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RT7295D Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT7295D Datasheet(HTML) 11 Page - Richtek Technology Corporation |
11 / 14 page RT7295D 11 DS7295D-05 July 2015 www.richtek.com © Copyright 2015 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Feed-forward Capacitor (Cff) The RT7295D is optimized for ceramic output capacitors and for low duty cycle applications. However for high-output voltages, with high feedback attenuation, the circuit's response becomes over-damped and transient response can be slowed. In high-output voltage circuits (VOUT > 3.3V) transient response is improved by adding a small “feed- forward” capacitor (Cff) across the upper FB divider resistor (Figure 1), to increase the circuit's Q and reduce damping to speed up the transient response without affecting the steady-state stability of the circuit. Choose a suitable capacitor value that following below step. Get the BW the quickest method to do transient response form no load to full load. Confirm the damping frequency. The damping frequency is BW. Figure 1. Cff Capacitor Setting Cff can be calculated base on below equation. ff 1 C 2 3.1412 R1 BW 0.8 Internal Soft-Start (SS) The RT7295D soft-start uses an internal soft-start time 800 μs. Enable Operation (EN) For automatic start-up the EN pin can be connected to VIN through a 100k Ω resistor. Its large hysteresis band makes EN useful for simple delay and timing circuits. EN can be externally pulled to VIN by adding a resistor- capacitor delay (REN and CEN in Figure 2). Calculate the delay time using EN's internal threshold where switching operation begins (1.4V, typical). An external MOSFET can be added to implement digital control of EN when no system voltage above 2V is available (Figure 3). In this case, a 100k Ω pull-up resistor, REN, is connected between VIN and the EN pin. MOSFET Q1 will be under logic control to pull down the EN pin. To prevent enabling circuit when VIN is smaller than the VOUT target value or some other desired voltage level, a resistive voltage divider can be placed between the input voltage and ground and connected to EN to create an additional input under voltage lockout threshold (Figure 4). RT7295D GND FB R1 R2 VOUT Cff BW Figure 4. Resistor Divider for Lockout Threshold Setting Figure 2. External Timing Control Figure 3. Digital Enable Control Circuit RT7295D EN GND VIN REN CEN EN RT7295D EN GND 100k VIN REN Q1 Enable RT7295D EN GND VIN REN1 REN2 |
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