Datasheet

NXP Semiconductors

UM11121

LPCXpresso51U68

UM11121

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User manual

Rev 1.1 — 27 April 27, 2018

13 of 22

LPC51U68 Vsense resistor current measurement

The voltage across a series 4.12Ω resistor with the target LPC51U68 VDD can be

manually measured at P2 on the PCB. The voltmeter positive probe is applied to P2 pin

1 (square pad) and negative probe to P2 pin 2. Use Ohm’s law to calculate the current

(LPC51U68 current = measured voltage / 4.12Ω). As an example, if the measured

voltage is 10mV, then 10e-3 / 4.12Ω = 2.44mA. Note that the input current to the

MAX9634 used in the on-board current measurement will be included in the voltage

measured across this resistor.

LPC51U68 VDD current measurement using a current meter

A current meter may be inserted at JP6 to measure the LPC51U68 VDD input current.

The 0Ω resistor at JS10 must be removed and the current meter connected at the

positive input at JP6 pin 1 (square pad) and negative input at pin 2.

LPC51U68 VDD current measurement

The LPCXpresso51U68 board has an on-board current measurement circuit consisting of

a MAX9634T current monitor chip and a 12-bit ADC (ADC122S021) with a 12-bit sample

at 50k to 200ksps. The on-board MAX9634T current monitor measures the voltage

across the LPC51U68 VDD vsense resistors; either 8.24Ω or 4.12 Ω if JP5 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 LPC51U68 current

monitor and the output on a MAX9634T current monitor chip on an expansion board (with

compatible current measurement circuit on-board). The current measurement circuit is

controlled by the Link2 processor and is not user programmable. Power measurement

utilities to use this feature are available in the MCUXpresso IDE installation.

Due to input offset voltage variations in the MAX9634, the current measurement circuit is

not recommended for measuring current below 150uA. See Fig 5 as a guideline for

measurement error versus measured current.