LTC3605_15 Datasheet, PDF (15/22 Pages) Linear Technology – 15V, 5A Synchronous Step-Down Regulator

LTC3605
Operation
Junction Temperature Measurement
The junction-to-ambient thermal resistance will vary de-
pending on the size and amount of heat sinking copper
on the PCB board where the part is mounted, as well as
the amount of air flow on the device. One of the ways to
measure the junction temperature directly is to use the
internal junction diode on one of the pins (PGOOD) to
measure its diode voltage change based on ambient
temperature change. First remove any external passive
component on the PGOOD pin, then pull out 100µA from
the PGOOD pin to turn on its internal junction diode and
bias the PGOOD pin to a negative voltage. With no output
current load, measure the PGOOD voltage at an ambient
temperature of 25°C, 75°C and 125°C to establish a slope
relationship between the delta voltage on PGOOD and
delta ambient temperature. Once this slope is established,
then the junction temperature rise can be measured as a
function of power loss in the package with corresponding
output load current. Keep in mind that doing so will violate
absolute maximum voltage ratings on the PGOOD pin,
however, with the limited current, no damage will result.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3605 (refer to Figure 3). Check the following in
your layout:
1. Do the capacitors C_IN connect to the power PV_IN and
power PGND as close as possible? These capacitors
provide 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 PGND and the (–) plate of C_IN.
3. The resistive divider, R1 and R2, must be connected
between the (+) plate of C_OUT and a ground line termi-
nated near SGND. The feedback signal V_FB should be
routed away from noisy components and traces, such
as the SW line, and its trace should be minimized. Keep
R1 and R2 close to the IC.
4. Solder the Exposed Pad (Pin 25) on the bottom of the
package to the PGND plane. Connect this PGND plane
to other layers with thermal vias to help dissipate heat
from the LTC3605.
5. Keep sensitive components away from the SW pin. The
R_T resistor, the compensation capacitor C_C and C_ITH
and all the resistors R1, R3 and R_C, and the INTV_CC
bypass capacitor, should be placed away from the SW
trace and the inductor L1. Also, the SW pin pad should
be kept as small as possible.
6. A ground plane is preferred, but if not available, keep
the signal and power grounds segregated with small-
signal components returning to the SGND pin which is
then connected to the PGND pin at the negative terminal
of the output capacitor, C_OUT.
C_IN L1
V_IN
V_OUT
C_OUT
GND
V_IN
V_OUT
GND
Figure 3a. Sample PCB Layout—Topside
Figure 3b. Sample PCB Layout—Bottom Side
3605fd
For more information www.linear.com/LTC3605
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