Datasheet LTC3403 (Analog Devices) - 10

LTC3403
U U W U APPLICATIO S I FOR ATIO Table 1. Representative Surface Mount Inductors
The selection of COUT is driven by the required effective
Part Value DCR MAX DC Size
series resistance (ESR). Typically, once the ESR require-
Number (
µ
H) (
Ω
MAX) Current (A) WxLxH (mm3)
ment for COUT has been met, the RMS current rating Sumida 1.5 0.068 0.90 3.2 x 3.2 x 1.2 generally far exceeds the IRIPPLE(P-P) requirement. The CDRH2D11 2.2 0.098 0.78 output ripple V 3.3 0.123 0.60 OUT is determined by: Sumida 2.2 0.041 0.85 3.2 x 3.2 x 2.0  1  CDRH2D18/LD 3.3 0.054 0.75 ∆VOUT ≅ ∆I ESR L + 4.7 0.078 0.63  f 8 COUT  Sumida 2.2 0.116 0.95 3.5 x 4.1 x 0.8 CMD4D06 3.3 0.174 0.77 where f = operating frequency, COUT = output capacitance 4.7 0.216 0.75 and IL = ripple current in the inductor. For a fixed output Murata 1.0 0.060 1.00 2.5 x 3.2 x 2.0 voltage, the output ripple is highest at maximum input LQH32C 2.2 0.097 0.79 4.7 0.150 0.65 voltage since IL increases with input voltage. Taiyo Yuden 1.0 0.080 0.78 1.8 x 2.5 x 1.8 Aluminum electrolytic and dry tantalum capacitors are LQLBC2518 1.5 0.110 0.66 both available in surface mount configurations. In the case 2.2 0.130 0.60 of tantalum, it is critical that the capacitors are surge tested Toko 2.2 0.14 1.14 4.6 x 4.6 x 1.2 D412F 3.3 0.20 0.90 for use in switching power supplies. An excellent choice is 4.7 0.22 0.80 the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give
C
the lowest ESR for a given volume. Other capacitor types
IN and COUT Selection
include Sanyo POSCAP, Kemet T510 and T495 series, and In continuous mode, the source current of the top MOSFET Sprague 593D and 595D series. Consult the manufacturer is a square wave of duty cycle VOUT/VIN. To prevent large for other specific recommendations. voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum The bulk capacitance values in Figure 1(a) (CIN = 10µF, RMS capacitor current is given by: COUT = 4.7µF) are tailored to mobile phone applications, in which the output voltage is expected to slew quickly V V V according to the needs of the power amplifier. Holding the C required I ≅ [ ( – )] / 1 2 I OUT IN OUT IN RMS OMAX output capacitor to 4.7µF facilitates rapid charging and VIN discharging. When the output voltage descends quickly in This formula has a maximum at V forced continuous mode, the LTC3403 will actually pull IN = 2VOUT, where IRMS = I current from the output until the command from V OUT/2. This simple worst-case condition is commonly REF is used for design because even significant deviations do not satisfied. On alternate half cyles, this current actually exits offer much relief. Note that the capacitor manufacturer’s the VIN terminal, potentially causing a rise in VIN and ripple current ratings are often based on 2000 hours of life. forcing current into the battery. To prevent deterioration This makes it advisable to further derate the capacitor, or of the battery, use sufficient bulk capacitance with low choose a capacitor rated at a higher temperature than ESR; at least 10µF is recommended. required. Always consult the manufacturer if there is any question. 3403f 10