Datasheet LTC3621, LTC3621-2 (Analog Devices) - 9

LTC3621/LTC3621-2
operaTion
selected, the part will transition in and out of sleep mode
Low Supply Operation
depending on the output load current. This significantly The LTC3621 incorporates an undervoltage lockout circuit reduces the quiescent current, thus prolonging the use which shuts down the part when the input voltage drops of the input supply. below 2.7V. As the input voltage rises slightly above the
V
undervoltage threshold, the switcher will begin its basic
IN Overvoltage Protection
operation. However, the RDS(ON) of the top and bottom In order to protect the internal power MOSFET devices switch will be slightly higher than that specified in the against transient voltage spikes, the LTC3621 constantly electrical characteristics due to lack of gate drive. Refer monitors the VIN pin for an overvoltage condition. When to graph of RDS(ON) versus VIN for more details. VIN rises above 19V, the regulator suspends operation by shutting off both power MOSFETs. Once VIN drops below
Soft-Start
18.7V, the regulator immediately resumes normal opera- The LTC3621 has an internal 800µs soft-start ramp. During tion. The regulator executes its soft-start function when start-up soft-start operation, the switcher will operate in exiting an overvoltage condition. pulse-skipping mode.
applicaTions inForMaTion Output Voltage Programming
VOUT VIN For non-fixed output voltage parts, the output voltage is IRMS ≅IOUT(MAX) –1 VIN V set by external resistive divider according to the following OUT equation: This formula has a maximum at VIN = 2VOUT, where:  R2 I V I OUT OUT = 0.6V • 1+   RMS ≅ 2 R1 The resistive divider allows the FB pin to sense a fraction This simple worst-case condition is commonly used for of the output voltage as shown in Figure 1. design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor VOUT manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, R2 CFF or choose a capacitor rated at a higher temperature than FB required. Several capacitors may also be paralleled to meet LTC3621 R1 size or height requirements in the design. For low input SGND voltage applications, sufficient bulk input capacitance is 3621 F01 needed to minimize transient effects during output load
Figure 1. Setting the Output Voltage
changes.
Input Capacitor (CIN) Selection Output Capacitor (COUT) Selection
The input capacitance, CIN, is needed to filter the square The selection of COUT is determined by the effective series wave current at the drain of the top power MOSFET. To resistance (ESR) that is required to minimize voltage ripple prevent large voltage transients from occurring, a low and load step transients as well as the amount of bulk ESR input capacitor sized for the maximum RMS current capacitance that is necessary to ensure that the control should be used. The maximum RMS current is given by: loop is stable. Loop stability can be checked by viewing 3621fc For more information www.linear.com/LTC3621 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 Package Description Revision History Typical Application Related Parts