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CS5111 Datasheet(PDF) 5 Page - Cherry Semiconductor Corporation |
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CS5111 Datasheet(HTML) 5 Page - Cherry Semiconductor Corporation |
5 / 10 page The 5V linear regulator consists of an error amplifier, bandgap voltage reference, and a composite pass transistor. The 5V linear regulator circuitry is shown in Figure 2. When an unregulated voltage greater than 6.6V is applied to the VREG input, a 5V regulated DC voltage will be pre- sent at VLIN. For proper operation of the 5V linear regula- tor, the IBIAS lead must have a 64.9kΩ pull down resistor to ground. A 100µF or larger capacitor with an ESR <8Ω must be connected between VLIN and ground. To operate the 5V linear regulator as an independent regulator (i.e. separate from the switching supply), the input voltage must be tied to the VREG lead. As the voltage at the VREG input is increased, Q1 is turned on. Q1 provides base drive for Q2 which in turn provides base current for Q3. As Q3 is turned on, the output voltage, VLIN, begins to rise as Q3’s output current charges the out- put capacitor, COUT. Once VLIN rises to a certain level, the error amplifier becomes biased and provides the appropri- ate amount of base current to Q1. The error amplifier mon- itors the scaled output voltage via an internal voltage divider, R2 through R5, and compares it to the bandgap voltage reference. The error amplifier output or error sig- nal is an output current equal to the error amplifier’s input differential voltage times the transconductance of the amplifier. Therefore, the error amplifier varies the base current to Q1, which provides bias to Q2 and Q3, based on the difference between the reference voltage and the scaled VLIN output voltage. The watchdog timer circuitry monitors an input signal (WDI) from the microprocessor. It responds to the falling edge of this watchdog signal which it expects to see within an externally programmable time (see Figure 3). The watchdog time is given by: tWDI = 1.353 × CDelay RBIAS Using CDelay = 0.1µF and RBIAS = 64.9kΩ gives a time rang- ing from 6.25ms to 11ms assuming ideal components. Based on this, the software must be written so that the watchdog arrives at least every 6.25ms. In practice, the tolerance of CDelay and RBIAS must be taken into account when calculat- ing the minimum watchdog time (tWDI). Figure 3. Timing diagram for normal regulator operation. Figure 4. Timing diagram when WDI fails to appear within the preset time interval, tWDI. VLIN WDI RESET VREG tPOR A B A: Watchdog waiting for low-going transition on WDI 50% Duty Cycle B: RESET stays low for tWDI time. VLIN WDI RESET VREG tPOR Normal Operation Control Functions 5V Linear Regulator Circuit Description Over Temperature Linear Error Amplifier 1.25V VREG VLIN Cdelay Over Voltage RESET & Watchdog Timer Current Limit WDI RESET Bandgap Reference IBIAS RBIAS 64.9kΩ R1 R2 R3 R4 R5 COUT = 100µF ESR < 8Ω Q1 Q2 Q3 Figure 2. Block diagram of 5V linear regulator portion of the CS5111. 5 |
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