TFE-4/RV-1 Company Confidential
TF5 Technical Information Nokia Customer Care
Page 8a-28 Copyright © 2005 Nokia Corporation. Issue 2.0 Mar/2005
Company Confidential
CRM Levelshifter
This level shifter shall transform the 1.8V signal coming from the phone engine to 13V
Vcar level. The CRMX output is drawn within car environment to VCAR by an external
resistor of about 200R. This circuit provides overload protection for the case that this
resistor value is too low.
Functional Description:
B2BCRM = low
When B2BCRM signal is low, V102, Pin 5 is low. Now VEBPWR reaches over R116 to
V123, pin 1 and V123 will be open. That means that CRMx will be held to Vcar high by
the external resistor.
B2BCRM = high
When B2BCRM signal is high (1.8V), V102, pin 6 will be drawn by R116 to VEBPWR,
because pin 2 is set to ground. Now Base of V123 is connected to VEBPWR via R116, so it
will be conducting. CRMX will be drawn to ground.
Short circuit protection:
V123 includes self-protection, which will activate at about 1.2A. The transistor switches
off when over current exists. Depending on type of transistor, transistor will activate
after some time again, but current will be limited to value of below 1.5A.
Figure 28: CRM Realization
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