Forum: Analog Circuits Trouble with MAX712

Author: Nico Van Duijn (nicovanduijn)
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Hey guys,

I've been fooling around with a charging circuitry for a while now, and 
I can't get it to run for the life of me. Here's what I'm doing:

I am trying to charge a 12-cell NimH 5000mAh battery pack.

1) I'm charging with C/4, to 5000mAh, thus
- Ifast around 1.2 A
- Using voltage slope detection

It follows:


2) Since I use 12 cells:


3) For DCin I use a 24V wallwart. Should suffice (12x1.45 + 1.5=18.9)

4) Rsense of 0.22R seems reasonable

5) The datasheet states that for >11 cells you need a cascode connection 
for BATT+ to avoid exceeding its max specs.

6) And since my supply is larger than 20V, I need a cascode for DRV as 

7) I'm trying to use status LEDs for POWER and FASTCHARGE, each using 
around 10mA. This means there should be a total of about 30-45mA flowing 
into the V+ node, right?
 (24-5)/R1=40, so I chose 470R for R1.

8) I connected Vlimit (pin 1) to V+, according to the datasheet page 5 
that makes the max voltage 1.65V. (Not sure if I should use Vref instead 
here, but I thought that would make the max cell voltage 2V, in which 
case my supply voltage wouldn't be large enough. Besides, if I charge a 
NiMH cell to 2V, god know what happens)

I attached a schematic of my circuit.

The whole thing doesn't work at all. My LEDs remain dark, I measure 1.7V 
at V+ (should be 5, right?), Vref is at 0.707V (Should be 2V).

I've been messing around with this so much now, that I am fairly certain 
that I somehow burned out all four of my MAX712s that I had. Is there a 
simple test circuit to determine its proper functionality?

Is there something fundamentally wrong with my circuit? I'm uncertain on 
a few issues:
- Whether I did the two cascode connections properly
- Whether I am powering the MAX712 right (Taking into account the two 
LEDs, base current for T1 and the 5-20mA for the MAX)
- Whether I'm doing the LED thing correctly
- Point 8 from above
- Whether my DCin is high enough
- Whether my MAX712 are defect

I'm pretty lost, any help would be greatly appreciated!


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