LTC3643
19
3643fb
For more information www.linear.com/LTC3643
The QFN package junction-to-ambient thermal resistance,
θ
JA
, is around 46°C/W. Therefore, the junction temperature
of the regulator operating in a 25°C ambient temperature
is approximately:
T
J
= 0.587W • 46°C/W + 25°C = 52°C
Remembering that the above junction temperature is
obtained from an R
DS(ON)
at 25°C, we might recalculate
the junction temperature based on a higher R
DS(ON)
since
it increases with temperature. Redoing the calculation
assuming that R
SW
increased 10% at 52°C yields a new
junction temperature of 54.4°C. If the application calls
for a higher ambient temperature and/or higher
switching
frequency, care should be taken to reduce the temperature
rise of the part by using a heat sink or air flow.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3643. Check the following in your layout:
1. Do the capacitors C
CAP
connect to the CAP and GND
terminals as close as possible?
2. Are C
IN
and L closely connected? The (–) plate of C
IN
returns current to GND and the (–) plate of C
CAP
.
3. Solder the Exposed Pad (Pin 25) on the bottom of the
package to the GND plane. Connect this GND plane to
other layers with thermal vias to help dissipate heat
from the LTC3643.
4. Keep sensitive components away from the SW pin. The
compensation components C
ITH
and R
ITH
, all resistor
dividers, and the INTV
CC
bypass caps should be routed
away from the SW trace and the inductor.
5. A ground plane is required.
6. Flood all unused areas on all layers with copper, which
reduces the temperature rise of power components.
These copper areas should be connected to GND.
APPLICATIONS INFORMATION
Holdup Time Calculation
The amount of energy available in the energy reservoir
capacitor before the voltage droops below the desired
backup voltage is equal to:
E
CAP
=
C
CAP
V
CAP
2
– V
IN
2
The amount of energy necessary to complete the backup
is equal to:
E
LOAD
= I
SYS
• V
IN
• t
HT
Where t
HT
is the amount of backup time needed.
Assuming an efficiency of η (this number will vary de-
pending on the application), the total amount of backup
time will be equal to:
t
HT
=
E
CAP
• η
I
SYS
• V
IN
=
C
CAP
V
CAP
2
– V
IN
2
• η
2•I
SYS
• V
IN
Where I
SYS
is the system load during backup.
Design Example
As a design example, consider the LTC3643 in an appli-
cation with the following specifications:
V
IN
= 4V to 8V
V
CAP
= 40V
I
CAP
(Input) = 2A
Given the internally programmed switching frequency of
1MHz, we can calculate the inductor value for about 40%
ripple current (800mA based on an average of 2A output
current) at maximum V
IN
:
L =
8V
1MHz •0.8A
⎛
⎝
⎜
⎞
⎠
⎟
1−
8V
40V
⎛
⎝
⎜
⎞
⎠
⎟
= 8µH
Given this, an 8.2µH inductor would suffice.
C
CAP
and C
IN
will be selected based on what is required to
satisfy the output voltage ripple requirement for the boost
and buck modes respectively. A 22µF or 47µF capacitor
on both nodes is adequate for most applications.
