Datasheet LTC3408 (Analog Devices) - 8

LTC3408
U U W U APPLICATIO S I FOR ATIO
At output voltages below 0.6V, the switching frequency maximum RMS current must be used. The maximum decreases linearly to a minimum of approximately 700kHz. RMS capacitor current is given by: This places the maximum ripple current (in forced con- tinuous mode) at the highest input voltage and the lowest V V V C required I ≅ [ ( – )] / 1 2 I OUT IN OUT IN RMS OMAX output voltage. In practice, the resulting ouput ripple VIN voltage is 10mV to 15mV using the components specified This formula has a maximum at VIN = 2VOUT, where IRMS in Figure 1. = IOUT/2. This simple worst-case condition is commonly The DC current rating of the inductor should be at least equal used for design because even significant deviations do not to the maximum load current plus half the ripple current to offer much relief. Note that the capacitor manufacturer’s prevent core saturation. Thus, a 660mA rated inductor ripple current ratings are often based on 2000 hours of life. should be enough for most applications (600mA + 60mA). This makes it advisable to further derate the capacitor, or For better efficiency, choose a low DC-resistance inductor. choose a capacitor rated at a higher temperature than re- quired. Always consult the manufacturer if there is any
Inductor Core Selection
question. Different core materials and shapes will change the size/ The selection of COUT is driven by the required effective current and price/current relationship of an inductor. series resistance (ESR). Typically, once the ESR Toroid or shielded pot cores in ferrite or permalloy mate- requirement for COUT has been met, the RMS current rials are small and don’t radiate much energy but generally rating generally far exceeds the IRIPPLE(P-P) requirement. cost more than powdered iron core inductors with similar The output ripple VOUT is determined by: electrical characteristics. The choice of which style induc-  1  tor to use often depends more on the price versus size ∆VOUT ≅ ∆I ESR L + requirements and any radiated field/EMI requirements  f 8 COUT  than on what the LTC3408 requires to operate. Table 1 shows some typical surface mount inductors that work where f = operating frequency, COUT = output capacitance and I well in LTC3408 applications. L = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input
Table 1. Representative Surface Mount Inductors
voltage since IL increases with input voltage.
PART VALUE DCR MAX DC SIZE NUMBER (
µ
H) (
Ω
MAX) CURRENT (A) WxLxH (mm3)
Aluminum electrolytic and dry tantalum capacitors are Sumida 4.7 0.135 0.5 3.2 x 3.2 x 1.2 both available in surface mount configurations. In the case CDRH2D11 of tantalum, it is critical that the capacitors are surge tested Sumida 4.7 0.078 0.63 3.2 x 3.2 x 2.0 for use in switching power supplies. An excellent choice is CDRH2D18/LD Sumida 4.7 0.216 0.75 3.5 x 4.1 x 0.8 the AVX TPS series of surface mount tantalum. These are CMD4D06 specially constructed and tested for low ESR so they give Murata 4.7 0.150 0.65 2.5 x 3.2 x 2.0 the lowest ESR for a given volume. Other capacitor types LQH32C include Sanyo POSCAP, Kemet T510 and T495 series, and Taiyo Yuden 4.7 0.250 0.210 1.6 x 2.0 x 1.6 Sprague 593D and 595D series. Consult the manufacturer LBLQ2016 Toko 4.7 0.20 0.79 3.6 x 3.6 x 1.2 for other specific recommendations. D312C The bulk capacitance values in Figure 1(a) (CIN = 10µF, COUT = 4.7µF) are tailored to mobile phone applications, in
CIN and COUT Selection
which the output voltage is expected to slew quickly In continuous mode, the source current of the top MOSFET according to the needs of the power amplifier. Holding the is a square wave of duty cycle V output capacitor to 4.7µF facilitates rapid charging and OUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the discharging. When the output voltage descends quickly in 3408f 8