Datasheet

LM25037, LM25037-Q1

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SNVS572D –JULY 2008–REVISED MARCH 2013

If C

= 0.01 µF, t3 is ≊ 400 µs.

The time t2 provides a periodic cool-down time for the power converter in the event of a sustained overload or

short circuit. This off time results in lower average input current and lower power dissipation within the power

components. It is recommended that the ratio of t2 / (t1 + t3) be in the range of 5 to 10 to take advantage of this

feature.

If the application requires no delay from the first detection of a current limit condition to the onset of the hiccup

mode (t1 = 0), the RES pin can be left open (no external capacitor). If it is desired to disable the hiccup mode

entirely, the RES pin should be connected to ground (AGND).

PRINTED CIRCUIT BOARD LAYOUT

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 via 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 in order 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.

APPLICATION EXAMPLE

The following schematic shows an example of a 50W push-pull converter controlled by LM25037. The operating

input voltage range is 16V to 32V, and the output voltage is 5V. The output current capability is 10 Amps. The

converter is configured for current mode control with external slope compensation. An auxiliary winding of the

output filter inductor L3 is used to raise the VCC voltage to reduce the power dissipation.

Product Folder Links: LM25037 LM25037-Q1