ADM696 behavior at opened VBATT pin

Question

Hello, 
 Regarding to VBATT pin in ADM696ARZ, my customer is asking a question. 
 Basically they will use VBATT pin for back-up battery but there will be in case of not used for back-up battery. 
 (VBATT=open.) 
 According to datasheet 10 page (Replacing the Back-Up Battery), 
 "When changing the back-up battery with system power on, spurious resets can occur when the battery is removed. 
 If the voltage on VBATT reaches within 50 mV of VCC, a reset pulse is generated." 
 When the back-up battery will not be connected to VBATT(=open), 
 how will AD696 do to behavior during system power on? 
 ･In same above reason, will the reset be occurred? (Vout=Vcc) 
 ･In this case, is there any impact in ADM696? (Only reset occurs?) 
 Thanks 
 TAO_2199

Ren · Accepted Answer

Hi Tao, 
 Reading from the datasheet, it implies the device will also attempt to switch VOUT to VBATT during that transition. 
 But even if the internal switch between VCC and VOUT is turned off, the diode on the switch will be forward biased so VOUT can still be powered from VCC with a voltage minus a diode drop. So I expect potentially a slight glitch on VOUT (won't go below VCC-0.7V) before recovering during the battery change. 
 The same behavior may not occur during device power up. 
 All these concerns can be addressed by following the recommendations listed in the Replacing the Back-Up Battery section of the datasheet. 
 another way to avoid this issue is to consider using ADM8696. It is a pin to pin replacement part for ADM696 with better specification and a different reset/battery switch over mechanism that prevents the above issue from happening. 
 With ADM8696, the switch over will depend on both VCC and VBAT voltage comparison and LLIN input threshold, and with LLIN voltage not affected by the battery change, the switch will not occur. Also the reset is only depending on LLIN input, and no longer depends on VCC and VBATT voltage comparison. 
 The ADM8696 also has more details on the RESET output voltage with various supply voltages, which could help you with the issue in the other topic. 
 Ren)

Partner Forums

Partner Libraries

Products Mentioned

Associated Drivers

ADM696 behavior at opened VBATT pin