Other Parts Discussed in Thread: TPS61027
Boost convertor troubles
TPS61020, actually it is the family member TPS61027 (fixed 5V version)
I have designed this into a new product and built some boards
I have carefully studied the data sheet and followed the design guidelines, I have done numerous similar designs and consider myself experienced in this area
My trouble is that this part will simply not start into a load which should be well within limits
If i apply power without a load the chip starts up fine, is 5.0Vout and is clean,
if i then apply the load after startup everything is fine, clean 5.0Vout, full power delivered
The Load is a purely resistive test load @400mA, no external load capacitance
Vin 3.5 - 5V (no difference noted over this range), again Vout = 5.0V
If i apply power with the 400mA test load attached the unit will not start, i spent a day playing with options
NOTE: the chip WILL start into load when the load is about 80-100mA
The output cap is a high quality low ESR 47uF polymer tantalum in parallel with an MLCC 4.7uF
Input cap 10uF MLSS
I tried with both PWM forced and power autosave, no difference
i played with input and output caps, verified inductor ratings per equations, Ind = 10uH, 0.84A, 0.2 Ohms DCR
I read the area about startup current limit and as far as i can tell i am well within the limits
Finally out of desperation i built a circuit with a few Fets that applies the load 100mS after startup and it works fine into 400mA, even 500mA (400mA is max target)
but we shouldn't have to do this (100mS was arbitrary, don't know how short might work, 100mS doesn't really matter to this design)
any help please ?
