Datasheet LTC1707 (Analog Devices) - 9

LTC1707
U U W U APPLICATIO S I FOR ATIO
size or height requirements in the design. Always consult the and T495 series, Nichicon PL series and Sprague 593D manufacturer if there is any question. and 595D series. Consult the manufacturer for other The selection of C specific recommendations. OUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement is
Output Voltage Programming
satisfied, the capacitance is adequate for filtering. The output ripple ∆V The output voltage is set by a resistive divider according OUT is determined by: to the following formula:  1  ∆VOUT ≅ ∆I ESR L +  2  R 8 fCOUT V 0. V OUT = 8 1 +  (2) R1 where f = operating frequency, COUT = output capacitance and ∆I The external resistive divider is connected to the output, L = ripple current in the inductor. The output ripple is highest at maximum input voltage since ∆I allowing remote voltage sensing as shown in Figure 4. L increases with input voltage. For the LTC1707, the general rule for
Run/Soft-Start Function
proper operation is: The RUN/SS pin is a dual purpose pin that provides the COUT required ESR < 0.25Ω soft-start function and a means to shut down the LTC1707. Manufacturers such as Nichicon, United Chemicon and Soft-start reduces surge currents from VIN by gradually Sanyo should be considered for high performance through- increasing the internal current limit. Power supply hole capacitors. The OS-CON semiconductor dielectric sequencing can also be accomplished using this pin. capacitor available from Sanyo has the lowest ESR/size An internal 2.25µA current source charges up an external ratio of any aluminum electrolytic at a somewhat higher capacitor CSS. When the voltage on RUN/SS reaches price. Once the ESR requirement for COUT has been met, 0.7V the LTC1707 begins operating. As the voltage on the RMS current rating generally far exceeds the RUN/SS continues to ramp from 0.7V to 1.8V, the inter- IRIPPLE(P-P) requirement. Remember ESR is typically a nal current limit is also ramped at a proportional linear direct function of the volume of the capacitor. rate. The current limit begins at 25mA (at VRUN/SS ≤ 0.7V) In surface mount applications multiple capacitors may and ends at the Figure 3 value (VRUN/SS ≈ 1.8V). The have to be paralleled to meet the ESR or RMS current output current thus ramps up slowly, charging the output handling requirements of the application. Aluminum capacitor. If RUN/SS has been pulled all the way to electrolytic and dry tantalum capacitors are both avail- ground, there will be a delay before the current starts able in surface mount configurations. In the case of increasing and is given by: tantalum, it is critical that the capacitors are surge tested . CSS for use in switching power supplies. An excellent choice tDELAY = 0 7 2 2 . 5 A µ is the AVX TPS series of surface mount tantalum, avail- able in case heights ranging from 2mm to 4mm. Other Pulling the RUN/SS pin below 0.4V puts the LTC1707 into capacitor types include Sanyo POSCAP, KEMET T510 a low quiescent current shutdown (IQ < 15µA). This pin can be driven directly from logic as shown in Figure 5. Diode 0.8V ≤ VOUT ≤ 8.5V R2 3.3V OR 5V RUN/SS RUN/SS VFB D1 LTC1707 R1 GND C C SS SS 1707 F04 1707 F05
Figure 4. Setting the LTC1707 Output Voltage Figure 5. RUN/SS Pin Interfacing
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