NXP Semiconductors
UM11227
NTM88 family of tire pressure monitor sensors
UM11227 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2020. All rights reserved.
User manual Rev. 6 — 24 April 2020
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10.16.9 Pre-charge function
Pre-charge function can be enabled before a transmission to reduce the slew rate of
current consumption during power up sequence, avoiding current surges which might
otherwise result in LVD activation or reduce battery life. If the Pre-Charge function
is enabled by setting AREGPC = 1, then when the analog regulator is activated, the
pre-charge function starts after the digital regulator voltage is established. As long as
AREGOK bit is 0, the pre-charge function is ON and the radio transmitter is OFF. When
the voltage across the external capacitance reaches the threshold voltage 1.5 V, then
AREGOK bit is set to 1, the pre-charge circuitry is automatically disabled and the power
up sequence continues as usual.
aaa-031082
SEND = 1 or CKREF = 1
Enable Comparator
AREGPC = 1
?
Is comparator
output high
?
Continue power
up sequence
no
yes
yes
Enable Current source
Disable current source
Disable comparator
no
Figure 38. Pre-charge diagram
10.16.10 VCO calibration machine
The RFM incorporates a VCO calibration machine which works in conjunction with the
VCO. The calibration machine selects the optimal VCO sub-band with respect to a
predefined reference voltage applied to the VCO.
• Calibration supports max band VCO sub-bands. Max band corresponds to the band
where the VCO frequency is maximum.
• A successive approximation algorithm is used to calculate the optimum sub-band.
• F
c
, the Center Frequency (AFREQ+BFREQ)/2 is used as the reference frequency for
the VCO calibration in FSK mode (MOD = 1).
• BFREQ is used as the reference frequency for the VCO calibration in OOK mode
(MOD = 0).
• Calibration occurs every time the VCO is enabled.
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