Datasheet LTC3402 (Analog Devices) - 8

LTC3402
U U W U APPLICATIO S I FOR ATIO DETAILED DESCRIPTION
the error amplifier output to provide a peak current control command for the PWM. The slope compensation in the IC The LTC3402 provides high efficiency, low noise power is adaptive to the input and output voltage. Therefore, the for applications such as portable instrumentation. The converter provides the proper amount of slope compensa- current mode architecture with adaptive slope compensa- tion to ensure stability and not an excess causing a loss of tion provides ease of loop compensation with excellent phase margin in the converter. transient load response. The low RDS(ON), low gate charge synchronous switches provide the pulse width modula-
Error Amp
. The error amplifier is a transconductance tion control at high efficiency. amplifier with gm = 0.1ms. A simple compensation net- work is placed from the V The Schottky diode across the synchronous PMOS switch C pin to ground. provides a lower drop during the break-before-make time
Current Limit
. The current limit amplifier will shut the (typically 20ns) of the NMOS to PMOS transition. The NMOS switch off once the current exceeds its threshold. addition of the Schottky diode will improve efficiency (see The current amplifier delay to output is typically 50ns. graph “Efficiency Loss Without Schottky vs Frequency”).
Zero Current Amp
. The zero current amplifier monitors the While the IC’s quiescent current is a low 38μA, high inductor current to the output and shuts off the synchro- efficiency is achieved at light loads when Burst Mode nous rectifier once the current is below 50mA, preventing operation is entered. negative inductor current.
Low Voltage Start-Up Antiringing Control
. The anitringing control will place an impedance across the inductor to damp the ringing on the The LTC3402 is designed to start up at input voltages of SW pin during discontinuous mode operation. The LC typically 0.85V. The device can start up under some load, SW ringing (L = inductor, C (see graph Start-Up vs Input Voltage). Once the output SW = capacitance on the switch pin) is low energy, but can cause EMI radiation. voltage exceeds a threshold of 2.3V, then the IC powers itself from VOUT instead of VIN. At this point, the internal
Burst Mode Operation
circuitry has no dependency on the input voltage, eliminat- ing the requirement for a large input capacitor. The input Burst Mode operation is when the IC delivers energy to the voltage can drop below 0.5V without affecting the opera- output until it is regulated and then goes into a sleep mode tion, but the limiting factor for the application becomes the where the outputs are off and the IC is consuming only availability of the power source to supply sufficient energy 38μA. In this mode, the output ripple has a variable to the output at the low voltages. frequency component with load current and the steady state ripple will be typically below 3%.
Low Noise Fixed Frequency Operation
During the period where the device is delivering energy to
Oscillator
. The frequency of operation is set through a the output, the peak current will be equal to 1/6 the current resistor from the Rt pin to ground where f = 3 • 1010/Rt. An limit value and the inductor current will terminate at zero internally trimmed timing capacitor resides inside the IC. current for each cycle. In this mode the maximum output The oscillator can be synchronized with an external clock current is given by: inserted on the MODE/SYNC pin. When synchronizing the V oscillator, the free running frequency must be set to I IN ≈ Amps OUT MAXBURST ( ) approximately 30% lower than the desired synchronized 6 • VOUT frequency. Keeping the sync pulse width below 2μs will Burst Mode operation is user controlled by driving the ensure that Burst Mode operation is disabled. MODE/SYNC pin high to enable and low to disable. It is
Current Sensing
. Lossless current sensing converts the recommended that Burst Mode operation be entered after peak current signal to a voltage to sum in with the internal the part has started up. slope compensation. This summed signal is compared to 3402fb 8