Datasheet LTC3528-2 (Analog Devices) - 10

LTC3528-2
OPERATION (Refer to Block Diagram) Error Amplifier Anti-Ringing Control
The error amplifier is a transconductance type. The nonin- The anti-ringing control connects a resistor across the verting input is internally connected to the 1.20V reference inductor to prevent high frequency ringing on the SW pin and the inverting input is connected to FB. Clamps limit during discontinuous current mode operation. The ringing the minimum and maximum error amp output voltage for of the resonant circuit formed by L and CSW (capacitance improved large-signal transient response. Power converter on SW pin) is low energy, but can cause EMI radiation. control loop compensation is provided internally. A voltage divider from V
Output Disconnect
OUT to ground programs the output voltage via FB from 1.6V to 5.25V. The LTC3528-2 is designed to allow true output discon-  R2 nect by eliminating body diode conduction of the internal V  P-channel MOSFET rectifier. This allows for V OUT = 1.20V • 1+ OUT to go to  R1 zero volts during shutdown, drawing no current from the
Current Sensing
input source. It also enables inrush current limiting at turn- on, minimizing surge currents seen by the input supply. Lossless current sensing converts the peak current signal Note that to obtain the advantages of output disconnect, of the N-channel MOSFET switch into a voltage which a Schottky diode cannot be connected between SW and is summed with the internal slope compensation. The VOUT. The output disconnect feature also allows VOUT summed signal is compared to the error amplifier output to be forced above the programmed regulation voltage, to provide a peak current control command for the PWM. without any reverse current into the input power source.
Current Limit Thermal Shutdown
The current limit comparator shuts off the N-channel MOS- If the die temperature exceeds 160°C, the LTC3528-2 FET switch once its threshold is reached. The current limit enters thermal shutdown. All switches will be turned off comparator delay to output is typically 60ns. Peak switch and the soft-start capacitor will be discharged. The device current is limited to approximately 1.5A, independent of will be enabled again when the die temperature drops by input or output voltage, unless VOUT falls below 0.7V, in approximately 15°C. which case the current limit is cut in half.
Burst Mode OPERATION Zero Current Comparator
The LTC3528-2 will automatically enter Burst Mode op- The zero current comparator monitors the inductor cur- eration at light load current and return to fixed frequency rent to the output and shuts off the synchronous rectifier PWM mode when the load increases. Refer to the Typical when this current reduces to approximately 20mA. This Performance Characteristics to see the output load Burst prevents the inductor current from reversing in polarity, Mode threshold vs VIN. The load at which Burst Mode improving efficiency at light loads. operation is entered can be changed by adjusting the inductor value. Raising the inductor value will lower the
Synchronous Rectifier
load current at which Burst Mode operation is entered. To control inrush current and to prevent the inductor In Burst Mode operation, the LTC3528-2 continues switch- current from running away when VOUT is close to VIN, the ing at a fixed frequency of 2MHz, using the same error P-channel MOSFET synchronous rectifier is only enabled amplifier and loop compensation for peak current mode when VOUT > (VIN + 0.24V). control. This control method minimizes output transients 35282fb 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts