Datasheet LTC3602 (Analog Devices) - 10

LTC3602
APPLICATIONS INFORMATION
characteristics. The choice of which style inductor to use density than other types. Tantalum capacitors have the mainly depends on the price vs size requirements and any highest capacitance density but it is important to only radiated fi eld/EMI requirements. New designs for surface use types that have been surge tested for use in switching mount inductors are available from Coiltronics, Coilcraft, power supplies. Aluminum electrolytic capacitors have Toko and Sumida. signifi cantly higher ESR but can be used in cost-sensitive applications provided that consideration is given to ripple
CIN and COUT Selection
current ratings and long term reliability. Ceramic capacitors The input capacitance, C have excellent low ESR characteristics but can have a high IN, is needed to fi lter the trapezoi- dal current at the source of the top MOSFET. To prevent voltage coeffi cient and audible piezoelectric effects. The large ripple voltage, a low ESR input capacitor sized for high Q of ceramic capacitors with trace inductance can the maximum RMS current should be used. RMS current also lead to signifi cant ringing. is given by:
Using Ceramic Input and Output Capacitors
V V I =I OUT IN Higher values, lower cost ceramic capacitors are now RMS OUT MA ( X) • • – 1 V V becoming available in smaller case sizes. Their high ripple IN OUT current, high voltage rating and low ESR make them ideal This formula has a maximum at VIN = 2VOUT, where for switching regulator applications. However, care must IRMS = IOUT/2. This simple worst-case condition is com- be taken when these capacitors are used at the input and monly used for design because even signifi cant deviations output. When a ceramic capacitor is used at the input and do not offer much relief. Note that ripple current ratings the power is supplied by a wall adapter through long wires, from capacitor manufacturers are often based on only a load step at the output can induce ringing at the input, 2000 hours of life which makes it advisable to further VIN. At best, this ringing can couple to the output and be derate the capacitor, or choose a capacitor rated at a mistaken as loop instability. At worst, a sudden inrush higher temperature than required. Several capacitors may of current through the long wires can potentially cause a also be paralleled to meet size or height requirements in voltage spike at VIN large enough to damage the part. the design.
Output Voltage Programming
The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple The output voltage is set by an external resistive divider and load step transients, as well as the amount of bulk according to the following equation: capacitance that is necessary to ensure that the control ⎛ ⎞ loop is stable. Loop stability can be checked by viewing V = 0.6V • 1+ R2 OUT the load transient response as described in a later section. ⎝⎜ ⎠⎟ R1 The output ripple, ΔVOUT, is determined by: The resistive divider allows the VFB pin to sense a fraction ⎛ ⎞ of the output voltage as shown in Figure 1. ΔV ≤ ΔI • ESR+ 1 OUT L ⎝⎜ 8fC ⎠⎟ OUT VOUT The output ripple is highest at maximum input voltage R2 since ΔIL increases with input voltage. Multiple capacitors VFB placed in parallel may be needed to meet the ESR and LTC3602 R1 RMS current handling requirements. Dry tantalum, special SGND polymer, aluminum electrolytic and ceramic capacitors are 3602 F01 all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance
Figure 1. Setting the Output Voltage
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