LTC3607
13
3607fb
For more information www.linear.com/LTC3607
As an example, consider the case when the LTC3607 is in
dropout on both channels at an input voltage of 5V with
a load current of 600mA and an ambient temperature
of 25°C. From the Typical Performance Characteristics
graph of Switch Resistance, the R
DS(ON)
resistance of
the main switch is 0.9Ω. Therefore, power dissipated by
each channel is:
P
D
= I
2
• R
DS(ON)
= 324mW
Running the two regulator channels under the same con-
ditions will result in a total power dissipation of 0.648W.
T
he MS
E package junction-to-ambient thermal resistance,
θ
JA
, is 37°C/W. Therefore, the junction temperature of
the regulator operating in a 25°C ambient temperature is
approximately:
T
J
= 0.648W • 37°C/W + 25°C = 49°C
Design Example
As a design example, consider using the LTC3607 in a
portable application with a dual lithium-ion battery. The
battery provides a V
IN
= 5.6V to 8.4V. The loads require a
maximum of 600mA in active mode and 2mA in standby
mode. The output voltages are V
OUT1
= 3.3V and V
OUT2
= 2.5V. Since the load still needs power in standby, Burst
Mode operation is selected for good light load efficiency.
First, calculate the inductor values for about 240mA ripple
current at maximum V
IN
:
L1=
3.3V
2.25MHz•240mA
• 1–
3.3V
8.4V
=3.7µH
Choosing the closest standardized inductor value of 3.3μH
results in a maximum ripple current of:
∆I
L1
=
3.3V
2.25MHz•3.3µH
• 1–
3.3V
8.4V
=270mA
The same calculations for L2 result in a standard inductor
value of 3.3µH and a maximum current ripple of 236mA.
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
OUT1
≈5•
2.25MHz•(5%•3.3V)
=8.1µF
C
OUT2
≈5•
2.25MHz•(5%•2.5V)
=10.7µF
For both outputs, a close standard value is 10µF. Since
the output impedance of a lithium-ion battery is very low,
each C
IN
is chosen to be 10µF also.
The output voltages can now be programmed by choosing
the values of R1 thru R4. To maintain high efficiency, the
current in these resistors should be kept small. Choosing
5µA with the 0.6V feedback voltage makes R2 and R4 ~
120k. Close standard 1% resistor values is 121k and then
R1 and R3 are 549k and 383k, respectively.
The PGOOD pins are common drain outputs, thus requir
-
ing pull-up resistors. Two 100k resistors are used for
ad
equa
te speed.
Figure 1 shows the complete schematic for this design
example. The specific passive components chosen allow
for a 1mm height power supply that maintains a high ef
-
ficiency across load.
Board Layout Considerations
W
hen l
aying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3607. These items are also illustrated graphically
in the layout diagram of Figure 2. Check the following in
your layout:
1. Do the input capacitors C
IN
connect to PV
IN1
, PV
IN2
,
PGND1, and PGND2 as closely as possible? These ca-
pacitors provides the AC current to the internal power
MOS
FET
s and their drivers.
2. Are C
OUT
and L closely connected? The (–) plate of
C
OUT
returns current to GND and the (–) plate of C
IN
.
applicaTions inForMaTion
