Datasheet LT1940, LT1940L (Analog Devices) - 9

LT1940/LT1940L
U U W U APPLICATIO S I FOR ATIO Table 1. Inductors.
at the LT1940 and to force this very high frequency
Part Number Value ISAT DCR Height
switching current into a tight local loop, minimizing EMI.
(
µ
H) (A) DC (
Ω
) (mm)
The input capacitor must have low impedance at the
Sumida
switching frequency to do this effectively, and it must have CR43-1R4 1.4 2.52 0.056 3.5 an adequate ripple current rating. With two switchers CR43-2R2 2.2 1.75 0.071 3.5 operating at the same frequency but with different phases CR43-3R3 3.3 1.44 0.086 3.5 and duty cycles, calculating the input capacitor RMS CR43-4R7 4.7 1.15 0.109 3.5 current is not simple. However, a conservative value is the CDRH3D16-1R5 1.5 1.55 0.040 1.8 RMS input current for the channel that is delivering most CDRH3D16-2R2 2.2 1.20 0.050 1.8 power (VOUT • IOUT). This is given by: CDRH3D16-3R3 3.3 1.10 0.063 1.8 CINRMS = IOUT √[VOUT • (VIN – VOUT)]/VIN < IOUT/2 CDRH4D28-3R3 3.3 1.57 0.049 3.0 and is largest when VIN = 2VOUT (50% duty cycle). As the CDRH4D28-4R7 4.7 1.32 0.072 3.0 second, lower power channel draws input current, the CDRH5D28-5R3 5.3 1.9 0.028 3.0 input capacitor’s RMS current actually decreases as the CDRH5D18-4R1 4.1 1.95 0.042 2.0 out-of-phase current cancels the current drawn by the
Coilcraft
higher power channel. Considering that the maximum DO1606T-152 1.5 2.10 0.060 2.0 load current from a single channel is ~1.4A, RMS ripple DO1606T-222 2.2 1.70 0.070 2.0 current will always be less than 0.7A. DO1606T-332 3.3 1.30 0.100 2.0 The high frequency of the LT1940 reduces the energy DO1606T-472 4.7 1.10 0.120 2.0 storage requirements of the input capacitor, so that the DO1608C-152 1.5 2.60 0.050 2.9 capacitance required is less than 10µF. The combination DO1608C-222 2.2 2.30 0.070 2.9 of small size and low impedance (low equivalent series DO1608C-332 3.3 2.00 0.080 2.9 resistance or ESR) of ceramic capacitors make them the DO1608C-472 4.7 1.50 0.090 2.9 preferred choice. The low ESR results in very low voltage 1812PS-222M 2.2 1.7 0.070 3.81 ripple and the capacitors can handle plenty of ripple 1008PS-182M 1.8 2.1 0.090 2.74 current. They are also comparatively robust and can be
Murata
used in this application at their rated voltage. X5R and X7R LQH32CN1R0M11L 1.0 1.00 0.078 2.2 types are stable over temperature and applied voltage, and LQH32CN2R2M11L 2.2 0.79 0.126 2.2 give dependable service. Other types (Y5V and Z5U) have LQH43CN1R5M01L 1.5 1.00 0.090 2.8 very large temperature and voltage coefficients of capaci- LQH43CN2R2M01L 2.2 0.90 0.110 2.8 tance, so they may have only a small fraction of their LQH43CN3R3M01L 3.3 0.80 0.130 2.8 nominal capacitance in your application. While they will still handle the RMS ripple current, the input voltage ripple
Input Capacitor Selection
may become fairly large, and the ripple current may end up Bypass the input of the LT1940 circuit with a 4.7µF or flowing from your input supply or from other bypass higher ceramic capacitor of X7R or X5R type. A lower value capacitors in your system, as opposed to being fully or a less expensive Y5V type can be used if there is sourced from the local input capacitor. additional bypassing provided by bulk electrolytic or An alternative to a high value ceramic capacitor is a lower tantalum capacitors. The following paragraphs describe value along with a larger electrolytic capacitor, for ex- the input capacitor considerations in more detail. ample a 1µF ceramic capacitor in parallel with a low ESR Step-down regulators draw current from the input supply tantalum capacitor. For the electrolytic capacitor, a value in pulses with very fast rise and fall times. The input larger than 10µF will be required to meet the ESR and capacitor is required to reduce the resulting voltage ripple ripple current requirements. Because the input capacitor 1940fa 9