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CS8140YTHA7 데이터시트(PDF) 8 Page - Cherry Semiconductor Corporation |
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CS8140YTHA7 데이터시트(HTML) 8 Page - Cherry Semiconductor Corporation |
8 / 12 page 8 As long as ENABLE is high or ENABLE is low and the Watchdog signal is normal, VOUT will be at 5V (typ). If ENABLE is low and the Watchdog signal moves outside programmable limits, the output transistor turns off and the IC goes into SLEEP mode. Only the ENABLE circuitry in the IC remains powered up, drawing a quiescent cur- rent of 250µA. The Watchdog monitors the frequency of an incoming WDI signal. If the signal falls outside of the WDI window, a frequency programmable pulse train is generated at the lead (Figure 3) until the correct Watchdog input signal reappears at the lead (ENABLE = HIGH). The lower and upper window threshold limits of the watchdog function are set by the value of CDELAY. The lim- its are determined according to the following equations for the CS8140: (a) tWDILOWER = (1.3 x 105)CDELAY or fWDI(LOWER) = (7.69 x 10-6)CDELAY-1 (b) tWDI(UPPER) = (3.82 x 10-4)CDELAY or fWDI(UPPER) = (2.62 x 10-5)CDELAY-1 For the CS8141 the lower limit is determined by the equa- tions in (a) above. The capacitor CDELAY also determines the frequency of the signal and the POWER-ON- (POR) delay period. Function The function is activated when the Watchdog sig- nal is outside of its preset window (Figure 3), when the regulator is in its power up state (Figure 4a) or when VOUT drops below VOUT -4.5% for more than 2µs (Figure 4b.) If the Watchdog signal falls outside of the preset voltage and frequency window, a frequency programmable pulse train is generated at the lead (Figure 3) until the correct Watchdog input signal reappears at the lead. The duration of the pulse is determined by CDELAY according to the following equation: tWDI( ) = (1 x10 4)CDELAY 4a: Power and Power Down 4b: Undervoltage Triggered If an undervoltage condition exists, the voltage on the lead goes low and the delay capacitor, CDELAY, is discharged. remains low until output is in regula- tion, the voltage on CDELAY exceeds the upper switching threshold and the Watchdog input signal is within its set window limits (Figure 4). The delay after the output is in regulation is: tPOR(typ) = (4.75 x 105) CDELAY The delay circuit is also programmed with the external cap CDELAY. The output of the reset circuit is an open collector NPN. is operational down to VOUT = 1V. Both and its delay are governed by comparators with hysteresis to avoid undesirable oscillations. RESET RESET RESET RESET RESET RESET VOUT VOUT -4.5% <2 mS RESET 5V tPOR ³2ms RESET VOUT VR HI VR LO VR LO tPOR RESET VR PEAK RESET Circuit Waveforms with Delays Indicated RESET RESET RESET RESET RESET RESET RESET RESET Circuit Description: continued Application Notes The CS8140 with its unique integration of linear regulator and control features: , ENABLE and WATCHDOG, provides a single IC solution for a microprocessor power supply. The reset delay, reset duration and watchdog fre- quency limits are all determined by a single capacitor. For a particular microprocessor the overriding requirement is usually the reset delay (also known as power on reset). The capacitor is chosen to meet this requirement and the reset duration and watchdog frequency follow. The reset delay is given by: tPOR(typ) = (4.75 x 105)CDELAY Assume that the reset delay must be 200ms minimum. From the CS8140 data sheet the reset delay has a ±37% tol- erance due to the regulator. Assume the capacitor tolerance is ±10%. tPOR (min) = (4.75 x 105 x 0.63) x CDELAY x 0.9 CDELAY (min) = CDELAY = (min) = 0.743 µF Closest standard value is 0.82µF. Minimum and maximum delays using 0.82µF are 220ms and 586ms. tPOR (min) 2.69 x 105 RESET CS8140 Design Example |
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