Datasheet LTC3528, LTC3528B (Analog Devices) - 9

LTC3528/LTC3528B
OPERATION (Refer to Block Diagram) Error Amplifier Synchronous Rectifier
The error amplifier is a transconductance type. The nonin- To control inrush current and to prevent the inductor verting input is internally connected to the 1.20V reference current from running away when VOUT is close to VIN, the and the inverting input is connected to FB. Clamps limit P- channel MOSFET synchronous rectifier is only enabled the minimum and maximum error amp output voltage for when VOUT > (VIN + 0.24V). improved large-signal transient response. Power converter control loop compensation is provided internally. A voltage
Anti-Ringing Control
divider from VOUT to ground programs the output voltage The anti-ringing control connects a resistor across the via FB from 1.6V to 5.25V. inductor to prevent high frequency ringing on the SW pin  R2 during discontinuous current mode operation. The ringing V  OUT = 1.20V • 1+ of the resonant circuit formed by L and CSW (capacitance  R1 on SW pin) is low energy, but can cause EMI radiation.
Current Sensing Output Disconnect
Lossless current sensing converts the peak current signal The LTC3528/LTC3528B is designed to allow true output of the N-channel MOSFET switch into a voltage which disconnect by eliminating body diode conduction of the is summed with the internal slope compensation. The internal P-channel MOSFET rectifier. This allows for VOUT summed signal is compared to the error amplifier output to go to zero volts during shutdown, drawing no current to provide a peak current control command for the PWM. from the input source. It also enables inrush current limiting at turn-on, minimizing surge currents seen by
Current Limit
the input supply. Note that to obtain the advantages of The current limit comparator shuts off the N-channel output disconnect, a Schottky diode cannot be connected MOSFET switch once its threshold is reached. The cur- between SW and VOUT. The output disconnect feature also rent limit comparator delay to output is typically 60ns. al ows VOUT to be forced above the programmed regulation Peak switch current is limited to approximately 1.5A, voltage, without any reverse current into a battery on VIN. independent of input or output voltage, unless VOUT falls below 0.7V, in which case the current limit is cut in half.
Thermal Shutdown
If the die temperature exceeds 160°C, the LTC3528/
Zero Current Comparator
LTC3528B will enter thermal shutdown. All switches will be The zero current comparator monitors the inductor cur- turned off and the soft-start capacitor will be discharged. rent to the output and shuts off the synchronous rectifier The device will be enabled again when the die temperature when this current reduces to approximately 20mA. This drops by approximately 15°C. prevents the inductor current from reversing in polarity, improving efficiency at light loads. 3528fd 9 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