I am using adum5000. the VDD1 is supply with 3.3v.
I notice when i supply 3.3v to adum5000, Viso measures around 1.6v and after many seconds will increase to 3.3v?
Would need help why this happen?
Bryan: The problem appears to be too much VDD1 supply current at 125mA causing the LT1735-3.3V to limit the current, since it's page 6 of the datasheet says:
During short-circuit conditions, the LTC1754 will draw between 100mA and 400mA from VIN causing a rise in the junction temperature. On-chip thermal shutdown circuitry disables the charge pump once the junction temperature exceeds approximately 150°C and reenables the charge pump once the junction temperature drops back to approximately 140°C. The LTC1754 will cycle in and out of thermal shutdown indefinitely without latchup or damage until the short circuit on VOUT is removed.
The datasheet page 2 specification at typical (25C) and 2.5V for short circuit current is 118mA, which is close to your 125mA measured current. The above Thermal protection description shows that if the LT1735 gets hot from too large a load current, it could "cycle in and out of thermal shutdown" "until the short circuit on VOUT is removed".
The ADuM5000 datasheet shows in figure 5 that the input current on VDD1 will be about 0.120A when the output current on Viso is about 0.04A.
It seems you may have designed your circuit with the LT1754-3.3V for 40mA output current, which may be the output current load on the ADuM5000, but the ADuM5000 input current is about 3 times this or 120mA, which is too much for the LT1754-3.3V, which is rated for 40mA.
To capture the voltage waveform with a scope, you need to trigger the scope on a single event before the changing voltage level of VDD1 or Viso waveform is above about 2V, before Viso has risen to 3.3V.
I will need some more information to try to see what could be wrong:
1. A schematic of your circuit of the ADuM5000.
2. A waveform of the VDD1 startup and VISO voltage.
3. What is the load on VISO in startup?
4. The data code on the ADuM5000 part.
Attached are the schematics.
Some additional infos.
The board wakes up for approximate 3sec and goes to sleep. The LTC1754-3.3v
will be pulled low at the SHDN pin. LTC1754-3.3 is providing the 3.3v to my
- The waveform for VDD1 and Viso is a straight line at 3.3v
- The load on Viso is at 125mA on startup and drops to 89mA while running
- The data code is
I saw this from LTC1754, could it this feature which allows the charge pump
to provide the needed current by ADum5000?
On Tue, Dec 20, 2016 at 12:48 AM, bkennedy <
Looking at your circuit, the LTC1754ES6-3.3V is only rated for 40mA Iout, and this powers the ADuM5000 which in startup you say Viso load is 125mA, so the current at VDD1 will be much more than 125mA and therefore the Viso load will be too much.
1. Even 89mA Viso load is too much, the ADum5000 at 3.3V to 3.3V is only rated for 60mA.
2. Can you provide a scope waveform of Viso load measurements?
3. Can you provide a scope waveform of Viso and VDD1 in startup and in operation?
I will snap the waveform and send you.
Sorry I made a mistake. The load of 125ma is on the VDD1.
On Dec 20, 2016 03:23, "bkennedy" <email@example.com>
Sorry I am quite new in electronics. The waveform at Viso is a straight
line at 3.3v using my scope. Can I ask how or what type of waveform should
I provide you?
Thank you for your help
Thank you for your reply.
I will snap the waveform and send to you later today.
Can i conclude that when ADum5000 needs 120mA, LTC1754 will work in an abnormal state(Short-circuit and draw current from Vin) to provide ADum5000 ?
I calculated as below for the power requirements
AD5934 @ 3.3V: 15mA
AD8606 @ 3.3V: 2.8mA
ADG715 @ 3.3V: 20uA
FXO-HC536R-1 crystal oscillator @ 3.3V: 32mA
ADuM1250 @3.3V: 3mA/side, total = 6mA
Total not including ADuM5000: 55.82mA
ADuM5000 with 55.82mA load:
Efficiency of ADuM5000 supply is 28% and it must supply 55.82mA to the load.
Therefore I must supply 55.82/0.28 = 200mA to the ADuM5000 to get 55.82mA out.
Am i correct?
Bryan: I get a different result. The supply current of 55.82 divided by the 35% (efficiency for 3.3V supplies) yields VDD1 supply current = 55.82/0.35 = 159mA. Either way, this VDD1 supply current is beyond the LTC1754 rating.
Retrieving data ...