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ST779ACD Datasheet(PDF) 4 Page - STMicroelectronics |
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ST779ACD Datasheet(HTML) 4 Page - STMicroelectronics |
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4 / 11 page  ST777/778/779 4/11 TYPICAL APPLICATION CIRCUIT APPLICATIONS INFORMATION R1 and R2 must be placed only in ST779 applications to set the output voltage according to the following equation: VOUT = (1.23) [(R1+R2)/R2] and to simplify the resistor selection: R1 = R2 [(VOUT/1.23)-1] It is possible to use a wide range of values for R2 (10K Ω to 50KΩ) with no significant loss of accuracy thanks to the very low FB input current. To have 1% error, the current through R2 must be at least 100 times FB’s bias current. When large values are used for the feedback resistors (R1>50K Ω), stray output impedance at FB can incidentally add "lag" to the feedback response, destabilizing the regulator and creating a larger ripple at the output. Lead lengths and circuit board traces at the FB node should be kept short. Compensate the loop by adding a "lead" compensation capacitor (C3, 100pF to 1nF) in parallel with R1. The typical value of the L1 inductor is 22µH, enough for most applications. However, are also suitable values ranging from 10µF to 47µF with a saturation rating equal to or greater than the peak switch -current limit. Efficiency will be reduced if the inductor works near its saturation limit, while will be maximized using an inductor with a low DC resistance, preferably under 0.2 Ω. Connecting ILIM to VIN the maximum LX current limit (1A) is set. If this maximum value is not required is possible to reduce it connecting a resistor between ILIM and VIN (See Figure16to choose the right value). The current limit value is misured when the switch current through the inductor begins to flatten and does’nt coincide with the max short circuit current. Even if the device is designed to tolerate a short circuit without any damage, it is strictly recommended to avoid a continuos and durable short circuit of the output to GND. To achieve the best performances from switching power supply topology, particular care to layout drawing is needed, in order to minimize EMI and obtain low noise. Moreover, jitter free operation ensures the full device functionality. Wire lengths must be minimized, filter and by-pass capacitors must be low ESR type, placed as close as possible to the integrated circuit. Solder AGND and PGND pins directly to a ground plane. ST777 ST778 ST779 ILIM L1 22µH SEL / N.C. / FB R1 V+ LX Vout SHDN ____ AGND C1 22µF C2 100µF C3 Vin 1÷6V 1 3 8 7 2 5 6 4 Vo PGND R2 |
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Similar Description - ST779ACD |
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