NXP Semiconductors MPC5746R - Page 9

Power Supply
MPC5746R Hardware Design Guide, Rev. 1
NXP Semiconductors 9
There are several options available: 1) use a heat sink 2) populate a copper layer(s) beneath the transistor with
thermal VIAS to improve the heat dissipation. NXP recommends consulting the transistor vendor to obtain
optimal thermal design.
As another option, an external resistor can be used to minimize the extra heat. The resistor will drop the
collector voltage to mitigate the heat dissipation of the pass transistor (BJT). In this case, the collector voltage
needs to be maintained at 3 V or above.
NXP recommends a low inductance between the transistor base and the VRC_CTRL pin/pad. This should be
less than 100nH. An inductance of 15nH is an optimal value.
The following examples explain how to calculate the power dissipation of the BJT with different resistor (R)
values.
Example.1
:
Supply voltage is 5.0 V, R=1.8
Ω, Max current consumption at VDD_LV (I
vdd_lv
) = 700 [mA] and
nominal VDD_LV = 1.25 [V]
Voltage drop across the R = 1.8
Ω x 700mA = 1.260 [v], voltage at collector 5 V-1.26 V = 3.74 [V] (>
3 V)
Power dissipation on BJT transistor P (Tr): P (Tr) = (5 - 1.25 - 1.26) x 0.7 = 1.743 [w]
Power dissipation on resistor P(R): P(R)= 1.26 x 0.7 = 0.882 [w]
Example.2
:
Supply voltage is 5.0 V, R=2.0
Ω, Max current consumption at VDD_LV (I
vdd_lv
) = 700 [mA] and
nominal VDD_LV = 1.25 [V]
Voltage drop across the R = 2.0
Ω x 700mA = 1.400 [V], voltage at collector 5 V-1.4 V = 3.6 [V] (> 3
V)
Power dissipation on BJT transistor P (Tr) given by: P (Tr) = (5 - 1.25 - 1.40) x 0.7 = 1.645 [w]
Power dissipation on resistor P(R) given by: P(R)= 1.40 x 0.7 = 0.980 [w]
3.2.1 Recommended Pass Transistor
The on-chip 1.25 V regulator requires an external BJT for operation. Refer to the following table for
recommended pass transistors. Please refer to the MPC5746R Data Sheet to obtain the requirements for
the BJT device.
Table 7. Recommended pass Transistors
Transistor Collector capacitor (C
C
) Base capacitor Resistor Supply options
ON semiconductor
NJD2873T4
4.7 uF (min) ESR<200 mΩ
-
2 Ω 5.0 V
ON semiconductor
NJD2873T4
4.7 uF (min) ESR<200 mΩ - - 3.3 V
ROHM 2SCR574D3 A07
4.7 uF (min) ESR<200 mΩ 1 uF - 3.3 V