Datasheet LTC3624, LTC3624-2 (Analog Devices) - 10

LTC3624/LTC3624-2
operaTion
alive. However, if Burst Mode operation or pulse-skipping LTC3624-2 (FSW = 2.25MHz). In the rare cases that this mode is selected, the part will transition in and out of minimum on-time is violated, the output voltage may lose sleep mode depending on the output load current. This regulation. In such situation, the user must choose either significantly reduces the quiescent current, thus prolong- Burst Mode or pulse-skipping mode operation, or apply a ing the use of the input supply. slower external clock to force a slower switching frequency in order to adhere to the minimum on-time limitation.
VIN Overvoltage Protection
In order to protect the internal power MOSFET devices
Low Supply Operation
against transient voltage spikes, the LTC3624/LTC3624-2 The LTC3624 incorporates an undervoltage lockout circuit constantly monitors the VIN pin for an overvoltage condi- which shuts down the part when the input voltage drops tion. When VIN rises above 19V, the regulator suspends below 2.7V. As the input voltage rises slightly above the operation by shutting off both power MOSFETs. Once VIN undervoltage threshold, the switcher will begin its basic drops below 18.5V, the regulator immediately resumes operation. However, the RDS(ON) of the top and bottom normal operation. The regulator executes its soft-start switch will be slightly higher than that specified in the function when exiting an overvoltage condition. electrical characteristics due to lack of gate drive. Refer to graph of RDS(ON) versus VIN for more details.
Minimum On-Time
The minimum on-time is the smallest duration of the time
Soft-Start
the top power switch is allowed to be in its on state. This The LTC3624/LTC3624-2 has an internal 1ms soft-start time is typically 60ns. In forced continuous mode operation, ramp. During start-up soft-start operation, the switcher the minimum on-time limit imposes a minimum duty cycle will operate in pulse-skipping mode. of 6% for the LTC3624 (FSW = 1MHz) and 13.5% for the
applicaTions inForMaTion Output Voltage Programming
VOUT The output voltage is set by external resistive divider ac- cording to the following equation for adjustable output R2 CFF versions: FB LTC3624 R1  R2 SGND V  OUT = 0.6V • 1+ 36242 F01  R1
Figure 1. Setting the Output Voltage (Adjustable Version)
The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. VOUT For fixed V FB OUT options, connect FB pin directly to VOUT. LTC3624 (FIXED VOUT)
Input Capacitor (CIN) Selection
SGND 36242 F02 The input capacitance, CIN, is needed to filter the square wave current at the drain of the top power MOSFET. To
Figure 2. Setting the Output Voltage (Fixed VOUT Option)
36242fd 10 For more information www.linear.com/LTC3624 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts