CM3212 Datasheet, PDF (9/11 Pages) ON Semiconductor – VTT Termination Voltage Regulator

CM3212
APPLICATION INFORMATION (Cont’d)
Adjusting VDDQ Output Voltage
The CM3212 internal bandgap reference is set at 1.25 V. The V_DDQ voltage is adjustable by using a resistor divider, R1 and
R2:
V_DDQ + V_ADJ
R1 ) R2
R2
where V_ADJ = 1.25 V. The recommended divider value is R₁ = R₂ = 10 kW for DDR−1 application, and R1 = 4.42 kW,
R2 = 10 kW for DDR−2 application (V_DDQ = 1.8 V, V_TT = 0.9 V).
Shutdown
ADJSD also serves as a shutdown pin. When this is pulled high (SHDN_H), both the VDDQ and the VTT outputs tri−state
and could sink/source less than 10 mA. During shutdown, the quiescent current is reduced to less than 0.5 mA, independent
of output load.
It is recommended that a low leakage Schottky diode be placed between the ADJSD Pin and an external shutdown signal
to prevent interference with the ADJ pin’s normal operation. When the diode anode is pulled low, or left open, the CM3212
is again enabled.
For Shutdown operation, observe the following:
V_DDQ
Under ADJSD Shutdown Condition, V_DDQ Should Go to Tri−State.
Under EN_VTT Shutdown Condition, V_DDQ Should Keep State (2.5 V).
V_TT
Under ADJSD or EN_VTT Shutdown Condition, V_TT Should Go to Tri−State and Should Sink or Source less than 10 mA.
V_REF
Under ADJSD Shutdown Condition, V_REF Should Go to Zero.
Under EN_VTT Shutdown Condition, V_REF Should Keep State (1.2 V or V_DDQ/2).
Current Limit and Over−temperature Protection
The CM3212 features internal current limiting with thermal protection. During normal operation, V_DDQ limits the output
current to approximately 2 A and V_TT limits the output current to approximately ±2 A. When V_TT is current limiting into a hard
short circuit, the output current folds back to a lower level (~1 A) until the over−current condition ends. While current limiting
is designed to prevent gross device failure, care should be taken not to exceed the power dissipation ratings of the package.
If the junction temperature of the device exceeds 170°C (typical), the thermal protection circuitry triggers and tri−states both
VDDQ and VTT outputs. Once the junction temperature has cooled to below about 120°C the CM3212 returns to normal
operation.
Typical Thermal Characteristics
The overall junction to ambient thermal resistance (q_JA) for device power dissipation (P_D) primarily consists of two paths
in the series. The first path is the junction to the case (q_JC) which is defined by the package style and the second path is case
to ambient (q_CA) thermal resistance which is dependent on board layout. The final operating junction temperature for any
condition can be estimated by the following thermal equation:
T_JUNC + T_AMB ) P_D
          + T_AMB ) P_D
(q_JC) ) P_D
(q_CA)
(q_CA)
When a CM3212 using WDFN8 package is mounted on a double−sided printed circuit board with four square inches of
copper allocated for “heat spreading,” the q_JA is approximately 55°C/W. Based on the over temperature limit of 170°C with
an ambient temperature of 85°C, the available power of the package will be:
P_D
+
170° C
55°
* 85°
CńW
C
+
1.5W
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