RT9166/A
For recommended operating conditions specification of
RT9166/A, where T
J(MAX)
is the maximum junction
temperature of the die (125°C) and T
A
is the operated
ambient temperature. The junction to ambient thermal
resistance
θ
JA
is layout
dependent. For SOT-23-3
packages, the thermal resistance
θ
JA
is 250°C/W on the
standard JEDEC 51-3 single-layer thermal test board. The
maximum power dissipation at T
A
= 25°C can be calculated
by following formula :
P
D (MAX)
= ( 125°C
−
25°C) / 250°C/W = 0.400W for
SOT-23-3 packages
P
D (MAX)
= ( 125°C
−
25°C) / 175°C/W = 0.571W for
SOT-89 packages
P
D (MAX)
= ( 125°C
−
25°C) / 135°C/W = 0.740W for
SOT-223 packages
P
D
(MAX)
PCB Layout
Good board layout practices must be used or instability
can be induced because of ground loops and voltage drops.
The input and output capacitors
MUST
be directly
connected to the input, output, and ground pins of the
device using traces which have no other currents flowing
through them.
The best way to do this is to layout C
IN
and C
OUT
near the
device with short traces to the V
IN
, V
OUT
, and ground pins.
The regulator ground pin should be connected to the
external circuit ground so that the regulator and its
capacitors have a
“single
point ground”.
It should be noted that stability problems have been seen
in applications where
“vias”
to an internal ground plane
were used at the ground points of the device and the input
and output capacitors. This was caused by varying ground
potentials at these nodes resulting from current flowing
through the ground plane. Using a single point ground
technique for the regulator and it’ s capacitors fixed the
problem. Since high current flows through the traces going
into V
IN
and coming from V
OUT
, Kelvin connect the capacitor
leads to these pins so there is no voltage drop in series
with the input and output capacitors.
Optimum performance can only be achieved when the
device is mounted on a PC board according to the diagram
below :
= ( 125°C
−
25°C) / 68°C/W = 1.470W for
TO-252 packages
The maximum power dissipation depends on operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance
θ
JA
. For RT9166/A packages, Figure 1 of
derating curves allows the designer to see the effect of
rising ambient temperature on the maximum power allowed.
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
0
Maximum Power Dissipation (mW)
TO-252
Single Layer PCB
SOT-223
SOT-89
SOT-23-3
V
IN
GND
25
50
75
100
125
Ambient Temperature (°C)
Figure 1. Derating Curves for RT9166/A Packages
V
OUT
Figure 2. SOT-23-3 Board Layout
www.richtek.com
10
DS9166/A-19 April 2008
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
