10.1 Layout Guidelines

The LM25037 Current Sense and PWM comparators are very fast, and respond to short duration noise pulses. The components at the CS, COMP, SS, UVLO, RT2 and the RT1 pins should be as physically close as possible to the IC, thereby minimizing noise pickup on the PC board trace inductances.

Layout considerations are critical for the current sense filter. If a current sense transformer is used, both leads of the transformer secondary should be routed to the sense filter components and to the IC pins. The ground side of the transformer should be connected through a dedicated PC board trace to the AGND pin, rather than through the ground plane.

If the current sense circuit employs a sense resistor in the drive transistor source, low inductance resistors should be used. In this case, all the noise sensitive, low-current ground trace should be connected in common near the IC, and then a single connection made to the power ground (sense resistor ground point).

While employing current mode control, RAMP pin capacitor and CS pin capacitor must be placed close to the IC. Also, a short direct trace should be employed to connect RAMP capacitor to the CS pin.

The gate drive outputs of the LM25037 should have short, direct paths to the power MOSFETs to minimize inductance in the PC board The two ground pins (AGND, PGND) must be connected together with a short, direct connection, to avoid jitter due to relative ground bounce.

If the internal dissipation of the LM25037 produces high junction temperatures during normal operation, the use of multiple vias under the IC to a ground plane can help conduct heat away from the IC. Judicious positioning of the PC board within the end product, along with use of any available air flow (forced or natural convection) will help reduce the junction temperatures. If using forced air cooling, avoid placing the LM25037 in the airflow shadow of tall components, such as input capacitors.

10.2 Layout Example

Figure 35. Layout Recommendation