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LTC3440 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3440 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3532 9 3532fc 7 VIN A 3 SW1 5 GND 4 SW2 L +– 6 VOUT D C 250mA 0mA 3532 F03 T1 B dI dt VIN L ≈ OPERATIO Figure 3. Inductor Charge Cycle During Burst Mode Operation 7 VIN A 3 SW1 5 GND 4 SW2 L –+ 6 VOUT D C 250mA 0mA 3532 F04 T2 B dI dt VOUT L ≈ – Figure 4. Inductor Disharge Cycle During Burst Mode Operation Note that the peak efficiency during Burst Mode operation is less than the peak efficiency during fixed frequency 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 control mode, therefore full-time 4-switch mode is required 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 88% 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 Operation: I = 10.5V RBURST Leave Burst Mode Operation: I = 7V RBURST where RBURST is in kΩ and IBURST is the load transition current in Amps. For automatic operation, a filter capaci- tor should also be connected from BURST to ground to prevent ripple on BURST from causing the IC to oscillate in and out of Burst Mode operation. The equation for the minimum capacitor value is: CBURST(MIN) ≥ COUT •VOUT 60,000V where CBURST(MIN) and COUT are in μF. In the event that a load transient causes the feedback pin to drop by more than 4% from the regulation value while in Burst Mode operation, the IC will immediately switch to fixed frequency mode and an internal pull-up will be momentarily applied to BURST, rapidly charging the BURST capacitor. This prevents the IC from immediately reentering Burst Mode operation once the output achieves regulation. Manual Burst Mode Operation For manual control of Burst Mode operation, the RC net- work connected to BURST can be eliminated. To force fixed frequency mode, BURST should be connected to VOUT. To force Burst Mode operation, BURST should be grounded. When commanding Burst Mode operation manually, the circuit connected to BURST should be able to sink up to 2mA. For optimum transient response with large dynamic loads, the operating mode should be controlled manually by the host. By commanding fixed frequency operation prior to a sudden increase in load, output voltage droop can |
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Similar Description - LTC3440 |
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