MIC94095YMT TR Micrel Inc, MIC94095YMT TR Datasheet - Page 13

• Manufacturer Part Number: MIC94095YMT TR
• Description: IC LOAD SW HGH SIDE 1.2A 4-TMLF
• Manufacturer: Micrel Inc
• Type: High Side Switchr

Specifications of MIC94095YMT TR

• Number Of Outputs: 1
• Rds (on): 225 mOhm
• Internal Switch(s): Yes
• Current Limit: 1.2A
• Voltage - Input: 1.7 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 4-TMLF®, QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 576-3495-2

Micrel, Inc.
Application Information
Power Dissipation Considerations
As with all power switches, the ultimate current rating of
the switch is limited by the thermal properties of the
package and the PCB it is mounted on. There is a
simple, ohms law type relationship between thermal
resistance, power dissipation and temperature which are
analogous to an electrical circuit:
From this simple circuit we can calculate Vx if we know
Isource, Vz and the resistor values, Rxy and Ryz using
the equation:
Thermal circuits can be considered using these same
rules and can be drawn similarly replacing current
sources with Power dissipation (in Watts), Resistance
with Thermal Resistance (in
with temperature (in
Now replacing the variables in the equation for Vx, we
can find the junction temperature (Tj) from power
dissipation, ambient temperature and the known thermal
resistance of the PCB (Rθ
It is this equation that is used to determine the graphs on
page 7. P
found in the operating ratings section of the datasheet
and Rθ
various PCB copper areas can be taken from ‘Designing
with Low Dropout Voltage Regulators’
the Micrel website (LDO Application Hints).
Example:
March 2009
T
Vx
J
=
=
P
Isource
DISS
CA
DISS
⋅
Figure 2. Thermal Equivalent Circuit
(
R
(the PCB thermal resistance) values for
⋅
θ
(
is calculated as (I
Rxy
JC
Figure 1. Electrical Circuit
+
+
R
Ryz
o
θ
C).
CA
)
)
+
+
CA
Vz
T
) and the package (Rθ
AMB
o
C/W) and Voltage sources
SWITCH
2
x R
(1)
available from
SWmax
). Rθ
(1)
JA
).
JC
is
13
A switch is intended to drive a 500mA load and is placed
on a printed circuit board which has a ground plane area
of at least 25mm square. The Voltage source is a Li-ion
battery with a lower operating threshold of 3V and the
ambient temperature of the assembly can be up to 50
Summary of variables:
I
VIN = 3V to 4.2V
Tamb = 50
Rθ
Rθ
P
The worst case switch resistance (R
VIN of 3V is not available in the datasheet, so the next
lower value of VIN is used.
R
If this were a figure for worst case R
additional consideration is to allow for the maximum
junction temperature of 125
resistance in this case will be 30% higher (See R
variance vs. temperature graph).
R
Therefore junction temperature (T
T
T
This is well below the maximum 125
SW
J
J
DISS
SWmax
SWmax
= 0.5
= 62
JC
CA
= 0.5A
= 60
= 53
= I
@ 2.5v (@ 125’C) = 315 x 1.3 = 410mΩ
o
@ 2.5v = 315mΩ
2
SW
C
Figure 3. Excerpt from the LDO Book
x 0.41 x (60+53) + 50
o
o
C/W from Datasheet (P. 3)
2
C/W Read from Graph in Fig. 3
o
C
x R
SWmax
o
C, the actual worst case
J
):
o
MIC94090/1/2/3/4/5
C.
SWmax
SWmax
M9999-033109-A
from (Eqn. 1)
) at the lowest
for 25
(1)
o
C, an
DS(on)
o
C.