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LTC3442 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3442 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3442 10 3442fb For more information www.linear.com/LTC3442 OPERATION Boost Region (VIN < VOUT) Switch A is always on and switch B is always off during this mode. When the internal control voltage, VCI, is above voltageV3,switchpairCDwillalternatelyswitchtoprovide a boosted output voltage. This operation is typical to a synchronous boost regulator. The maximum duty cycle of the converter is limited to 88% typical and is reached when VCI is above V4. BURST MODE OPERATION Burst Mode operation occurs when the IC delivers energy to the output until it is regulated and then goes into a sleep mode where the outputs are off and the IC is consuming only 35µA of quiescent current from VIN. In this mode the output ripple has a variable frequency component that depends upon load current, and will typically be about 2% peak-to-peak. Burst Mode operation ripple can be reduced slightly by using more output capacitance (47µF or greater). Another method of reducing Burst Mode operation ripple is to place a small feedforward capacitor across the upper resistor in the VOUT feedback divider network (as in Type III compensation). During the period where the device is delivering energy to the output, the peak switch current will be equal to 900mA typical and the inductor current will terminate at zero current for each cycle. In this mode the typical maximum average output current is given by: IOUT(MAX)BURST ≈ 0.2 • VIN VOUT + VIN A Note that the peak efficiency during Burst Mode operation is less than the peak efficiency during fixed frequency 9 VIN A 4 SW1 5 GND 6 SW2 L + – 8 VOUT D C 900mA 0mA 3442 F03 T1 B dI dt VIN L Figure 3. Inductor Charge Cycle During Burst Mode Operation Figure 4. Inductor Discharge Cycle During Burst Mode Operation 9 VIN A 4 SW1 5 GND 6 SW2 L – + 8 VOUT D C 900mA 0mA 3442 F04 T2 B dI dt VOUT L – because the part enters full-time 4-switch mode (when servicing the output) with discontinuous inductor cur- rent as illustrated in Figures 3 and 4. During Burst Mode operation, the control loop is nonlinear and cannot utilize the control voltage from the error amp to determine the controlmode,thereforefull-time4-switchmodeisrequired to maintain the buck/boost function. The efficiency below 1mA becomes dominated primarily by the quiescent cur- rent. The Burst Mode operation efficiency is given by: EFFICIENCY ≅ n •ILOAD 35µA +ILOAD where n is typically 82% during Burst Mode operation. Automatic Burst Mode Operation Control Burst Mode operation can be automatic or manually con- trolled with a single pin. In automatic mode, the IC will enter Burst Mode operation at light load and return to fixed frequency operation at heavier loads. The load current at which the mode transition occurs is programmed using a single external resistor from the BURST pin to ground, according to the following equations: Enter Burst Mode: I = 17.6 RBURST Leave Burst Mode: I = 22.4 RBURST where RBURST is in kΩ and IBURST is the load transition current in Amps. Do not use values of RBURST greater than 250k. |
Similar Part No. - LTC3442_15 |
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Similar Description - LTC3442_15 |
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