PC BOARD LAYOUT

The LM5033 current sense and PWM comparators are very

fast, and as such will respond to short duration noise pulses.

Layout considerations are critical for the current sense filter.

The components at pins 3, 8, 9, and 10 should be as

physically close as possible to the IC, thereby minimizing

noise pickup in the PC tracks.

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

track to pin 7 of the IC rather than through the ground plane.

If the current sense circuit employs a sense resistor in the

drive transistor sources, a low inductance resistor should be

used. In this case all the noise sensitive low power grounds

should be connected in common near the IC, and then a

single connection made to the power ground (sense resistor

ground point).

The outputs of the LM5033, or of the high voltage gate driver

(if used), should have short direct paths to the power MOS-

FETs in order to minimize the effects of inductance in the PC

board traces.

If the internal dissipation of the LM5033 and any of the power

devices produces high junction temperatures during normal

operation, good use of the PC board’s ground plane can help

considerably to dissipate heat. The exposed pad on the

bottom of the LLP-10 package can be soldered to ground

plane on the PC board, and the ground plane should extend

out from beneath the IC to help dissipate the heat. The

exposed pad is internally connected to the IC substrate.

Additionally, the use of wide PC board traces where possible

can help conduct heat away from the IC. Judicious position-

ing of the PC board within the end product, along with use of

any available air flow (forced or natural convection) can help

reduce the junction temperatures.

APPLICATION CIRCUIT EXAMPLE

Figure 6 shows an example circuit for a half-bridge 200W

DC/DC converter built in a quarter brick format. The circuit is

that of an intermediate bus converter (IBC) which operates

open-loop (unregulated output), converting a nominal 48V

input to a nominal 9.0V output with a 30 m

Ω output imped-

ance. The current sense transformer (T2), and the associ-

ated filter at the CS pin, provide overcurrent detection at

approximately 23A. The auxiliary winding on T1 powers V

CC

and the LM5100’s V+ pin (once the outputs are enabled) to

reduce power dissipation within the LM5033. The LM5100

provides appropriate level shifting for Q1. Synchronous rec-

tifiers Q3 and Q4 minimize conduction losses in the output

stage. Dual comparators U2 and U3 provide under-voltage

and over-voltage sensing at Vin. The under-voltage sense

levels are 37V increasing, and 33V decreasing. The over-

voltage sense levels are 63V increasing, and 61.5V decreas-

ing. The circuit can be shut down by taking the ON/OFF input

below 0.8V. An external synchronizing frequency can be

applied to the SYNC input. Measured efficiency and output

characteristics for this circuit are shown in Figure 4 and

Figure 5.

20035416

FIGURE 3. Deadtime Adjustment

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