NXP Semiconductors
UM10855
LPCXpresso board for LPC54100 family of MCUs
UM10855_OM1377.docm
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User manual
Rev. 1.0
— 4th November 2014
10 of 19
5.1.1
LPC54102 Vsense resistor current measurement
The voltage across a series 8.2Ω resistor with the target LPC54102 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, see
. Use Ohm’s law to calculate
the
current (LPC54102 current = measured voltage / 8.2Ω). As an example if the
measured voltage is 20mV, then 20e-
3 / 8.2Ω = 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.
+
V
-
Figure 4 - Vsense measurement
5.1.2
LPC54102 external sense resistor for VDD current measurement
It is not required to use an external sense resistor for LPC54102 current measurement.
To measure LPC54102 VDD current of less than 4 uA an external resistor is required.
The external resistor should be
installed between the JP3 terminals, and the 0Ω resistor
at JS5 must be removed. The external resistor will be in series with the on-board
8.2Ω
(R18), so the external resistor value is added to 8.2Ω to get the combined Vsense
resistance. The voltage measurement is made across the total series resistance from
P2-1 to JP3-2, as shown in Figure 5
To calculate the LPC54102 current divide the
voltage read by the combined sense resistance. To restore the JS5 short around JP3,
use a solder bridge across JS5.
V
+
-
remove
Ext resistor
Figure 5 - External vSense resistor
5.1.3
LPC54102 VDD current measurement using a current meter
A current meter may be inserted at JP4 to measure the LPC54102 VDD input current.
The 0Ω resistor at JS6 must be removed and the current meter connected at the positive
input at JP4 pin 1 (square pad) and negative input at JP2 pin 2, as shown in Figure 6