Datasheet LTM4624 (Analog Devices) - 10

LTM4624
APPLICATIONS INFORMATION Discontinuous Current Mode (DCM)
If any operating frequency other than 1MHz is required In applications where low output ripple and high efficiency by application, the operating frequency can be increased at intermediate current are desired, discontinuous current by adding a resistor, RFSET, between the FREQ pin and mode (DCM) should be used by connecting the MODE pin SGND, as shown in Figure 21. The operating frequency to SGND. At light loads the internal current comparator can be calculated as: may remain tripped for several cycles and force the top MOSFET to stay off for several cycles, thus skipping cycles. f H ( z) = 1.6e11 162k ||RFSET (Ω) The inductor current does not reverse in this mode. The operating frequency can also be decreased by adding
Forced Continuous Current Mode (CCM)
a resistor between the FREQ pin and INTVCC, calculated as: In applications where fixed frequency operation is more 2.8e11 critical than low current efficiency, and where the lowest f H ( z) = 1MHz – R ( ) output ripple is desired, forced continuous operation FSET Ω should be used. Forced continuous operation can be The programmable operating frequency range is from enabled by tying the MODE pin to INTVCC. In this mode, 800kHz to 4MHz. inductor current is allowed to reverse during low output loads, the COMP voltage is in control of the current
Soft-Start And Output Voltage Tracking
comparator threshold throughout, and the top MOSFET always turns on with each oscillator pulse. During start-up, The TRACK/SS pin provides a means to either soft start forced continuous mode is disabled and inductor current the regulator or track it to a different power supply. A ca- is prevented from reversing until the LTM4624’s output pacitor on the TRACK/SS pin will program the ramp rate voltage is in regulation. of the output voltage. An internal 2.5µA current source will charge up the external soft-start capacitor towards
Operating Frequency
INTVCC voltage. When the TRACK/SS voltage is below 0.6V, it will take over the internal 0.6V reference voltage The operating frequency of the LTM4624 is optimized to to control the output voltage. The total soft-start time can achieve the compact package size and the minimum out- be calculated as: put ripple voltage while still keeping high efficiency. The default operating frequency is internally set to 1MHz. In C t SS SS = 0.6 • most applications, no additional frequency adjusting is 2.5µA required. where CSS is the capacitance on the TRACK/SS pin. Cur- rent foldback and forced continuous mode are disabled during the soft-start process. 4624fd 10 For more information www.linear.com/LTM4624 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Package Description Package Photo Design Resources Related Parts