Datasheet LTC3621, LTC3621-2 (Analog Devices) - 10

LTC3621/LTC3621-2
applicaTions inForMaTion
the load transient response. The output ripple, ∆VOUT, is requirement. During a load step, the output capacitor must determined by: instantaneously supply the current to support the load until the feedback loop raises the switch current enough V  1  ∆ to support the load. Typically, five cycles are required to OUT < I ∆ L +ESR   8 • f •COUT respond to a load step, but only in the first cycle does the output voltage drop linearly. The output droop, V The output ripple is highest at maximum input voltage DROOP, is usually about three times the linear drop of the first cycle. since ∆IL increases with input voltage. Multiple capaci- Thus, a good place to start with the output capacitor value tors placed in parallel may be needed to meet the ESR is approximately: and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic, and ceramic ∆I C OUT capacitors are all available in surface mount packages. OUT = 3 f• VDROOP Special polymer capacitors are very low ESR but have lower capacitance density than other types. Tantalum More capacitance may be required depending on the duty capacitors have the highest capacitance density but it is cycle and load-step requirements. In most applications, important to only use types that have been surge tested the input capacitor is merely required to supply high for use in switching power supplies. Aluminum electrolytic frequency bypassing, since the impedance to the supply capacitors have significantly higher ESR, but can be used is very low. A 10μF ceramic capacitor is usually enough in cost-sensitive applications provided that consideration for these conditions. Place this input capacitor as close is given to ripple current ratings and long-term reliability. to the VIN pin as possible. Ceramic capacitors have excellent low ESR characteristics and small footprints.
Output Power Good
In the MS8E package, when the LTC3621’s output voltage
Using Ceramic Input and Output Capacitors
is within the ±7.5% window of the regulation point, the Higher values, lower cost ceramic capacitors are now output voltage is good and the PGOOD pin is pulled high becoming available in smaller case sizes. Their high ripple with an external resistor. Otherwise, an internal open-drain current, high voltage rating and low ESR make them ideal pull-down device (275Ω) will pull the PGOOD pin low. for switching regulator applications. However, care must To prevent unwanted PGOOD glitches during transients be taken when these capacitors are used at the input and or dynamic VOUT changes, the LTC3621’s PGOOD fall- output. When a ceramic capacitor is used at the input ing edge includes a blanking delay of approximately 32 and the power is supplied by a wall adapter through long switching cycles. wires, a load step at the output can induce ringing at the VIN input. At best, this ringing can couple to the output and
Frequency Sync Capability
be mistaken as loop instability. At worst, a sudden inrush The LTC3621 has the capability to sync to a frequency within of current through the long wires can potentially cause a ±40% range of the internal programmed frequency. It a voltage spike at VIN large enough to damage the part. takes 2 to 3 cycles of external clock pulses to engage the When choosing the input and output ceramic capacitors, sync mode. If the external clock signal were to stop switch- choose the X5R and X7R dielectric formulations. These ing during operation, it will take roughly 7μs for the part’s dielectrics have the best temperature and voltage char- internal sync signal to go low and respond accordingly. acteristics of all the ceramics for a given value and size. Once engaged in sync, the LTC3621 immediately runs at the external clock frequency in forced continuous mode. Since the ESR of a ceramic capacitor is so low, the input and output capacitor must instead fulfill a charge storage 3621fc 10 For more information www.linear.com/LTC3621 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