Datasheet LTM4614 (Analog Devices) - 9

LTM4614
applicaTions inForMaTion Fault Conditions: Current Limit and Overcurrent
0.01µF. Soft-start time is approximately given by:
Foldback
 V  The LTM4614 has current mode control, which inher- t IN SOFTSTART = In ( )  V  •RSS • CSS +CSSEXT ently limits the cycle-by-cycle inductor current not only IN – 1.8V in steady-state operation, but also in transient. where RSS and CSS are shown in the Block Diagram of Along with foldback current limiting in the event of an Figure 1, and 1.8V is the soft-start upper range. The soft- overload condition, the LTM4614 has overtemperature start function can also be used to control the output ramp- shutdown protection that inhibits switching operation up time, so that another regulator can be easily tracked around 150°C for each channel. to it. To turn off the module while VIN remains active, the RUN/SS pin should be pulled low with a falling edge ≤ 1µs
Run Enable and Soft-Start
to ensure the device does not transition slowly through The RUN/SS pins provide a dual function of enable and the internal undervoltage lockout threshold. soft-start control for each channel. The RUN/SS pins are
Output Voltage Tracking
used to control turn on of the LTM4614. While each enable pin is below 0.6V, the LTM4614 will be in a low quiescent Output voltage tracking can be programmed externally current state. At least a 0.9V level applied to the enable using the TRACK pins. Either output can be tracked up pins will turn on the LTM4614 regulators. The voltage on or down with another regulator. The master regulator’s the RUN/SS pins clamp the control loop’s current com- output is divided down with an external resistor divider parator threshold. A RUN/SS pin voltage of 2.375V upon that is the same as the slave regulator’s feedback divider completion of soft-start guarantees the regulator can deliver to implement coincident tracking. The LTM4614 uses a full output current. These pins can be used to sequence very accurate 4.99k resistor for the internal top feedback the regulator channels. Soft-start control is provided by resistor. Figure 2 shows an example of coincident tracking. a 1M pull-up resistor (RSS) and a 1000pF capacitor (CSS) Equations: as shown in the Block Diagram for each channel. Option- ally, an external capacitor (CSSEXT) can be applied to the  RFB1  RUN/SS pin to increase soft-start time. A typical value is TRACK1=  4.99k+RFB1 •Master  4.99k  Slave = 1+  RFB1  • TRACK1 VIN 3V TO 5.5V C1 C2 22µF 22µF 6.3V 6.3V PGOOD1 PGOOD2 VIN1 VIN2 R3 PGOOD1 PGOOD2 R4 10k 10k 1.2V V 1.5V OUT1 VOUT2 4A 4A FB1 FB2 C4 R LTM4614 TB C7 22µF 4.99k COMP1 COMP2 100µF 1.5V V 6.3V IN OR 6.3V C3 TRACK1 TRACK2 CONTROL C9 100µF R RAMP R 22µF FB1 FB2 6.3V RTA RUN/SS1 RUN/SS2 10k 5.76k 6.3V 10k GND1 GND2 CSSEXT1 4614 F02
Figure 2. Dual Outputs (1.5V and 1.2V) with Tracking
4614fb 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Simplified Block Diagram Decoupling Requirements Operation Applications Information Package Description Revision History Package Photograph Related Parts