Datasheet

LTC3548A
14
3548afa
APPLICATIONS INFORMATION
The junction temperature, T
J
, is given by:
T
J
= T
RISE
+ T
AMBIENT
As an example, consider the case when the LTC3548A is
in dropout on both channels at an input voltage of 2.7V
with a load current of 400mA and 800mA and an ambi-
ent temperature of 70°C. From the Typical Performance
Characteristics graph of Switch Resistance, the R
DS(ON)
resistance of the main switch is 0.425Ω. Therefore, power
dissipated by each channel is:
P
D
= I
2
• R
DS(ON)
= 272mW and 68mW
The MS package junction-to-ambient thermal resistance,
θ
JA
, is 45°C/W. Therefore, the junction temperature of
the regulator operating in a 70°C ambient temperature is
approximately:
T
J
= (0.272 + 0.068) • 45 + 70 = 85.3°C
which is below the absolute maximum junction tempera-
ture of 125°C.
Design Example
As a design example, consider using the LTC3548A in a
portable application with a Li-Ion battery. The battery pro-
vides a V
IN
= 2.8V to 4.2V. The load requires a maximum
of 800mA in active mode and 2mA in standby mode. The
output voltage is V
OUT
= 2.5V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the inductor value for about 30% ripple
current at maximum V
IN
:
L ≥
2.5V
2.25MHz • 360mA
•1–
2.5V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
=1.25µH
Choosing the next highest standardized inductor value of
2.2μH, results in a maximum ripple current of:
ΔI
L
=
2.5V
2.25MHz • 2.2µH
•1−
2.5V
4.2V
⎛
⎝
⎜
⎞
⎠
⎟
= 204mA
For cost reasons, a ceramic capacitor will be used. C
OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C
OUT
≈2.5
800mA
2.25MHz •(5% • 2.5V)
= 7.1µF
The closest standard value is 10μF. Since the output imped-
ance of a Li-Ion battery is very low, C
IN
is typically 10μF.
The output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
current in these resistors should be kept small. Choosing
2μA with the 0.6V feedback voltage makes R1~300k. A close
standard 1% resistor is 280k, and R2 is then 887k.
The POR pin is a common drain output and requires a pull-
up resistor. A 100k resistor is used for adequate speed.
Figure 3 shows the complete schematic for this design
example. The specific passive components chosen allow
for a 1mm height power supply that maintains a high
efficiency across load.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3548A. These items are also illustrated graphically
in the layout diagram of Figure 2. Check the following in
your layout:
1. Does the capacitor C
IN
connect to the power V
IN
(Pin 3)
and GND (Exposed Pad) as closely as possible? This
capacitor provides the AC current to the internal power
MOSFETs and their drivers.
2. Are C
OUT
and L1 closely connected? The (–) plate of
C
OUT
returns current to GND and the (–) plate of C
IN
.
3. The resistor divider formed by R1 and R2 must be
connected between the (+) plate of C
OUT
and a ground
sense line terminated near GND (Exposed Pad). The
feedback signals V
FB1
and V
FB2
should be routed away
from noisy components and traces, such as the SW lines
(Pins 4 and 7), and their traces should be minimized.