Comparator A5 then trips and a sleep signal is generated. 1174fe 7