SNOSB31J July 2009 – December 2014 LMX2541
PRODUCTION DATA.
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The charge pump is tested in external VCO mode (MODE=1), although it is no external VCO hooked up. The CPout pin should be disconnected from the any external VCO tuning pin, external loop filter, and also the Vtune pin on the device. A signal is then applied to the OSCin pin to ensure that the R counter is oscillating. This signal does not have to be clean and the frequency is very critical. These currents at the CPout pin are typically measured with a semiconductor parameter analyzer.
In order to test the TRI-STATE current, the CPT bit is set to one and the current is measured. Aside from having no other sources of leakage attached to this pin, it is also important that the board be well cleaned before doing this test. The temperature and voltage at the charge pump can then be varied and the resulting leakage current is then recorded. Typically, the leakage currents are worst at higher temperatures and higher charge pump voltages.
In order to test the source and sink currents, the CPT bit is set to active mode and the frequency is programmed to something much higher than can be achieved in order to force the charge pump to rail. The reason why this is necessary is that the duty cycle of the charge pump is not 100% unless it is forced against one of the rails. If the charge pump polarity bit (CPP) is set to positive, then the charge pump source current is measured. To measure the sink current, the CPT bit is set to negative. The part is then programmed and the charge pump will rail in one direction. The semiconductor parameter analyzer measures the current at a particular charge pump voltage. The phase detector polarity bit, CPP, can be toggled to test between the negative and positive charge pump gains. In order to test leakage, set the TRI-STATE bit, CPT, to 1 so that this can be measured. For the most accurate measurements, it is desirable that the CPout and Vtune pin are not shorted together for these measurements. Once these currents are measured, then the datasheet parameters can be calculated.
A summary of these charge pump tests is given in the table below.
MEASUREMENT | PLL_R | PLL_N | CPG | CPT | CPP |
---|---|---|---|---|---|
Leakage Current | X | X | X | 1 (TRI-STATE) | X |
Source Current | 1 | 4000 | 0 - 31 | 0 (Active) | 1 (Positive) |
Sink Current | 1 | 4000 | 0 - 31 | 0 (Active) | 0 (Negative) |
I1 = Charge Pump Sink Current at VCPout = Vcc - ΔV
I2 = Charge Pump Sink Current at VCPout = Vcc/2
I3 = Charge Pump Sink Current at VCPout = ΔV
I4 = Charge Pump Source Current at VCPout = Vcc - ΔV
I5 = Charge Pump Source Current at VCPout = Vcc/2
I6 = Charge Pump Source Current at VCPout = ΔV
ΔV = Voltage offset from the positive and negative supply rails. Defined to be 0.4 volts for this part.