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ADM8690 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADM8690 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 20 page ADM8690/ADM8691/ADM8692/ADM8693/ADM8695 Rev. B | Page 10 of 20 CIRCUIT INFORMATION BATTERY SWITCHOVER SECTION The battery switchover circuit compares VCC to the VBATT input, and connects VOUT to whichever is higher. Switchover occurs when VCC is 50 mV higher than VBATT as VCC falls, and when VCC is 70 mV greater than VBATT as VCC rises. This 20 mV of hysteresis prevents repeated rapid switching if VCC falls very slowly or remains nearly equal to the battery voltage. GATE DRIVE VCC VBATT VOUT BATT ON (ADM8690, ADM8695) 100 mV 700 mV INTERNAL SHUTDOWN SIGNAL WHEN VBATT > (VCC + 0.7V) Figure 14. Battery Switchover Schematic During normal operation, with VCC higher than VBATT, VCC is internally switched to VOUT through an internal PMOS tran- sistor switch. This switch has a typical on resistance of 0.7 Ω and can supply up to 100 mA at the VOUT terminal. VOUT is normally used to drive a RAM memory bank, requiring instantaneous currents of greater than 100 mA. If this is the case, a bypass capacitor should be connected to VOUT. The capacitor provides the peak current transients to the RAM. A capacitance value of 0.1 μF or greater can be used. If the continuous output current requirement at VOUT exceeds 100 mA, or if a lower VCC − VOUT voltage differential is desired, an external PNP pass transistor can be connected in parallel with the internal transistor. The BATT ON output (ADM8691/ ADM8693/ADM8695) can directly drive the base of the external transistor. A 7 Ω MOSFET switch connects the VBATT input to VOUT during battery backup. This MOSFET has very low input-to-output differential (dropout voltage) at the low current levels required for battery back up of CMOS RAM or other low power CMOS circuitry. The supply current in battery back up is typically 0.4 μA. The ADM8690/ADM8691/ADM8695 operate with battery voltages from 2.0 V to 4.25 V, and the ADM8692/ ADM8693 operate with battery voltages from 2.0 V to 4.0 V. High value capacitors, either standard electrolytic or the farad- size, double-layer capacitors, can also be used for short-term memory backup. A small charging current of typically 10 nA (0.1 μA maximum) flows out of the VBATT terminal. This current is useful for maintaining rechargeable batteries in a fully charged condition. This extends the life of the backup battery by compensating for its self-discharge current. Also note that this current poses no problem when lithium batteries are used for backup because the maximum charging current (0.1 μA) is safe for even the smallest lithium cells. If the battery switchover section is not used, VBATT should be connected to GND and VOUT should be connected to VCC. POWER-FAIL RESET OUTPUT RESET is an active low output that provides a RESET signal to the microprocessor whenever VCC is at an invalid level. When VCC falls below the reset threshold, the RESET output is forced low. The nominal reset voltage threshold is 4.65 V (ADM8690/ ADM8691/ADM8695) or 4.4 V (ADM8692/ADM8693). RESET LOW LINE V1 V2 V2 V1 VCC t1 t1 t1 = RESET TIME V1 = RESET VOLTAGE THRESHOLD LOW V2 = RESET VOLTAGE THRESHOLD HIGH HYSTERESIS = V2–V1 Figure 15. Power-Fail Reset Timing On power-up, RESET remains low for 50 ms (200 ms for ADM8695) after VCC rises above the appropriate reset threshold. This allows time for the power supply and microprocessor to stabilize. On power-down, the RESET output remains low with VCC as low as 1 V. This ensures that the microprocessor is held in a stable shutdown condition. This RESET active time is adjustable on the ADM8691/ADM8693/ ADM8695 by using an external oscillator or by connecting an external capacitor to the OSC IN pin. Refer to Table 5 and Figure 17, Figure 18, Figure 19, and Figure 20. The guaranteed minimum and maximum thresholds of the ADM8690/ADM8691/ADM8695 are 4.5 V and 4.73 V, and the guaranteed thresholds of the ADM8692/ADM8693 are 4.25 V and 4.48 V. The ADM8690/ADM8691/ADM8695 are, therefore, compatible with 5 V supplies with a +10%, −5% tolerance and the ADM8692/ADM8693 are compatible with 5 V ± 10% supplies. The reset threshold comparator has approximately 50 mV of hysteresis. The response time of the reset voltage comparator is less than1 μs. If glitches are present on the VCC line that could cause spurious reset pulses, VCC should be decoupled close to the device. |
Podobny numer części - ADM8690_15 |
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Podobny opis - ADM8690_15 |
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