MCP1701 Datasheet, PDF (11/20 Pages) Microchip Technology – 2uA Low Dropoout Positive Voltage Refulator

5.0 THERMAL CONSIDERATIONS
5.1 Power Dissipation
The amount of power dissipated internal to the LDO
linear regulator is the sum of the power dissipation
within the linear pass device (P-channel MOSFET) and
the quiescent current required to bias the internal refer-
ence and error amplifier. The internal linear pass
device power dissipation is calculated as shown in
Equation 5-1.
EQUATION 5-1:
P_D (Pass Device) = (V_IN – V_OUT) x I_OUT
The internal power dissipation, which is due to the bias
current for the LDO internal reference and error ampli-
fier, is calculated as shown in Equation 5-2.
EQUATION 5-2:
P_D (Bias) = V_IN x I_GND
The total internal power dissipation is the sum of P_D
(Pass Device) and P_D (Bias).
EQUATION 5-3:
P_TOTAL = P_D (Pass Device) + P_D (Bias)
For the MCP1701, the internal quiescent bias current is
so low (2 µA, typical) that the P_D (Bias) term of the
power dissipation equation can be ignored. The
maximum power dissipation can be estimated by using
the maximum input voltage and the minimum output
voltage to obtain a maximum voltage differential
between input and output. The next step would be to
multiply the maximum voltage differential by the
maximum output current.
EQUATION 5-4:
P_D = (V_INMAX – V_OUTMIN) x I_OUTMAX
Given:
V_IN = 3.3V to 4.1V
V_OUT = 3.0V ± 2%
I_OUT = 1 mA to 100 mA
T_AMAX = 55°C
P_MAX = (4.1V – (3.0V x 0.98)) x 100 mA
P_MAX = 116.0 milliwatts
MCP1701
To determine the junction temperature of the device, the
thermal resistance from junction-to-ambient must be
known. The 3-pin SOT-23 thermal resistance from
junction-to-air (R_θJA) is estimated to be approximately
335°C/W. The SOT-89 R_θJA is estimated to be
approximately 52°C/W when mounted on 1 square inch
of copper. For the TO-92, R_θJA is estimated to be
131.9°C/W. The R_θJA will vary with physical layout,
airflow and other application-specific conditions.
The device junction temperature is determined by
calculating the junction temperature rise above
ambient, then adding the rise to the ambient
temperature.
EQUATION 5-5:
JUNCTION
TEMPERATURE - SOT-23
EXAMPLE:
T_J = P_DMAX × R_θJA + T_A
T_J = 116.0 milliwatts × 335°C/W + 55°C
T_J = 93.9°C
EQUATION 5-6:
JUNCTION
TEMPERATURE - SOT-89
EXAMPLE:
T_J = 116.0 milliwatts × 52°C/W + 55°C
T_J = 61°C
EQUATION 5-7:
JUNCTION
TEMPERATURE - TO-92
EXAMPLE:
T_J = 116.0 milliwatts × 131.9°C/W + 55°C
T_J = 70.3°C
 2004 Microchip Technology Inc.
DS21874A-page 11