Datasheet LTM4602 (Analog Devices) - 10

LTM4602
APPLICATIONS INFORMATION Output Capacitors Soft-Start and Latchoff with the RUN/SS pin
The LTM4602 is designed for low output voltage ripple. The The RUN/SS pin provides a means to shut down the bulk output capacitors COUT is chosen with low enough LTM4602 as well as a timer for soft-start and overcurrent effective series resistance (ESR) to meet the output voltage latchoff. Pulling the RUN/SS pin below 0.8V puts the ripple and transient requirements. COUT can be low ESR LTM4602 into a low quiescent current shutdown (IQ ≤ tantalum capacitor, low ESR polymer capacitor or ceramic 100μA). Releasing the pin allows an internal 1.2μA cur- capacitor (X5R or X7R). The typical capacitance is 200μF rent source to charge up the timing capacitor CSS. Inside if all ceramic output capacitors are used. The internally LTM4602, there is an internal 1000pF capacitor from RUN/ optimized loop compensation provides suffi cient stability SS pin to ground. If RUN/SS pin has an external capacitor margin for all ceramic capacitors applications. Additional CSS_EXT to ground, the delay before starting is about: output fi ltering may be required by the system designer, if further reduction of output ripple or dynamic transient tDELAY = 1.5V •(CSS_EXT +1000pF) spike is required. Refer to Table 2 for an output capaci- 1.2μA tance matrix for each output voltage droop, peak to peak When the voltage on RUN/SS pin reaches 1.5V, the LTM4602 deviation and recovery time during a 3A/μs transient with internal switches are operating with a clamping of the a specifi c output capacitance. maximum output inductor current limited by the RUN/SS
Fault Conditions: Current Limit and Overcurrent
pin total soft-start capacitance. As the RUN/SS pin voltage
Foldback
rises to 3V, the soft-start clamping of the inductor current is released. The LTM4602 has a current mode controller, which inher- ently limits the cycle-by-cycle inductor current not only in
VIN to VOUT Step-Down Ratios
steady-state operation, but also in transient. There are restrictions in the maximum VIN to VOUT step To further limit current in the event of an over load condi- down ratio that can be achieved for a given input voltage. tion, the LTM4602 provides foldback current limiting. If the These constraints are shown in the Typical Performance output voltage falls by more than 50%, then the maximum Characteristics curves labeled “VIN to VOUT Step-Down output current is progressively lowered to about one sixth Ratio”. Note that additional thermal derating may apply. See of its full current limit value. the Thermal Considerations and Output Current Derating sections of this data sheet. 4602fa 10