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TC120 Datasheet(PDF) 6 Page - Microchip Technology |
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TC120 Datasheet(HTML) 6 Page - Microchip Technology |
6 / 14 page TC120 DS21365B-page 6 © 2002 Microchip Technology Inc. 3.6 Low Power Shutdown Mode/Soft Start Input The SHDN/SS input acts as both the shutdown control and the node for the external soft start capacitor, which is charged by an internal 1 µA current source. A value of 4700pF (100pF minimum) is recommended for the soft start capacitor. Failure to do this may cause large overshoot voltages and/or large inrush currents result- ing in possible instability. The TC120 enters a low power shutdown mode when SHDN/SS is brought low. While in shutdown, the oscillator is disabled and the output discharge switch is turned on, discharging the output capacitor. Figure 3-3 shows the recommended interface circuits to the SHDN/SS input. As shown, the SHDN/SS input should be controlled using an open collector (or open drain) device, such that the SHDN/ SS input is grounded for shutdown mode, and open- circuited for normal operation (Figure 3-3a). If a CMOS device is used to control shutdown (Figure 3-3b), the value of R1 and CSS should be chosen such that the voltage on SHDN/SS rises from ground to 0.65V in 1.5msec (Figure 3-4). If shutdown is not used, CSS must still be connected as shown in Figure 3-3c and Figure 3-3d. SHDN/SS may be pulled up with a resistor (Figure 3-3c) as long as the values of RSS and CSS provide the approximate charging characteristic on power up shown in Figure 3-4. CSS only may also be connected as shown in Figure 3-3d with CSS chosen at 4700pF (minimum 100pF). 3.7 Undervoltage Lockout (UVLO) The TC120 is disabled whenever VIN is below the undervoltage lockout threshold. This threshold is equal to the guaranteed minimum operating voltage for the TC120 (i.e., 2.2V). When UVLO is active, the TC120 is completely disabled. 3.8 Short Circuit Protection Upon detection of an output short circuit condition, the TC120 reduces the PWM duty cycle to a minimum value using its internal protection timer. The sequence of events is as follows: when an output voltage decrease to near zero is detected (as the result of an overload), the internal (5msec) protection timer is started. If the output voltage has not recovered to nominal value prior to the expiration of the protection timer, the TC120 is momentarily shut down by dedicated, internal circuitry. Immediately following this action, the soft start sequence is engaged in an attempt to re-start the TC120. If the output short circuit is removed, normal operation is automatically restored. If the short circuit is still present, the timed self-shutdown sequence described above is repeated. 3.9 External Switching Transistor Selection EXT is a complimentary output with a maximum ON resistances of 32 Ω to VDD when high and 26Ω to ground when low, at VOUT = 5V. It is designed to directly drive a P-channel MOSFET (Figure 3-5). The P-channel MOSFET selection is determined mainly by the on-resistance, gate-source threshold and gate charge requirements. Also, the drain-to-source and gate-to-source breakdown voltage ratings must be greater than VINMAX. The total gate charge specification should be less than 100nC for best efficiency. The MOSFET must be capable of handling the required peak inductor current, and should have a very low on- resistance at that current. For example, a Si9430 MOSFET has a drain-to-source rating of -20V, and a typical on-resistance rDSON of 0.07Ω at 2A, with VGS = -4.5V. (EXTW (Figure 3-6) may be gated with external circuitry to add blanking, or as an auxiliary timing signal.) Table 3-1 lists suggested components and suppliers. 3.10 Board Layout Guidelines As with all inductive switching regulators, the TC120 generates fast switching waveforms, which radiate noise. Interconnecting lead lengths should be minimized to keep stray capacitance, trace resistance and radiated noise as low as possible. In addition, the GND pin, input bypass capacitor and output filter capacitor ground leads should be connected to a single point. The input capacitor should be placed as close to power and ground pins of the TC120 as possible. The length of the EXT trace must also be kept as short as possible. |
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