Datasheet LTC3421 (Analog Devices) - 10

LTC3421
OPERATION Zero Current Amplifier
by increasing the output capacitance. Another method The zero current amplifier monitors the inductor current of reducing Burst Mode ripple is to place a small feed- to the output and shuts off the synchronous rectifier once forward capacitor across the upper resistor in the VOUT the current is below 50mA typical, preventing negative feedback divider network. inductor current. During Burst Mode operation, the VC pin is disconnected from the error amplifier in an effort to hold the voltage on
Antiringing Control
the external compensation network where it was before The antiringing control places a resistor across the induc- entering Burst Mode operation. To minimize the effects of tor to damp the ringing on the SW pin in discontinu- leakage current and stray resistance, voltage clamps limit ous conduction mode. The LC the min and max voltage on V SW ringing (L = inductor, C during Burst Mode opera- C tion. This minimizes the transient experienced when a SW = capacitance on SW pin) is low energy, but can cause EMI radiation. heavy load is suddenly applied to the converter after being in Burst Mode operation for an extended period of time.
VREF
For automatic operation, an RC network should be con- The internal 1.22V reference is buffered and brought nected from BURST to ground. The value of the resistor out to VREF. It is active when the ENB pin is pulled high will control the average load current (IBURST) at which (above 1.2V). For stability, a minimum of a 0.1µF capaci- Burst Mode operation will be entered and exited (there tor must be placed on the pin. The output can source up is hysteresis to prevent oscillation between modes). The to 100µA and sink up to 8µA. For the lowest possible equation given for the capacitor on BURST is for the mini- quiescent current in Burst Mode operation, the reference mum value to prevent ripple on BURST from causing the output should be disabled by grounding the ENB pin. part to oscillate in and out of Burst Mode operation at the current where the mode transition occurs.
Burst Mode OPERATION
2 R = Burst Mode operation can be automatic or user con- BURST IBURST trolled. In automatic operation, the IC will automatically where R enter Burst Mode operation at light load and return to BURST is in kΩ and IBURST is in amps. fixed frequency PWM mode for heavier loads. The user C • V C OUT OUT can program the average load current at which the mode ≥ BURST 10,000 transition occurs using a single resistor. where C The oscillator is shut down in this mode, since the on time BURST(MIN) and COUT are in µF. is determined by the time it takes the inductor current to In the event that a sudden load transient causes FB to reach a fixed peak current and the off time is determined deviate by more than 4% from the regulation value, an by the time it takes for the inductor current to return to internal pull-up is applied to BURST, forcing the part zero. quickly out of Burst Mode operation. For optimum tran- sient response when going between Burst Mode opera- In Burst Mode operation, the IC delivers energy to the tion and PWM mode, the mode should be controlled output until it is regulated and then goes into a sleep mode manually by the host. This way PWM mode can be com- where the outputs are off and the IC is consuming only manded before the load step occurs, minimizing output 12µA of quiescent current. In this mode, the output ripple voltage droop. For manual control of Burst Mode opera- has a variable frequency component with load current and tion, the RC network can be eliminated. To force fixed will be typically 2% peak-peak. This maximizes efficiency frequency PWM mode, BURST should be connected to at very light loads by minimizing switching and quies- V cent losses. Burst Mode ripple can be reduced slightly OUT. To force Burst Mode operation, BURST should be Rev. A 10 For more information www.analog.com Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Package/Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Low Voltage Start-Up Low Noise Fixed Frequency Operation Burst Mode Operation Output Disconnect and Inrush Limiting Applications Information Component Selection Typical Application Package Description Revision History Typical Application Related Parts