LTC3589 Linear Technology, LTC3589 Datasheet - Page 39

• Manufacturer Part Number: LTC3589
• Description: 8-Output Regulator
• Manufacturer: Linear Technology

OPERATION
be measured or looked up in an effi ciency table for the
programmed output voltage.
The power dissipated by an LDO regulator is estimated
by:
Where V
is the LDO supply voltage, and I
current. If one of the switching regulator outputs is used
as an LDO supply voltage, remember to include the LDO
supply current in the switching regulator load current for
calculating power loss.
With θ
temperature of 85°C, the power dissipation must be kept
under 1.18W so that maximum junction temperature is
less than 125°C.
An example using the equations above with the parameters
in Table 18 shows an application that is at the maximum
junction temperature of 125°C at an ambient temperature
of 85°C. LDO2, LDO3, and LDO4 are powered by step-
down switching regulator 2 and the buck-boost switching
regulator. The total load on those two switching regulators
is the sum of the application load and the LDO load. This
example is with the LDO regulators at one half rated cur-
rent and the switching regulators at three quarters rated
current.
Table 18. T
OUTPUT
LDO1_VSTB 3.8V
LDO2
LDO3
LDO4
V
V
V
V
OUT1
OUT2
OUT3
OUT4
INTERNAL JUNCTION TEMPERATURE AT 85°C AMBIENT
P
D(LDOX)
JA
LDOX
J
of 34°C/W and maximum ambient operating
Calculation Example
3.3V
3.8V
3.8V
3.8V
3.8V
1.8V
3.3V
V
= (V
IN
is the programmed output voltage, V
IN(LDOX)
1.25V
V
1.2V
1.2V
1.8V
2.5V
1.2V
1.8V
3.3V
OUT
APP LOAD
100mA
100mA
100mA
0.65A
10mA
0.75A
0.70A
1.2A
– V
LDOX
100mA
100mA
100mA
TOTAL
LDOX
LOAD
10mA
0.75A
0.75A
0.90A
1.2A
)• I
TOTAL POWER
LDOX
is the output load
80%
90%
90%
85%
EFF
1180mW
IN(LDOX)
POWER
290mW
300mW
150mW
140mW
140mW
30mW
60mW
80mW
125°C
DISS
Printed Circuit Board Layout
When laying out the printed circuit board, the following
checklist should be followed to ensure proper operation
of the LTC3589:
1. Connect the exposed pad of the package (Pin 41)
2. The switching regulator input supply traces and their
3. Minimize the switching power traces connecting SW1,
4. Minimize the length of the connection between the
5. Minimize the length of the connection between the
directly to a large ground plane to minimize thermal
and electrical impedance.
decoupling capacitors should be as short as possible.
Connect the GND side of the capacitors directly to the
ground plane of the board. The decoupling capacitors
provide the AC current to the internal power MOSFETs
and their drivers. It is important to minimize inductance
from the capacitors to the LTC3589 pins.
SW2, SW3, and buck-boost switch pins SW4AB and
SW4CD to the inductors to reduce radiated EMI and
parasitic coupling. Keep sensitive nodes such as the
feedback pins away from or shielded from the large
voltage swings on the switching nodes.
step-down switching regulator inductors and the out-
put capacitors. Connect the GND side of the output
capacitors directly to the thermal ground plane of the
board.
buck-boost regulator output (BB_OUT) and the output
capacitor. Connect the GND side of the output capacitor
directly to the thermal ground plane of the board.
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LTC3589
39
3589p