MIC61150 Datasheet, PDF (13/22 Pages) Micrel Semiconductor – Low Input Voltage, Single-Supply High-Current LDO

Micrel, Inc.
Adjustable Regulator Design
The MIC61150 adjustable version allows programming
the output voltage from 0.5V to 3.0V by placing a resistor
divider network (R1, R2) from V_OUT to GND (see
Application Circuit). The high side of R1 should be
connected at the point-of-load for high-accuracy Kelvin
sensing. V_OUT is determined by the following equation:
V_OUT
=
0.5 × ⎜⎛ ^R1
⎝ ^R2
+ 1⎟⎞
⎠
Eq. 1
where V_OUT is the desired output voltage.
The resistor (R2) value between the FB pin and GND is
selected to maintain a minimum 10mA load on the
output.
The resistor values are calculated from the previous
equation, resulting in the following:
R1
=
R2
×
⎜⎜⎝⎛
V_OUT
0.5
− 1_{⎟⎟⎠⎞}
Eq. 2
Table 1 is a list of resistor combinations to set the output
voltage. A 1% tolerance is recommended for both R1
and R2. For a unity gain, 0.5V output voltage, connect
the FB pin directly to the output.
V_OUT
0.5V
0.6V
0.7V
0.8V
0.9V
1.0
1.1V
1.2V
1.5V
1.8V
2.2V
R1
−
10.0Ω
20.0Ω
30.1Ω
40.2Ω
49.9Ω
60.4Ω
69.8Ω
100Ω
130Ω
169Ω
R2
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
49.9Ω
Table 1. Resistor Selection for Specific V_OUT
MIC61150
Thermal Design
Linear regulators are simple to use. The most
complicated design parameters to consider are thermal
characteristics. To help reduce the thermal resistance,
the ePad (underneath the IC) should be soldered to the
PCB ground and the placement of thermal vias either
underneath or near the ePad is highly recommended.
Thermal design requires the following application-
specific parameters:
• Maximum ambient temperature (T_A)
• Output current (I_OUT)
• Output voltage (V_OUT)
• Input voltage (V_IN)
• Ground current (I_GND)
First, calculate the power dissipation of the regulator
from these numbers and the device parameters from this
datasheet:
P_D = (V_IN - V_OUT) × I_OUT + (V_IN × I_GND)
Eq. 3
where the ground current is approximated by using
numbers from the Electrical Characteristics or Typical
Characteristics sections
For example, given an expected maximum ambient
temperature (T_A) of 75°C with V_IN = 1.2V, V_OUT = 0.9V,
and I_OUT = 1.5A, first calculate the expected P_D using
Equation 1:
P_D = (1.2V – 0.9V) × 1.5A + 1.2V × 0.015A
= 0.468W
Eq. 4
Next, determnine the junction temperature for the
expected power dissipation above using the thermal
resistance (θ_JA) of the 10-pin 3mm × 3mm MLF^® (YML)
adhering to the following criteria for the PCB design:
1oz. copper and 100mm² copper area for the
MIC61150.
T_J = (θ_JA × P_D) + T_A
= (60.7°C/W × 0.468W) + 75°C
= 103.4°C
Eq. 5
November 2010
13
M9999-112210-A