Datasheet

-200
-160
-120
-80
-40
0
40
80
PHASE (°)
120
160
200
-60
-40
-20
0
20
40
60
GAIN (dB)
1.E+02 1.E+03 1.E+04
1.E+05
FREQUENCY (Hz)
LM25088
LM25088-Q1
www.ti.com SNVS609H –DECEMBER 2008–REVISED MARCH 2013
Figure 27. Overall Loop Gain and Phase
PCB BOARD LAYOUT AND THERMAL CONSIDERATIONS
In a buck regulator there are two loops where currents are switched very fast. The first loop starts from the input
capacitors, through the buck MOSFET, to the inductor then out to the load. The second loop starts from the
output capacitor ground, to the regulator PGND pins, to the current sense resistor, through the Schottky diode, to
the inductor and then out to the load. Minimizing the area of these two loops reduces the stray inductance and
minimizes noise which can cause erratic operation. A ground plane is recommended as a means to connect the
input filter capacitors of the output filter capacitors and the PGND pin of the regulator. Connect all of the low
power ground connections (C
SS
, R
T
, C
RAMP
) directly to the regulator GND pin. Connect the GND pin and PGND
pins together through to topside copper area covering the entire underside of the device. Place several vias in
this underside copper area to the ground plane. The input capacitor ground connection should be as close as
possible to the current sense ground connection.
In a buck converter, most of the losses can be attributed to MOSFET conduction and switching loss, re-
circulating diode conduction loss, inductor DCR loss and LM25088 VIN and VCC loss. The other dissipative
components in a buck converter produce losses but these other losses collectively account for about 2% of the
total loss. Formulae to calculate all the major losses are described in their respective sections of this datasheet.
The easiest method to determine the power dissipated within the LM25088 is to measure the total conversion
losses (Pin-Pout), then subtract the power losses in the Schottky diode, MOSFET, output inductor and snubber
resistor. When operating at 7A of output current and at 36V, the power dissipation of the LM25088 is
approximately 550 mW. The junction to ambient thermal resistance of the LM25088 mounted in the evaluation
board is approximately 40°C with no airflow. At 25°C ambient temperature and no airflow, the predicted junction
temperature will be 25+40*0.55 = 47°C. The LM25088 has an exposed thermal pad to aid in power dissipation.
Adding several vias under the device will greatly reduce the controller junction temperature. The junction to
ambient thermal resistance will vary with application. The most significant variables are the area of copper in the
PC board; the number of vias under the IC exposed pad and the amount of forced air cooling. The integrity of
solder connection from the IC exposed pad to the PC board is critical. Excessive voids will greatly diminish the
thermal dissipation capacity.
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