ADM1064 glitching on power up

Question

Hi, 
 I have a design that uses the ADM1064 super-sequencer device. Whilst tracing a fault with another device, I observed that two of the power supplies, controlled by the ADM1064 were glitching on power-up. I believe this is due to the ADM1064 experiencing a brown-out condition. 
 The card I was investigating is one of two PCBS in a module. The baseboard has a competitors power monitor, that switches +5V and +3V3 through power MOSFETs to the onboard circuitry. When it switches the supplies through, in a staggered mode, the large capacitance causes the +5V and the +3.3V supplies to dip. This causes the ADM1064 to dropout and re-start twice. 
 Comparing my design to the evaluation board I can see a difference. The VCCP, REFOUT and VDDCAP capacitors on your design are AVX TAJB Tantalum capacitors. On my design they are Kemet X7R ceramics. Can the internal LDO converter connected to the VDDCAP cope with a ceramic capacitor on it's output? 
 Regards, 
 Ian

Ren · Accepted Answer

Hi Ian, 
 I guess it's true that a picture is worth a thousand words. 
 i initially thought the glitch is negative and happens as these supplies are powering up, now i understand it. 
 It's worth noting the behavior of the PDO outputs during power up, as described in the datasheet: 
 DEFAULT OUTPUT CONFIGURATION 
 All of the internal registers in an unprogrammed ADM1066 device from the factory are set to 0. Because of this, the PDOx pins are pulled to GND by a weak (20 kΩ) on-chip pull-down resistor. 
 As the input supply to the ADM1066 ramps up on VPx or VH, all PDOx pins behave as follows: 
 • Input supply = 0 V to 1.2 V. The PDOs are high impedance. 
 • Input supply = 1.2 V to 2.7 V. The PDOs are pulled to GND by a weak (20 kΩ) on-chip pull-down resistor. 
 • Supply > 2.7 V. Factory-programmed devices continue to pull all PDOs to GND by a weak (20 kΩ) on-chip pull-down resistor. Programmed devices download current EEPROM configuration data, and the programmed setup is latched. The PDO then goes to the state demanded by the configuration. This provide 
 is the 20k pull down too weak to keep these converters off? 
 could you power the ADM106x from the 5V backplane? 
 Ren)

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ADM1064 glitching on power up