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LTC3458 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3458 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page LTC3458 10 3458fa transition occurs using a single resistor. During Burst Mode operation, the oscillator is shut down, since the on time is determined by the time it takes the inductor current to reach a fixed peak current, and the off time is deter- mined by the time it takes for the inductor current to return to zero. In Burst Mode operation, 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 15 μA of quiescent current. In this mode the output ripple voltage has a variable frequency component with load current and will be typically 2% peak-to-peak. This maxi- mizes efficiency at very light loads by minimizing switch- ing and quiescent losses. Burst Mode ripple can be re- duced slightly by using more output capacitance (22 μF or greater). This capacitor does not need to be a low ESR type if low ESR ceramics are also used. Another method of reducing Burst Mode ripple is to place a small feed- forward capacitor across the upper resistor in the VOUT feedback divider network. During Burst Mode operation, COMP is disconnected from the error amplifier in an effort to hold the voltage on the external compensation network where it was before entering Burst Mode operation. To minimize the effects of leakage current and stray resistance, voltage clamps limit the minimum and maximum voltage on COMP during Burst Mode operation. This minimizes the transient expe- rienced when a heavy load is suddenly applied to the converter after being in Burst Mode operation for an extended period of time. For automatic operation, an RC network should be con- nected from BURST to ground. The value of the resistor will control the average load current (IBURST) at which Burst Mode operation will be entered and exited (there is hysteresis to prevent oscillation between modes). The equation given for the capacitor on BURST is for the minimum value, to prevent ripple on the BURST pin from causing the part to oscillate in and out of Burst Mode operation at the current where the mode transition occurs. R I BURST BURST = 10 where RBURST is in kΩ and IBURST is in amps. C CV BURST OUT OUT = • , 10 000 where CBURST(MIN) and COUT are in μF. Note: the BURST pin only sources current based on current delivered to VOUT through the P-channel MOSFET. If current in the inductor is allowed to go negative (this can occur at very light loads and high step-up ratios), the burst threshold may become inaccurate, preventing the IC from entering Burst Mode operation. For RBURST values greater than 200k, a larger than recommended inductor value may be needed to ensure positive inductor current and auto- matic Burst Mode operation. In the event that a sudden load transient causes the voltage level on FB to drop by more than 4% from the regulation value, an internal pull-up is applied to BURST, forcing the part quickly out of Burst Mode operation. For optimum transient response when going between Burst Mode op- eration and PWM mode, Burst can be controlled manually by the host. This way PWM mode can be commanded before the load step occurs, minimizing output voltage drop. Note that Burst Mode operation is inhibited during start-up and soft-start. Manual Control For applications requiring fixed frequency operation at all load currents, connect the BURST pin to VOUT through a 50k Ω resistor. To force Burst Mode operation, ground the BURST pin. For applications where a large load step can be anticipated, the circuit below can be used to reduce the voltage transient on VOUT. Automatic operation is achieved when the external PMOS is off and fixed frequency operation is commanded when the external PMOS is on. In shutdown, the PMOS should be off. APPLICATIO S I FOR ATIO Figure 1 0.01 μF VIN BURST 133k PMOS HIGH: AUTO MODE LOW: FIXED FREQUENCY 3458 FO2 |
Similar Part No. - LTC3458_15 |
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Similar Description - LTC3458_15 |
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