Datasheet LTC1627 (Analog Devices) - 9

LTC1627
U U W U APPLICATIO S I FOR ATIO
0.8V ≤ VOUT ≤ 8.5V The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement is R2 satisfied, the capacitance is adequate for filtering. The output VFB ripple ∆VOUT is determined by: LTC1627 R1  1  GND ∆VOUT ≅ ∆I ESR L +  8 fC  1627 F04 OUT where f = operating frequency, COUT = output capacitance
Figure 4. Setting the LTC1627 Output Voltage
and ∆IL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ∆IL increases
Run/Soft-Start Function
with input voltage. For the LTC1627, the general rule for proper operation is: The RUN/SS pin is a dual purpose pin that provides the soft-start function and a means to shut down the LTC1627. COUT required ESR < 0.25Ω Soft-start reduces surge currents from VIN by gradually Manufacturers such as Nichicon, United Chemicon and increasing the internal current limit. Power supply Sanyo should be considered for high performance through- sequencing can also be accomplished using this pin. hole capacitors. The OS-CON semiconductor dielectric An internal 2.25µA current source charges up an external capacitor available from Sanyo has the lowest ESR/size capacitor CSS. When the voltage on RUN/SS reaches 0.7V ratio of any aluminum electrolytic at a somewhat higher the LTC1627 begins operating. As the voltage on RUN/SS price. Once the ESR requirement for COUT has been met, continues to ramp from 0.7V to 1.8V, the internal current the RMS current rating generally far exceeds the limit is also ramped at a proportional linear rate. The IRIPPLE(P-P) requirement. current limit begins at 25mA (at VRUN/SS ≤ 0.7V) and ends In surface mount applications multiple capacitors may at the Figure 3 value (VRUN/SS ≈ 1.8V). The output current have to be paralleled to meet the ESR or RMS current thus ramps up slowly, charging the output capacitor. If handling requirements of the application. Aluminum elec- RUN/SS has been pulled all the way to ground, there will trolytic and dry tantalum capacitors are both available in be a delay before the current starts increasing and is given surface mount configurations. In the case of tantalum, it is by: critical that the capacitors are surge tested for use in . C switching power supplies. An excellent choice is the AVX t SS DELAY = 0 7 2 2 TPS series of surface mount tantalum, available in case . 5 A µ heights ranging from 2mm to 4mm. Other capacitor types Pulling the RUN/SS pin below 0.4V puts the LTC1627 into include Sanyo POSCAP, KEMET T510 and T495 series, a low quiescent current shutdown (IQ < 15µA). This pin can Nichicon PL series and Sprague 593D and 595D series. be driven directly from logic as shown in Figure 5. Diode Consult the manufacturer for other specific recommenda- D1 in Figure 5 reduces the start delay but allows CSS to tions. ramp up slowly providing the soft-start function. This diode can be deleted if soft-start is not needed.
Output Voltage Programming
The output voltage is set by a resistive divider according 3.3V OR 5V RUN/SS RUN/SS to the following formula: D1  R2 C C SS SS V 0. V OUT = 8 1 + (2) 1627 F05  R1 The external resistive divider is connected to the output,
Figure 5. RUN/SS Pin Interfacing
allowing remote voltage sensing as shown in Figure 4. 9