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1. Brief description of the LDO voltage regulator

38 LDO(Low Drop-Out) voltage regulators on the Mother Board( 5 X 7 groups + 3 X 1 mother board ) condition incoming voltages from low voltage power supplies. Three different voltages, +5V digital, +5V analog, and -5V analog, are provided to activate the Mother Board. In the meantime, five different voltages, +5VD for Heap manager & ADC, +5VA_Comp for comparator, +5VA_TGV for TGV and AMU, -5VA for isum amp, and +2.5VA for Vmid, are provided to the front-end electronics.

From the regulator circuit, the monitoring voltages come out as inputs to multiplexed serial readout ADC's. All monitoring voltages go through some voltage divider getting to the input stage of the ADC. Monitoring voltages of +5V outputs and +2.5VA are divided by two. For example, if the outputs of the LDO voltage regulators are exactly +5V(or +2.5), then the monitoring voltages should be +2.5V(or +1.25V). In the case of -5VA, the monitoring voltage is divided by four third with the lower level at -5VA output and the upper level at +5VA output. For example, supposing normal operation of the regulators, i.e -5VA output is -5V and +5V output is +5V, then the monitoring voltage is +2.5V. If -5V output is changed to -4V, the monitoring voltage for -5VA should increase to +2.75V instead of decreasing.

2. Setup & procedure for the test

Fig 4. The simple setup for the LDO voltage regulator test

Two power cables were made with Panduit connectors and 18-gauge solid wire. Referring to the mother board schematic sheet #2, only wires from pins #1, 2, 5, 6, 10, 11 were hooked up to the power supply. DGND(pin #2) and AGNDs were all tied together externally for all remaining tests. Three kinds of input voltages were applied to pin #1(+5VD_IN), 5(+5VA_IN), 10(-5VA_IN). The LDO voltage regulator needs the differential voltage between the input and output stage to be greater than 1.1V. This means that the input amplitude should be greater than 6.1V. Therefore we used +6.5V and -6.5V as input voltages.

Output voltages were measured on the 160 pin(80 pin) fan out board for the groups 1-6(7).

• +5VD,HM & ADC -- pin #103, 104, 107, 111, 115, 119, 123, 127, 131, 135, 139, 143
• +5VA_Comp -- pin #108, 112, 116
• +5VA_TGV -- pin #120, 124, 128
• -5VA,isum amp -- pin #130, 132, 134
• +2.5VA,Vmid -- pin #136, 138, 140

LDO voltage regulator outputs for the Mother Board itself were directly measured from LDO chips(Q26, Q27, Q28).

• MB_5VD -- Q28, pin#3
• MB_5VA -- Q26, pin#3
• MB_-5VA -- Q27, pin#3

3. Results

At the beginning of the test, 8 LDO regulators for -5VA showed strange behavior. As soon as we applied -6.5V, the current increased slowly so that the input decreased to about -4V in a minute. The reason for this was due to capacitors with the wrong polarity. All 8 caps(c51, c55, c63, c71, c79, c87, c122, c178), which had driven too much current, were remounted with the correct polarity. We were able to verify that the poly-fuse at the input line was working. The measured values are listed in Table 3 and 4. Because there were no front-end electronics connected, currents for groups 1-7 were small enough to be negligible. The fluctuation of LDO regulator outputs from the target voltage are less than 1.2 %.

* Actually there were no outputs on pins 124 and 128 of the fan out board when group 6 was powered up. So we checked the outputs on the LDO chip(Q31) and could see the correct output. Finally we found missing outputs on pins 122 and 126, which should be AGND. This meant that the LDO chip outputs were wrongly connected to AGND on the mother board.This was confirmed by the connectivity test.

The LDO test results for the mother board itself are shown on table 4. Output voltages were within the expected range, and power consumption was 1.5W. The estimated power consumption of the mother board had been 1.0W.