Datasheet LTC1773 (Analog Devices) - 10

LTC1773
U U W U APPLICATIONS INFORMATION
The ITH pin OPTI-LOOP compensation components can be
Run/Soft-Start Function
optimized to provide stable, high performance transient The RUN/SS pin is a dual purpose pin that provides the response regardless of the output capacitors selected. soft-start function and a means to shut down the LTC1773. 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 se- hole capacitors. The OS-CON semiconductor dielectric quencing can also be accomplished using this pin. capacitor available from Sanyo has the lowest ESR/size An internal 1.5µA current source charges up an external ratio of any aluminum electrolytic at a somewhat higher capacitor C price. Once the ESR requirement for C SS. When the voltage on RUN/SS reaches 0.7V OUT has been met, the LTC1773 begins operating. As the voltage on RUN/SS the RMS current rating generally far exceeds the continues to ramp from 0.7V to 1.8V, the I I TH clamp is also RIPPLE(P-P) requirement. ramped at a proportionally linear rate. Depending on the In surface mount applications multiple capacitors may external RSENSE used, the peak inductor current, and thus have to be paralleled to meet the ESR or RMS current the internal current limit, rises with the RUN/SS voltage. handling requirements of the application. Aluminum elec- The output current thus ramps up slowly, charging the trolytic and dry tantalum capacitors are both available in output capacitor. If RUN/SS has been pulled all the way to surface mount configurations. In the case of tantalum, it is ground, there will be a delay before the current starts critical that the capacitors are surge tested for use in increasing and is given by: switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum, available in case 0 7 . CSS = = (0 4.7 /µ ) heights ranging from 2mm to 4mm. Other capacitor types t s F C DELAY SS 1 5 . A µ include Sanyo OS-CON and POSCAP, Nichicon PL series, Panisonic SP series and Sprague 593D and 595D series. Pulling the RUN/SS pin below 0.4V puts the LTC1773 into Consult the manufacturer for other specific recommenda- a low quiescent current shutdown mode (IQ < 10µA). This tions. pin can be driven directly from logic as shown in Figure 4. Diode D1 in Figure 4 reduces the start delay but allows CSS
Output Voltage Programming
to ramp up slowly providing the soft-start function. This The output voltage is set by a resistive divider according diode can be deleted if soft-start is not needed. to the following formula: 3.3V OR 5V RUN/SS RUN/SS ⎛ R2⎞ D1 V 0. V OUT = 8 1+ ⎝⎜ (2) R ⎠⎟ 1 CSS CSS The external resistive divider is connected to the output as 1773 F04 shown in Figure 3, allowing remote voltage sensing.
Figure 4. RUN/SS Pin Interfacing
0.8V ≤ VOUT ≤ 8.5V
Auxiliary Winding Control Using SYNC/FCB Pin
R2 The SYNC/FCB pin can be used as a secondary feedback VFB to provide a means of regulating a flyback winding output. LTC1773 R1 When this pin drops below its ground referenced 0.8V GND threshold, continuous mode operation is forced. In con- tinuous mode, the P-channel main and N-channel syn- 1773 F03 chronous switches are switched continuously regardless
Figure 3. Setting the LTC1773 Output Voltage
of the load on the main output. 1773fb 10