JN5139/001,515 NXP Semiconductors, JN5139/001,515 Datasheet - Page 76

• Manufacturer Part Number: JN5139/001,515
• Description: IC MCU 32BIT 56QFN
• Manufacturer: NXP Semiconductors

Specifications of JN5139/001,515

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

This can be used to give an equation for the required transconductance
Example: Using typical parameters of
equation above gives the required transconductance (
transconductance of 1.25mA/V
The example and equation illustrate the trade-off that exists between the load capacitance and crystal ESR. For
example, a crystal with a higher load capacitance can be used, but the value of max. ESR that can be tolerated is
reduced. Also note, that the circuit sensitivity to external capacitance [ C
Meeting the criteria for start-up is only one aspect of the way these parameters affect performance, they also affect
the time taken during start-up to reach a given, (or full), amplitude. Unfortunately, there is no simple mathematical
model for this, but the trend is the same. Therefore, both a larger load capacitance and larger crystal ESR will give a
longer start-up time, which has the disadvantages of reduced battery life and increased latency.
For this reason, we strongly recommend that for a 9pF load capacitance crystals be specified with a maximum ESR
of 40 ohms. For lower load capacitances the recommended maximum ESR rises, for example, CL=7pF the max ESR
is 61 ohms. For the lower cost crystals in the large HC49 package, a load capacitance of 10pF is widely available and
the max ESR of 30 ohms specified by many manufacturers is acceptable. Also available in this package style, are
crystals with a load capacitance of 12pF, but in this case the max ESR required is 25 ohms or better.
Below is measurement data showing the variation of the crystal oscillator amplifier transconductance with
temperature and supply voltage, notice how small the variation is. Circuit techniques have been used to apply
compensation, such that the user need only design for nominal conditions.
76
1.285
1.275
1.265
1.255
1.245
1.28
1.27
1.26
1.25
g
-40
Crystal Oscillator Transconductance Versus Temperature
m
≥
4
R
-20
R
m
=40Ω,
m
×
ω
2
JN-DS-JN5139 1v9
0
C
[
C
S
=1pF and
S
(VDD=3V)
g
Temperature (C)
(
m
C
C
20
) as 647uA/V. The JN5139 has a typical value for
T
T
1
1
×
+
C
C
C
T
T
T
40
1
2
2
=
)
C
+
.
T
1
C
2
, C
=18pF ( for a load capacitance of 9pF), the
T
60
2
1
×
] is a square law.
C
T
2
80
]
2
© NXP Laboratories UK 2010
100