6.1 LPCXpresso546x8/540xx/54S0xx current measurement
The LPC546x8/540xx/54S0xx current can be measured by measuring the voltage across
a sense resistor in series with the supply, a current meter or using the on board current
measurement circuit. Each of these methods will be described in subsections below.
There is no current monitoring of the Link2 section circuits on the board. The Target side
power going to LEDs and support ICs is not monitored by the current measurement
circuit. The LPC546x8/540xx/54S0xx BGA package has the core and IO power both
sourced from the same VDD pins.
When a shield board is attached, attempting to measure the lowest possible power the
LPC546x8/540xx/54S0xx IO pins must be configured according to how the software has
configured the shield board to ensure there is no extra current from the
LPC546x8/540xx/54S0xx IO ports that have external pull-up or pull-down resistors
enabled.
There are several leakage paths through the various devices on the
LPCXpresso546x8/540xx/54S0xx board, so this board should not be used to measure
the lowest possible leakage current achievable in a target application.
6.1.1 LPC546x8/540xx/54S0xx Vsense resistor current measurement
The voltage across a pair of 1Ω resistors in series with the target
LPC546x8/540xx/54S0xx VDD can be manually measured at JP4 across pins 1 and 4 on
the PCB. For a higher range of current a jumper should be installed between pins 3 and
4 of JP4 to short out on of these resistors. The voltmeter positive probe is applied to JP4
pin 1 (see silkscreen labelling) and pin 4. Use Ohm’s law to calculate the current
(LPC546x8/540xx/54S0xx current = measured voltage / 2 Ω). As an example, if the
measured voltage is 20 mV, then 20e-3 / 2 Ω = 10 mA.
Note: The input current to the MAX9634 used in the on-board current measurement will
be included in the voltage measured across this resistor.
6.1.2 LPC546x8/540xx/54S0xx VDD current measurement using a current meter
A current meter may be inserted at JP4 between pins 5 and 6 to measure the
LPC546x8/540xx/54S0xx VDD input current. Note that a jumper needs to be installed in
the 5-6 position when an ammeter is not present so power can reach the
LPC546x8/540xx/54S0xx target.
6.1.3 LPC546x8/540xx/54S0xx VDD current measurement
The LPCXpresso546x8/540xx/54S0xx board has an on-board current measurement
circuit consisting of a MAX9634T (U18) current monitor chip and a 12-bit ADC
(ADC122S021, U23) with a 12-bit sample at 50k to 200ksps. The on-board MAX9634T
current monitor measures the voltage across the LPC546x8/540xx/54S0xx VDD Vsense
resistors; either 2 Ω or 1 Ω if JP4 jumper 3-4 is installed. The MAX9634 multiplies the
sense voltage by 25 to provide a voltage range suitable for the ADC to measure. A 2-
input analog mux selects between the LPC546x8/540xx/54S0xx current monitor and the
output off a MAX9634T current monitor chip on an expansion board (with compatible
current measurement circuit on-board). The current measurement circuit is controlled by
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