Datasheet LTC3422 (Analog Devices) - 9

LTC3422
U OPERATIO
CSW = SW Capacitance) is low energy, but can cause EMI
Automatic Burst Mode Operation Control
radiation. For automatic operation, an RC network should be con- nected from BURST to ground. The value of the resistor
Burst Mode OPERATION
will control the average load current (IBURST) at which Burst Mode operation can be automatic or user controlled. Burst Mode operation will be entered and exited (there is In automatic operation, the LTC3422 will automatically hysteresis to prevent oscillation between modes). The enter Burst Mode operation at light load and return to fixed equation given for the capacitor on BURST is the minimum frequency PWM mode for heavier loads. The user can value to prevent ripple on BURST from causing the part to program the average load current at which the mode oscillate in and out of Burst Mode operation at the current transition occurs using a single resistor connected from where the mode transition occurs. The equation given for BURST to GND. the resistor on BURST is the typical average load current at which automatic Burst Mode operation is exited. The oscillator is shut down during Burst Mode operation, since the on time is determined by the time it takes the = 12 inductor current to reach a fixed 600mA peak current and RB IEXITBURST the off time is determined by the time it takes for the inductor current to return to zero. where RB is in kΩ and IEXITBURST is in amps. In Burst Mode operation, the LTC3422 delivers energy to C V OUT OUT the output until it is regulated and then enters a sleep state, CB ≥ • , 64 000 where the switches are kept off while the LTC3422 con- sumes only 25µA of quiescent current. In this mode the where CB(MIN) and COUT are in µF. output ripple has a variable frequency component with Please refer to the Burst Mode Output Current Threshold load current and will be typically 2% peak-peak. This vs R maximizes efficiency at very light loads by minimizing BURST Typical Performance Chacteristic curves. switching and quiescent losses. Burst Mode operation In the event that a load transient causes FB to drop by more ripple can be reduced slightly by increasing the output than 4% from the regulation value while in Burst Mode capacitance (47µF or greater). This additional capacitance operation, the LTC3422 will immediately switch to fixed does not need to be a low ESR type if low ESR ceramics are frequency operation and an internal pull-up will be mo- also used. Another method of reducing Burst Mode opera- mentarily applied to BURST, rapidly charging the BURST tion ripple is to place a small feed-forward capacitor (10pF capacitor. This prevents the LTC3422 from immediately to 100pF) across the upper resistor in the VOUT feedback re-entering Burst Mode operation once the output achieves divider network. regulation. In Burst Mode operation, the compensation network is not
Manual Burst Mode Operation
used and VC is disconnected from the error amplifier. During long periods of Burst Mode operation, leakage For optimum transient response with large dynamic loads, currents in the external components or on the PC board the operating mode should be controlled manually by the could cause the compensation capacitor to charge (or host. By commanding fixed frequency PWM operation discharge), which could result in a large output transient prior to a sudden increase in load, output voltage droop when returning to fixed frequency mode of operation, even can be minimized. For manual control of Burst Mode at the same load current. To prevent this, the LTC3422 operation, the RC network connected to BURST can be incorporates an active clamp circuit that holds the voltage eliminated. To force fixed frequency PWM mode, BURST on V should be connected to V C at an optimal voltage during Burst Mode operation. OUT. To force Burst Mode opera- This minimizes any output transient when returning to tion, BURST should be grounded. When commanding fixed frequency mode operation. Burst Mode operation manually, the circuit connected to 3422fa 9